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Genes are linearly arranged on the chromosome

Genes are linearly arranged on the chromosome


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Could someone please explain the meaning of the statement given by TH Morgan:

"genes are linearly arranged on chromosomes"

Since according to my knowledge there are noncoding parts in between.


This combines two important findings to which Morgan's lab contributed.

  • Genes on the chromosome: They could show that mutated phenotypes were linked to the physical entity of chromosomes.
  • Arranged linearly: They could show that there are distances between different phenotypes correlated to a specific linear arrangement. These distances correspond to the physical distance on these chromosomes (and thus how frequently they are co-inherited). More specifically they showed that distances suggest that the space is linear, e.g.: in ---A--B-------C-D- , A would be commonly inherited with B, but less so with C, and even less with D… ; Theoretically one could have also thought that the chromosomes are circular (as they indeed are in some bacteria) or adopt an even more complex shape, e.g.: some net-like meshwork on which mutated phenotypes / genes sit. In these other cases one would anticipate more complex patterns of co-inheritance among all pairs of genes relative to a linear arrangement.

Genes and Chromosome

Genes are functional segments of DNA, which is highly organized and packed into nucleosomes and then chromosomes in eukaryotes, existing on two homologous chromosomes (two copies). While in prokaryotes genes are “naked” and single copy. Genes are arranged linearly on chromosomes, with a promoter, exons and introns. One-gene-one-protein hypothesis provided a link between a mutation and an enzyme to explain the observed phenotype in a biochemical pathway.

Genes control the genetic traits, and genes are DNA, which is organized into chromosomes. Both prokaryotes and eukaryotes have chromosomes, although the organization level is different. A gene is a region of DNA segment that controls certain trait of inheritance, while chromosome is the basic inheritance unit in cells. Genes are arranged on chromosomes linearly.

Prokaryotic chromosome
Prokaryotes do not have nucleus but they have their own chromosomes, which contain either linear or circular DNA molecules. DNA is naked in prokaryotes, which means they do not associate with histones. Prokaryotes usually only have one copy of chromosome, therefore they are haploid. Virus and organelles in an eukaryotes also have chromosomes similar to chromosomes in prokaryotes.

Eukaryotes chromosome
Eukaryotes usually have large, linear and multiple chromosomes within a cell nucleus. DNA forms highly packed structure and wrapped with histones. These chromosomes are visible under light microscope when a cell is in metaphase of mitosis.

Genome organization in eukaryotes
Genes are arranged linearly on chromosomes. Usually a gene has a regulatory region which is called promoter and a coding sequence which determines the amino acid sequence of the gene. For majority of the genes in mammalian cells, the coding sequence is interrupted by non-coding sequence called introns (the coding sequence are called exons).

One-gene-one-protein hypothesis
Proposed by Beadle and Tatum in 1942, this hypothesis linked the enzyme function to genes. Based on the hypothesis, a mutation outcome can be predicted if genes are within a biochemical pathway.


Introduction

Genes, which are carried on (a) chromosomes, are linearly organized instructions for making the RNA and protein molecules that are necessary for all of processes of life. The (b) interleukin-2 protein and (c) alpha-2u-globulin protein are just two examples of the array of different molecular structures that are encoded by genes. (credit “chromosome: National Human Genome Research Institute credit “interleukin-2”: Ramin Herati/Created from PDB 1M47 and rendered with Pymol credit “alpha-2u-globulin”: Darren Logan/rendered with AISMIG)

Since the rediscovery of Mendel’s work in 1900, the definition of the gene has progressed from an abstract unit of heredity to a tangible molecular entity capable of replication, expression, and mutation (Figure). Genes are composed of DNA and are linearly arranged on chromosomes. Genes specify the sequences of amino acids, which are the building blocks of proteins. In turn, proteins are responsible for orchestrating nearly every function of the cell. Both genes and the proteins they encode are absolutely essential to life as we know it.


Genes are linearly arranged on the chromosome - Biology

Introduction
Genes control the genetic traits, and genes are DNA, which is organized into chromosomes. Both prokaryotes and eukaryotes have chromosomes, although the organization level is different. A gene is a region of DNA segment that controls certain trait of inheritance, while chromosome is the basic inheritance unit in cells. Genes are arranged on chromosomes linearly.

Prokaryotic chromosome
Prokaryotes do not have nucleus but they have their own chromosomes, which contain either linear or circular DNA molecules. DNA is naked in prokaryotes, which means they do not associate with histones. Prokaryotes usually only have one copy of chromosome, therefore they are haploid. Virus and organelles in an eukaryotes also have chromosomes similar to chromosomes in prokaryotes.

Eukaryotes chromosome
Eukaryotes usually have large, linear and multiple chromosomes within a cell nucleus. DNA forms highly packed structure and wrapped with histones. These chromosomes are visible under light microscope when a cell is in metaphase of mitosis.

Genome organization in eukaryotes
Genes are arranged linearly on chromosomes. Usually a gene has a regulatory region which is called promoter and a coding sequence which determines the amino acid sequence of the gene. For majority of the genes in mammalian cells, the coding sequence is interrupted by non-coding sequence called introns (the coding sequence are called exons).

One-gene-one-protein hypothesis
Proposed by Beadle and Tatum in 1942, this hypothesis linked the enzyme function to genes. Based on the hypothesis, a mutation outcome can be predicted if genes are within a biochemical pathway.

Genes are functional segments of DNA, which is highly organized and packed into nucleosomes and then chromosomes in eukaryotes, existing on two homologous chromosomes (two copies). While in prokaryotes genes are “naked” and single copy. Genes are arranged linearly on chromosomes, with a promoter, exons and introns. One-gene-one-protein hypothesis provided a link between a mutation and an enzyme to explain the observed phenotype in a biochemical pathway.

  • External concept map to describe the relationship of Mendelian genetics and other disciplines in biology
  • Internal concept map to reveal the relationships among the content within this tutorial
  • Step by step analysis of Mendel’s original experiments
  • Brief explanation of Mendelian genetics at chromosome level
  • Flow chart on calculating chi square
  • Diagrams and symbols for human pedigree reading
  • The Karyotype and Chromosome Banding
  • Organization of DNA into Chromosome
  • Nucleosomes
  • Organization of Genome
  • One-gene-one-enzyme Hypothesis
  • Enzyme deficient and genetic disease
  • Genetic Counseling

See all 24 lessons in Genetics, including concept tutorials, problem drills and cheat sheets:
Teach Yourself Genetics Visually in 24 Hours


Linkage and Crossing Over of Genes (With Diagram) | Genetics

In Mendelian experiment the common sweet peas with two or more “elements” (now called genes) are considered heterozy­gous. In a hybrid, the segregation of any one is completely independent of the rest. That means, all the genes are distributed to the gametes at random.

Mendel considered inheritance of seven characters and all the characters showed random assortment. It was later demon­strated cytologically that peas have seven pairs of chromosomes. It was genetically shown that the genes for the characters studied by Mendel are each located in a different member of the seven pairs of chromosomes.

The independent assortment between two gene pairs, each with complete dominance and affecting different charac­ters, results in 9: 3: 3: 1 ratio .among the offspring of heterozygotes for both pairs (AaBb x AaBb).

But Bateson and Punnet in England in 1906 found exception to this law when they bred a di-hybrid strain of sweet pea. They crossed purple (P) flowered-long pollen grained (L) sweet peas with red flowered (p) round pollen grained (1) ones.

The f1 gen­eration was purple flowered-long pollen grained (P-L-). But in f2 generation purple-long, purple- round, red-long and red-round appeared in the ratio of 11: 1: 1: 3 with too many parental types.

Bateson and Punnet explained that the gametic coupling between P and L and p and 1 lead to such high ratio of parental combination. Again, in the next generation, Pl and pL characters tend to come together more often. This is called repulsion where the parental associations did not form but new combinations tend to persist.

Both coupling and repulsion are possible when genes are arranged linearly along the length of chromosomes. When two genes remain in the same chromosome they tend to be inherited together to the offspring. This is called linkage and two genes are linked.

But, during reduction division, ex­change of segments of a pair of chromo­somes occur, as a result linked genes are separated and recombination occurs. This is called crossing over (Fig. 46.3). T. H. Mor­gan (1911) showed in his experiment with Drosophila, the linear arrangement of genes in a chromosome and gave explanation of both coupling and repulsion.

So linkage may be defined as the ten­dency of two or more genes in the same chromosome to remain in their original combination in the process of inheritance.

Thus, (i) The linked genes remain in the same chromosome and tend to be transmitted together generation after generation, (ii) Linked genes never show Mendel’s Principle of Independent Assortment, (iii) Less dis­tance between the genes show stronger link­age bond.

Example of Linkage:

In Drosophila, grey body (b + ) and long wing (v + ) characters are dominant over black body (b) and vestigeal wing (v). When a pure grey-long is crossed to a black vestigeal the offspring produced in fi generation are grey-long hybrid (Fig. 46.4).

In the f1 female genotype, the chromo­some might come together and separate without crossing over between b + and v + giving eggs of two non-crossover classes b + v + and b v. The second possibility is that crossing over might take place between b + and v + given eggs of two crossover classes b + v and b v +

If a cross is made between fi female and a black-vestigeal male (double recessive) four categories of offspring’s will be pro­duced (Fig. 46.4). The grey-long and black-vestigeal are the no crossovers (parental combination) and constitute more than 50% of the total offspring’s while the grey-vestigeal and black-long are crossover classes (new combination) and constitute less than 50%. So the tendency of two genes to remain in their original combination is always greater. In this case, as the crossing over breaks the linkage in certain.percentage, it is called incomplete linkage.

Complete Linkage:

The complete linkage or absence of cross­ing over between the genes on the same chromosome is a rarity in most sexually reproducing species. However, in male Drosophila, complete linkage is observed.

In other cases, when genes are so closely as­sociated that they are always transmitted together upon coming from the same parent, linkage between them is considered com­plete. The 4th chromosome mutations of fruit fly Drosophila melanogaster show little or no independent assortment during transmis­sion.

A Drosophila with 4th chromosome caring genes producing bent wings and shaven bristles crossed to a normal fly produce normal appearing heterozygous in f1 germination. When such f1 flies are crossed to the homozygous bent-shaven stock, the offspring’s are almost all phenotypically either bent and shaven or normal for both characters. The complete absence of new combination is the evidence of very strong and complete linkage between the two genes on the same homologue.

The cross of fi hybrids with a double recessive male or female is known as test cross or back cross. In this process, only recessive genes will be added so that all the mutations will be expressive. It is the sim­plest way to determine the per cent of crossing over.


Genes are linearly arranged on the chromosome - Biology

Introduction
Genes control the genetic traits, and genes are DNA, which is organized into chromosomes. Both prokaryotes and eukaryotes have chromosomes, although the organization level is different. A gene is a region of DNA segment that controls certain trait of inheritance, while chromosome is the basic inheritance unit in cells. Genes are arranged on chromosomes linearly.

Prokaryotic chromosome
Prokaryotes do not have nucleus but they have their own chromosomes, which contain either linear or circular DNA molecules. DNA is naked in prokaryotes, which means they do not associate with histones. Prokaryotes usually only have one copy of chromosome, therefore they are haploid. Virus and organelles in an eukaryotes also have chromosomes similar to chromosomes in prokaryotes.

Eukaryotes chromosome
Eukaryotes usually have large, linear and multiple chromosomes within a cell nucleus. DNA forms highly packed structure and wrapped with histones. These chromosomes are visible under light microscope when a cell is in metaphase of mitosis.

Genome organization in eukaryotes
Genes are arranged linearly on chromosomes. Usually a gene has a regulatory region which is called promoter and a coding sequence which determines the amino acid sequence of the gene. For majority of the genes in mammalian cells, the coding sequence is interrupted by non-coding sequence called introns (the coding sequence are called exons).

One-gene-one-protein hypothesis
Proposed by Beadle and Tatum in 1942, this hypothesis linked the enzyme function to genes. Based on the hypothesis, a mutation outcome can be predicted if genes are within a biochemical pathway.

Genes are functional segments of DNA, which is highly organized and packed into nucleosomes and then chromosomes in eukaryotes, existing on two homologous chromosomes (two copies). While in prokaryotes genes are “naked” and single copy. Genes are arranged linearly on chromosomes, with a promoter, exons and introns. One-gene-one-protein hypothesis provided a link between a mutation and an enzyme to explain the observed phenotype in a biochemical pathway.

  • External concept map to describe the relationship of genes and chromosome and other disciplines in biology
  • Internal concept map to reveal the relationships among the content within this tutorial
  • Side by side comparison of prokaryotic and eukaryotic chromosomes
  • Color diagram and scheme to describe DNA packing into eukaryotic chromosomes
  • Step by step deduction of one-gene-one-protein hypothesis
  • Color table listing some genetic disease and the mutated genes that caused these diseases.
  • The Karyotype and Chromosome Banding
  • Organization of DNA into Chromosome
  • Nucleosomes
  • Organization of Genome
  • One-gene-one-enzyme Hypothesis
  • Enzyme deficient and genetic disease
  • Genetic Counseling

See all 24 lessons in college biology, including concept tutorials, problem drills and cheat sheets:
Teach Yourself Genetics Visually in 24 Hours

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Biology Question Bank – 69 MCQs on “Mendelian & Post Mendelian Genetics” – Answered!

69 Questions with Answers and Explanations on Mendelian & Post Mendelian Genetics for Biology Students.

1. Which contribute to the success of Mendel?

(a) qualitative analysis of data

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(b) observation of distinct inherited traits

(c) his knowledge of biology

(d) consideration of one character at one time.

Answer and Explanation:

1. (c): Consideration of one character at one time contribute to the success of Mendel. Mendel’s contribution was unique because of his methodological approach to a definite problem, use of clear cut variables and application of mathematics (statistics) to the problem. Using pea plants and statistical methods, Mendel was able to demonstrate that traits were passed from each parent inheritance of genes.

2. Two linked genes a and b show 20% recombination, the individuals of a dihybrid cross between ++/+ + x ab/ab shall show gametes

(c) ++ 40 : ab 40 : + a 10 : + b : 10

(d) ++ 30 : ab 30 : + a 20 : + b : 20

Answer and Explanation:

2. (c): Two linked genes a and b show 20% recombination. The individuals of a dihybrid cross between + +/+ + x ab/ab shall show gametes + + 40 : + a 10 : + b : 10

3. A normal green male Maize is crossed with albino female. The progeny is albino because

(a) trait for a albinism is dominant

(b) the albinos have biochemical to destroy plastids derived from green male

(c) plastids are inherited from female parent

(d) green plastids of male mus. have mutated.

Answer and Explanation:

3. (c): normal green male maize is crossed with albino female. The progeny is albino because plastids are inherited from female parents.

4. tt mates with Tt. What will be characteristic of offspring?

Answer and Explanation:

4. (b): When tt mate with Tt, the characteristics of offspring will be 50% recessive. On mating tt and Tt, 50% individuals are recessive and 50% inhibit heterozygous dominant characteristics.

5. ABO blood group system is due to

(a) Multifactor inheritance

Answer and Explanation:

5. (c): ABO blood group system is due to multiple allelism. A gene can have more than two alleles or allelomorphs, which can be expressed by mutation in wild form in more than one ways. These alleles or allelomorphs make a series of multiple alleles. The mode of inheritance in case of multiple alleles is called multiple allelism. A well known and simplest example of multiple allelism is the inheritance of ABO blood groups in human beings. In human population, 3 different alleles for this character are found – I A I B and 1°. A person is having only two of these three alleles and blood type can be determined.

6. In a genetic cross having recessive epistasis, F2 phenotypic ratio would be

Answer and Explanation:

6. (c): In a genetic cross having recessive epistasis, F2 phenotypic ratio would be 9:3 :4. The recessive epistasis is illustrated by coat colour in mouse, the coat colour is determined by A/a pair, recessive allele b is epistatic over A/a. Thus, in the presence of bb, both A and aa give the same phenotype (albino). The F2 ratio is generally 9 : 3 : 4.

7. Cross between AaBB and aaBB will form

Answer and Explanation:

7. (a): Cross between AaBB and aaBB will form 1 AaBB : 1 aaBB. On crossing, AaBB x aaBB gives 50% individuals having genotype AaBB and 50% individuals having genotype aaBB.

8. RR (Red) Antirrhinum is crossed with white (WW) one. Offspring RW are pink. This is an example of

Answer and Explanation:

8. (b): RR (Red) Antirrhinum is crossed with white (ww) one. Offspring RW are pink. This is an example of incomplete dominance. Incomplete dominance may be defined as the partial expression of both alleles in heterozygote so that the phenotype is intermediate between those of homozygotes.

In plants like the Snapdragon (Antirrhinum majus), when a plant bearing red (RR) flowers is crossed with the plant bearing white (it) flowers, the flowers of F, plants were crossed to each other, F2 generation showed red (RR), pink (Rr) and white (rr) flowered plants in the ratio of 1 : 2 :1. Dominance thus appeared to be partial or incomplete.

9. The allele which is unable to express its effect in the presence of another is called

Answer and Explanation:

9. (d): The allele which is unable to express its effect in the presence of another is called recessive. A member of a pair of alleles that does not show its effect in the phenotype in the presence of any other allele. It is denoted by small letter.

10. Blue eye colour is recessive to brown eye colour. A brown eyed man whose mother was blue eyed marries a blue-eyed woman. The children will be

(a) both blue eyed and brown eyed 1 : 1

(d) blue eyed and brown eyed 3:1.

Answer and Explanation:

10. (a): Blue eye colour is recessive to brown eye colour. A brown eyed man whose mother was blue eyed marries a blue eyed woman. The children shall be both blue eyed and brown eyed 1:1. The brown eyed man will have the genotype Bb and his wife bb. Hence Bb x bb = Bb: bb. Hence, the ratio is 1 : 1.

11. A dihybrid condition is

Answer and Explanation:

11. (d): A dihybrid condition is TtRr. Dihybrid conditions involves simultaneous inheritance of two pairs of mendelian factors or genes.

12. Mendel’s last law is

Answer and Explanation:

12. (c): Mendel’s last law is independent assortment. The principle of independent assortment states that when two individuals differ from each other in two or more pairs of factors, the inheritance of one pair is quite independent of the inheritance of ethers.

13. First geneticist/father of genetics was

Answer and Explanation:

13. (b): An Australian Monk, Gregor Mendel, developed his theory of in heritance. He formulated the law of Heredity. Therefore, he is called the ‘father of genetics’.

14. The contrasting pairs of factors in Mendelian crosses are called

Answer and Explanation:

14. (b): The contrasting pairs of factors in Mendelian crosses are called allelomorphs. Alleles or allelomorphs are the different forms of a gene, having the same locus on homologous chromosomes and are subject to mendelian (alternative) inheritance.

15. Multiple alleles control inheritance of

Answer and Explanation:

15. (d): ABO blood group system is due to multiple allelism. A gene can have more than two alleles or allelomorphs, which can be expressed by mutation in wild form in more than one ways.

These alleles or allelomorphs make a series of multiple alleles. The mode of inheritance in case of multiple alleles is called multiple allelism. A well known and simplest example of multiple allelism is the inheritance of ABO blood groups in human beings. In human population, 3 different alleles for this character are found – I A I B and 1°. A person is having only two of these three alleles and blood type can be determined.

16. A man of A-blood group marries women of AB blood group. Which type of progeny would indicate that man is heterozygous A?

Answer and Explanation:

16. (d): I A I° x 1 A I B gives us the following genotypes I A I A , I°I B , I A I B . Hence, when a man of blood group A marries a women of AB blood group, B progeny would indicate that man is heterozygous A.

17. An organism with two identical alleles is

Answer and Explanation:

17. (d): An organism with two identical alleles is homozygous. Homozygous have identical genes at the same locus on each member of a pair of homologous/ chromosomes.

18. Segregation of Mendelian factors (no linkage, no crossing over) occurs during

Answer and Explanation:

18. (a): Segregation of Mendelian factors (no linkage, no crossing over) occurs during anaphase I. At anaphase I, actual segregation occurs, but two similar alleles occurs in the dyad chromosome which separate at anaphase II.

19. In a cross between AABB x aabb, the ratio of F2 genotypes between AABB, AaBB, Aabb and aabb would be

Answer and Explanation:

19. (c): In a cross between AABB x aabb, the ratio of F2 genotypes between AABB, AaBB, Aabb and aabb would be 1 : 2 : 2 : 1. Genotype is the genetic makeup of an individual. This may refer to just one trait or it may refer to the combination of many or all of the traits of the individuals.

20. A gene pair hides the effect of another. The phenomenon is

Answer and Explanation:

20. (a): A gene pair hides the effect of another. The phenomenon is epistasis. When one gene modifies or masks the action of another gene. This is known as epistasis, and it can give rise to unusual ratios in genetic crosses. The gene, which can override the expression of the other, is the epistasis gene and the other, which get masked, is the hypostatic gene.

21. An allele is dominant if it is expressed in

(a) both homozygous and heterozygous states

(c) heterozygous combination

Answer and Explanation:

21. (a): An allele is dominant if it is expressed in both homozygous states. Dominant alleles expresses itself in the homozygous as well as heterozygous condition. It is denoted by capital letter.

22. A child of O-group has B-group father. The genotype of father will be

Answer and Explanation:

22. (d): The child of O-group has B-group father. The genotype of father will be I B I°. The genotype of the child would be I°I° (recessive). Hence, the genotype of the father can only be I B I°.

23. Mendel studied inheritance of seven pairs of traits in Pea which can have 21 possible combinations.

If you are told that in one of these combinations, independent assortment is not observed in later studies, your reaction will be

(a) independent assortment principle may be wrong

(b) mendel might not have studied all the combinations

(d) later studies may be wrong.

Answer and Explanation:

23. (b): Law of independent assortment states that when two individuals differ from each other in two or more pairs of factors, the inheritance of one pair is quite’ independent of the inheritance of other. Law of independent assortment is applicable to only those factors or genes which are located on different chromosomes.

24. A polygenic inheritance in human beings is

Answer and Explanation:

24. (a): A polygenic inheritance in human beings is skin colour. Polygenic or quantitative inheritance occurs when several genes occupying different loci on the same homologous pairs of chromosomes govern one tract. Inheritance of colour in man is an example of polygenic inheritance. The presence of melanin pigment in the skin colour. The more the pigment the darker is the skin.

25. Two dominant nonallelic genes are 50 map units apart. The linkage is

Answer and Explanation:

25. (d): Two dominant nonallelic genes are 50 map units apart. The linkage is absent/incomplete. Chromosome mapping is based on the fact that genes are linearly arranged in the chromosome and frequency of crossing over is directly proportional to the distance between two genes. Dominant genes show cis arrangement. At 50 map units cis is changed to trans and vice-versa hence no fixed linkage is present.

26. Of a normal couple, half the sons are haemophiliac while half the daughters are carriers. The gene is located on

(c) one X-chromosome of mother

(d) both the X-chromosomes of mother.

Answer and Explanation:

26. (c): Of a normal couple, half the sons are haemophilic while half the daughters are carries. The gene is located on one X-chromosomes of mother. Cross between a haemophilic carrier female X h X and normal male would yield 50% of the sons being haemophilic and 50% of the daughter are carriers.

27. Which of the following is suitable for experiment on linkage?

Answer and Explanation:

27. (b): AABB x aabb is suitable for experiment on linkage. Linkage is the tendency for certain genes tend to be inherited together, because they are on the same chromosome. Thus, parental combinations of characters are found more frequently in offspring than non-parental.

28. Mr. Kapoor has Bb autosomal gene pair and d allele sex-linked. What shall be proportion of Bd in sperms?

Answer and Explanation:

28. (c): Genotype of Mr. Kapoor will be Bbd hence one fourth of the sperms will have Bd.

29. Of both normal parents, the chance of a male child becoming colour blind are

(b) possible only when all the four grand parents had normal vision

(c) possible only when father’s mother was colour blind

(d) possible only when mother’s father was colour blind.

Answer and Explanation:

29. (d): Of both normal parents, the chance of a male child becoming colour blind are possible only when mother’s father was colour blind. It is an example of criss cross inheritance. If a cross is made between two sexes differing in certain characters, in such a way that character of one sex remains hidden in the opposite sex of F, generation, but it is passed on to the same sex in the F, generation, it is said to exhibit criss cross inheritance.

30. The phenomenon, in which an allele of one gene suppresses the activity of an allele of another gene, is known as

Answer and Explanation:

30. (a): Epistasis is the phenomenon by which a gene suppresses the phenotypic expression of a nonal leiic gene. In this type of interaction one gene masks or changes the

Effect of another gene. This is a counterpart of dominance. While dominance works at interallelic but interagenic level, epistasis works at intergenic level.

31. When two genetic loci produce identical phenotypes in cis and trans position, they are considered to be

Answer and Explanation:

31. (c): E.B. Lewis in 1951 reported from a cross of apricot eyed and white eyed flies in Drosophila, he obtained F1 having intermediate eye colour. In F2, he had expected segregation only for apricot and white, but he recovered very low frequency of wild type. Since those alleles behaved as non-alleles, Lewis preferred to call them pseudoalleles and the phenomenon as pseudoallelism.

In pseudoallelism in cis position both the mutant alleles are on one chromosome. So the other chromosome will be normal and will be able to produce the end result. But in trans position the sequence of steps involved in synthesis will be interrupted due to mutations on either of the two homologous chromosomes thus leading to a mutant phenotype.

32. When two dominant independently assorting genes react with each other, they are called

Answer and Explanation:

32. (b): Complementary genes are those nonallelic genes which independently show a similar effect but produce a new trait when present together in the dominant form. Supplementary genes are a pair of nonallelic a gene, one of which produce its effect independently in the dominant state while the dominant allele of the second gene is without any independent effect but is able to modify the effect of the former to produce a new trait. Duplicate genes are independent genes producing the same or similar effect.

33. The polygenic genes show

Answer and Explanation:

33. (c): Polygenes are also called multiple factor. Polygene is a gene with an individually small effect on the phenotype that interacts with other polygenes controlling the same character to produce the continuous quantitative variation typical of such traits as height, weight and skin colour.

34. A fruit fly is heterozygous for sex-linked genes, when mated with normal female fruit fly, the males specific chromosome will enter egg cell in the proportion

Answer and Explanation:

34. (c): The female Drosophila possesses two homomorphic sex chromosomes, (XX) and the male Drosophila contains two heteromorphic sex chromosomes (XY). The differential or non-homologous region of Y-chromosome is mostly heterochromatic. The female parent produces only one type of eggs (22 + X).

The male parent produces two types of gametes (22 + Y) and (22 + X). They are produced in equal proportions. As the two types of sperms are produced in equal proportions, there are equal chances of getting a male or female fly in a particular mating.

35. How many different types of genetically different gametes will be produced by a heterozygous plant having the genotype AABbCc?

36. Crossing over in diploid organism is responsible for

(b) recombination of linked alleles

Answer and Explanation:

36. (b): Crossing over is the reciprocal exchange of segments between nonsister chromatids of a pair of homologous chromosomes. It results in recombination of genes.

The nonsister chromatids in which exchange of segments takes place are known as cross-overs or recombinants while other chromatids not involved in exchange of segments are called noncross-over or parental types. Single crossing-over at one point between two nonsister chromatids resulting in two cross-over and two parental types. Double crossing over at two points in a tetrad of chromatids.

37. If Mendel had studied the seven traits using a plant with 12 chromosomes instead of 14, in what way would his interpretation have been different?

(a) he would not have discovered the law of independent assortment

(b) he would have discovered sex linkage

(c) he could have mapped the chromosome

(d) he would have discovered blending or incomplete dominance.

Answer and Explanation:

37. (a): According to principle of independent assortment, the two factors of each trait assort at random and independent of the factors of other traits at the time of meiosis and get randomly as well as independently rearranged in the offspring. Principle of independent assortment is applicable to only those factors or genes which are present on different chromosomes. Chromosomes have hundreds of genes which show linked inheritance or linkage. Linkage is the phenomenon of certain genes (present on the same chromosome) to remain together and get inherited through generations. The seven characters that Mendel chooses were present on 14 chromosomes and so they did not show linkage but if present on 12 chromosomes they would have shown linkage and the principle of independent assortment would not have been discovered.

38. When a single gene influences more than one trait it is called

Answer and Explanation:

38. (b): The ability of a gene to have multiple phenotypic effect because it influences a number of characters simultaneously is known as pleiotrophy. The phenomenon by which a gene suppresses the phenotypic expression of a nonallelic gene is called epistasis.

39. In hybridization, Tt x tt gives rise to the progeny of ratio

40. A gene pair hides the effect of another gene. The phenomenon is called

Answer and Explanation:

40. (c): Epistasis is the phenomenon of suppression of phenotypic expression of gene by a nonallelic gene which shows its own effect. The gene which masks the effect of another is called epistatic gene while the one which is suppressed is termed hypostatic gene. Epistasis is of three types – dominant, recessive and dominant-recessive. The epistatic ratio is 12: 3: 1.

41. According to mendelism which character shows dominance?

(a) terminal position of flower

(b) green colour in seed coat

42. Due to the cross between TTRrxttrr the resultant progenies show what percent of tall, red flowered plants

43. Independent assortment of genes does not takes place when

(a) genes are located on homologous chromosomes

(b) genes are linked and located on same chromosome

(c) genes are located on non-homogenous chromosome

Answer and Explanation:

43. (b): According to the principle of law of independent assortment the two factors of each trait assort at random and independent of the factors of other traits at the time of meiosis and get randomly as well as independently rearranged in the offspring. Principle of law of independent assortment is applicable to only those factors or genes which are present on different chromosomes.

44. Ratio of complementary genes is

Answer and Explanation:

44. (d): If two genes present on different loci produce the same effect when present alone but interact to form a new trait when present together, they are called complementary genes. The F2 ratio is modified to 9: 7 instead of 9: 3: 3: 1.

45. When dominant and recessive alleles express itself together it is called

Answer and Explanation:

45. (a): Dominance is the phenomenon in which one member of a pair of allelic genes expresses itself as a whole (complete dominance) or in part (incomplete dominance).

Co dominance is when the two genes neither show dominant-recessive relationship nor show intermediate condition, but both of them express themselves simultaneously. This has been reported in roan character of cattle (i.e., patches of 2 different colours on the skin). Pseudo dominance is when a recessive allele of normal homologous chromosome will behave like a dominant allele and will be phenotypically expressed.

46. A and B genes are linked. What shall be genotype of progeny in a cross between AB/ab and ab/ab

47. Two nonallelic a gene produces the new phenotype when present together but fail to do so independently then it is called

Answer and Explanation:

47. (a): Epistasis is the phenomenon of suppression of phenotypic expression of gene by a nonallelic gene which shows its own effect. The gene which masks the effect of another is called epistatic gene while the one which is suppressed is termed hypostatic gene. Epistasis is of three types – dominant, recessive and dominant-recessive. Two or more different pairs of alleles, with a presumed cummulative effect governing quantitive traits like size, pigmentation etc., are called polygenes.

48. A gene is said to be dominant if

(a) it expresses its effect only in homozygous state

(b) it expresses its effect only in heterozygous condition

(c) it expresses its effect both in homozygous and heterozygous condition.

(d) it never expresses its effect in any condition.

(c) it expresses its effect both in homozygous and heterozygous condition.

49. On selfing a plant of F1 generation with genotype “AABbCC”, the genotypic ratio in F2-generation will be

(d) 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1.

Answer and Explanation:

49. (a): Selfing is the process of fertilization with polar or male gametes of the same individual. AABbCC will produce two type of gametes ABC and AbC. Thus in F2 generation three genotypes will be obtained. These are AABBCC, AABbCC and AAbbCC in the ratio of 1: 2: 1. Phenotypically AABBCC and AABbCC are same. So the phenotypic ratio in F2 generation will be 3: 1.

50. There are three genes a, b, c. Percentage of crossing over between a and b is 20%, b and c is 28% and a and c is 8%. What is the sequence of genes on chromosome?

Answer and Explanation:

50. (a): Crossing over and recombination frequencies : One crossing over produces 50% recombinant types. Therefore, frequency of crossing over would be double the frequency of recombinants. 1% recombinants (1% cross-over) mean crossing over in 2% meiocytes. Linkage/ Cross over/ Chromosome maps is a graphic representation of relative positions/ order and relative distances of genes in a chromosome in the form of line like a linear road map depicting different places and their relative distances without giving exact mileage.

It is based on Morgan’s hypothesis (1911) that frequency of crossing over/recombination between two linked genes is directly proportional to the physical distance between the two. 1 map unit or centrimorgan is equivalent 1% recombination between two genes. Percentage of crossing over between a and b is 20% so they are 20 map distance apart and b and c are 28 map distance apart. So that corrects sequence of genes chromosomes will be as

51. Which of the following is an example of pleiotropy ?

Answer and Explanation:

51. (c): Pleiotropic gene is such a gene which has a wider effect on phenotype i.e., it controls several phenotypic traits. Sickle cell anaemia is considered to be caused by one such pleiotropic gene.

Sickle cell anaemia is caused due to a mutation in P globin gene of haemoglobin. Single point mutation leads to one change of amino acid. Hb P chain 1 2 345678 i

But this small point mutation & change in the resulting P globin loads to sickling of RBCs, haemolytic anaemia, pain in joints, splenomegaly etc.

Moreover carrier of sickle cell anaemia gene remains protected from malaria. Thus the gene is naturally selected in tropical & subtropical Asia and Africa.

52. Two crosses between the same pair of genotypes or phenotypes in which the sources of the gametes are reversed in one cross, is known as

Answer and Explanation:

52. (b): A reciprocal cross means that the same two parent are used in two experiments in such a way that if in one experiment A is used as the female parent and B is used as the male parent then in the other experiment A will be used as the male parent and B as the female parent. Thus the sources of gametes are reversed. When the F, individuals obtained in a cross is crossed with the recessive parent is called a test cross. When inheritance of two pairs of contrasting character is studied simultaeneously it is called dihybrid cross.

53. The genes controlling the seven pea characters studied by Mendel are now known to be located on how many different chromosomes?

Answer and Explanation:

53. (d): Mendel worked on garden pea. He chose only those characters which showed consistent results. He worked on seven characters. These characters showed complete independent assortment despite the seven characters chosen by him were present on four chromosomes -1, 4, 5 and 7.

54. Which one of the following traits of garden pea studied by Mendel was a recessive feature?

Answer and Explanation:

54. (b): Mendel worked on garden pea and choose seven characters for this. Green seed colour is the recessive character in his experiment and the dominant character for seed colour is yellow. Axial flower position, green pod colour and round seed shape are all dominant characters.

55. One of the parents of a cross has a mutation in its mitochondria. In that cross, that parent is taken as a male. During segregation of F2 progenies that mutation is found in

(a) one-third of the progenies

(d) fifty percent of the progenies.

Answer and Explanation:

55. (b): Mutation is a sudden alteration of the chemical structure of a gene or the alteration of its position on the chromosome by breaking and rejoining of the chromosome. It has occurred in male parent. But organelles like mitochondria, chloroplast etc. are a part of cytoplasmic inheritance.

Cytoplasmic inheritance is the passage of traits from parents to offspring through structures present inside the cytoplasm of contributing gametes. Plasma genes occur in plastids, mitochondria, plasmids and some special particles like kappa particles, sigma particles, etc.

In higher organisms cytoplasmic inheritance is called maternal inheritance because the zygote receives most of its cytoplasm from the ovum. Therefore, cytoplasmic inheritance is usually unparental. “=n none of the progeny will show mutation as the mutation is male in mitochondria.

56. Lack of independent assortment of two genes A and B in fruit fly Drosophila is due to

Answer and Explanation:

56. (c): Mendel’s law of independent assortment states that when the parent differs from each other in two or more pairs of contrasting characters, the inheritance of one pair of factor is independent of the other. For the character to assort independently they should be located on separate non homologous chromosomes. Genes present on the same chromosome show linkage.

It means that these characters remain together and thus low numbers of combinations are formed. This phenomenon is called linkage such genes are called linked genes. So A and B are linked genes.

57. A male human is heterozygous for autosomal genes A and B and is also hemizygous for hemophilic gene h. What proportion of his sperms will be abh?

Answer and Explanation:

57. (a): The male human is heterozygous for autosomal gene A and B and also hemizygous for hemophilic gene h then his genotype will be AaBbX h Y because hemophilia is a sex linked trait that is present an X-chromosome. So the total number of gametes will be abX h , abY, ABX h , ABY, AbX h , AbY, aBX aBY. So the proportion of abX h sperm will be 1/8.

58. A nutritionally wild type organism, which does not required any additional growth supplement is known as

Answer and Explanation:

58. (d): Prototroph is the nutritionally wild strain which is unable to grow in the minimal medium unless additional nutrients were added to the medium. Phenotype is the kind of organism produced by the reaction of a given genotype with the environment. Holotype is one of the original type used to describe a new species.

59. In a plant, red fruit (R) is dominant over yellow fruit (r) and tallness (7) is dominant over shortness (t). If a plant with RRTt genotype is crossed with a plant that is rrtt,

(a) 25% will be tall with red fruit

(b) 50% will be tall with red fruit

(c) 75% will be tall with red fruit

(d) all the offspring will be tall with red fruit.

(b) 50% will be tall with red fruit

60. In order to find out the different types of gametes produced by a pea plant having the genotype AaBb it should be crossed to a plant with the genotype

Answer and Explanation:

60. (c): A test cross involving the crossing of F, individual with the homozygous recessive parent. It is done to find out homozygous and heterozygous individuals. So AaBb, should be crossed with aabb.

61. Which one of the following is an example of polygenic inheritance?

(b) flower colour in Mirabilis jalapa

(c) production of male honey bee

(d) pod shape in garden pea

Answer and Explanation:

61. (a): Quantitative or polygenic inheritance is that type of inheritance in which the complete expression of a trait is controlled by two or more genes in which a dominant allele of each gene contributes only a unit fraction of the trait and the total phenotypic expression is the sum of total effect of all the dominant alleles of genes/polygenes. The complete trait developes only by the cumulative effect of all homologous and nonhomologous dominant alleles influencing that trait. Human skin colour (melanin) is controlled by three pairs of polygenes A, B and C.

62. Phenotype of an organism is the result of

(a) genotype and environment interactions

(c) cytoplasmic effects and nutrition

(d) environmental changes and sexual dimorphism

Answer and Explanation:

62. (a): The external manifestation, morphological or physiological expression of an individual with regard to one or more characters is cali-d phenotype. For recessive genes, phenotype and genotype are similar. For dominant genes, the phenotype is same for both homozygous states. Phenotype is influenced by environment as well as age.

A child definitely differs from adolescent, the latter from adult and ar. adult from aged one. Many phenotypes are determined by multiple genes. Thus, the identity of phenotype is determined by genotype and environment.

63. How many different kinds of gametes will be produced by a plant having the genotype AABbCC?

64. In Mendel’s experiments with garden pea, round seed shape (RR) was dominant over wrinkled seeds (rr), yellow cotyledon (YY) was dominant over green cotyledon (yy). What are the expected phenotypes in the F2 generation of the cross RRYY x rryy?

(a) round seeds with yellow cotyledons, and wrinkled seeds with yellow cotyledons

(b) only round seeds with green cotyledons

(c) only wrinkled seeds with yellow cotyledons

(d) only wrinkled seeds with green cotyledons

(a) round seeds with yellow cotyledons, and wrinkled seeds with yellow cotyledons

65. Test cross involves

(a) crossing between two genotypes with dominant trait

(b) crossing between two genotypes with recessive trait

(c) crossing between two F, hybrids

(d) crossing the F, hybrid with a double recessive genotype

Answer and Explanation:

65. (d): Test cross is obtained when F, plant is crossed with homozygous recessive individual. By this cross the heterozygocity and homozygocity of the organisms can be tested. The offsprings will be 100% dominant if the individual was homozygous dominant, the ratio will be 50% dominant and 50% recessive in case of hybrid or heterozygous individual.

66. A common test to find the genotype of a hybrid is by

(a) crossing of one F2 progeny with female parent

(b) studying the sexual behaviour of F. progenies

(c) crossing of one F, progeny with male parent

(d) crossing of one F2 progeny with male parent.

Answer and Explanation:

66. (c): A common test to find the genotypes of a hybrid is by crossing of one F, progeny with male parent.

67. In the hexaploid wheat, the haploid («) and basic (a – ) numbers of chromosomes are

Answer and Explanation:

67. (c): Hexaploid wheat is a result of allopolyploidy induced by doubling the chromosome number of the hybrid produced by crossing two different plants. In hexaploid wheat triticale 2m = 6x = 42. So x stands for basic chromosome number and n for haploid chromosome number. So, n- 21 and x = l for hexaploid wheat.

68. Inheritances of skin colour in humans is an example of

Answer and Explanation:

68. (b): Polygenic inheritance is that type of inheritance in which the complete expression of a trait is controlled by two or more genes in which a dominant allele of each gene contribution only a unit fraction of the trait and total phenotypic expression is the sum total of a additive or cumulative effect of all the dominant alleles of genes/polygenes. Human skin colour is an example of such polygenic inheritance which is controlled by three pairs of polygenes A, B and C. Negro/black colour is due to presence of all the six dominant contributing alleles AABBCC. Very light colour or white olour is due to presence of all six recessive noncontributing alleles aabbcc.

69. In pea plants, yellow seeds are dominant to green. If a heterozygous yellow seeded plant is crossed with a green seeded plant, what ratio of yellow and green seeded plants would you expect in F[ generation?


Significance of crossing over

1. Crossing over leads to the production of new combination of genes and provides basis for obtaining new varieties of plants.

2. It plays an important role in the process of evolution.

3. The crossing over frequency helps in the construction of genetic maps of the chromosomes.

4. It gives us the evidence for linear arrangement of linked genes in a chromosome.

Genes are arranged linearly in a chromosome. The point in a chromosome where the gene is located is called locus. The diagrammatic representation of location and arrangement of genes and relative distance between linked genes of a chromosome is called linkage or genetic map.

The unit of genetic map is Morgan or centimorgan. When the percentage of crossing over between two linked genes is 1 per cent, then the map distance between the linked genes is one morgan.

There is a greater probability of occurrence of crossing over, when the two genes are farther apart in a chromatid. The probability of crossing over between two genes is directly proportional to the distance between them.

When two genes are nearer, the probability of occurrence of crossing over between them is limited.

Let A, B, C, D and E be five knots on a string separated by the distances as shown. The probability of making a random cut between two knots is directly proportional to the distance between them. Every cut separates A from E, whereas 5/100 th cut only separates C from D.If the knots or genes linearly arranged on a chromosome in randoms are the cross overs, then C and D remain linked, whereas A and E will not show linkage in this situation.


1. Coupling and Repulsion theory

The two dominant alleles or recessive alleles occur in the same homologous chromosomes, tend to inherit together into same gamete are called coupling or cis configuration (Figure: 3.5 ). If dominant or recessive alleles are present on two different, but homologous chromosomes they inherit apart into different gamete are called repulsion or trans configuration (Figure: 3. 6).




The Embryo Project Encyclopedia

In 1913, Alfred Henry Sturtevant published the results of experiments in which he showed how genes are arranged along a chromosome. Sturtevant performed those experiments as an undergraduate at Columbia University, in New York, New York, under the guidance of Nobel laureate Thomas Hunt Morgan. Sturtevant studied heredity using Drosophila, the common fruit fly. In his experiments, Sturtevant determined the relative positions of six genetic factors on a fly’s chromosome by creating a process called gene mapping. Sturtevant’s work on gene mapping inspired later mapping techniques in the twentieth and twenty-first centuries, techniques that helped scientists identify regions of the chromosome that when mutated cause organisms to develop abnormally and to create treatments to cure those kinds of disorders.

Throughout the early 1900s, many scientists studied the physical materials and processes that control heredity. In 1902, researchers Theodor Boveri working in Germany and Walter Sutton working in the US each proposed that chromosomes, the threadlike structures found in the nucleus of cells, carry the physical materials that underlie heredity. Although Boveri and Suttons’ hypotheses were later supported, many scientists initially rejected or were skeptical of their theory. Morgan was among those skeptics. In 1910, Morgan published an article outlining his disagreements. However, later that year, he published research results that provided experimental evidence that physical heritable factors, or genes, are found on chromosomes.

After that experiment, Morgan and the researchers in his laboratory at Columbia University continued to learn more about the processes that control heredity. Morgan began studying a process called crossing over, or recombination, which occurs between two of the paired chromosomes in the gamete cells of organisms. During crossing over, one of the paired chromosomes, inherited from that father, comes together and exchanges genetic information with its chromosome pair, which was inherited from the mother. That exchange of information forms two new chromosomes that contain a mix of paternal and maternal genetic information. A mixed chromosome is called a recombinant. As Morgan, Sturtevant, and the rest of Morgan’s research team showed, bits of the chromosomes, which they called factors and later called genes, were organized along chromosomes in some way.

As a result of that organization, in 1911, Morgan hypothesized that the likelihood of any two factors crossing over together depended on how far apart they were on the chromosome. From what Morgan and other scientists had observed, the location of genetic factors that crossed over seemed random. Given that randomness, Morgan hypothesized that the farther apart any two factors were on a chromosome, the more likely they were to be separated by recombination. In other words, factors near each other on a chromosome were more likely to remain together when crossing over than were factors that were farther apart. Morgan further proposed that the frequency of recombination between any two factors was proportional to the distance they were separated on the chromosome.

Later that same year, in 1911, Morgan gave recombination data to Sturtevant, an undergraduate researcher in Morgan’s lab. Morgan had studied Drosophila and had observed the frequency of recombination between various factors controlling eye color, body color, and wing shape. According to Sturtevant, he realized that those frequencies could be used to create a genetic map of where factors were located on chromosomes. Sturtevant reported that he went home that night and, to the neglect of his homework, determined the order in which the factors were arranged along the chromosome. Over the next several years, Sturtevant performed further experiments to validate his hypothesized arrangement. In 1913, he published his results in the Journal of Experimental Zoology. Sturtevant’s article was titled “The linear arrangement of six sex-linked factors in drosophila, as shown by their mode of association.”

In his experiments, Sturtevant obtained flies with six different types of genetic factors, some of which Morgan had discovered among the flies he had bred in his lab. Sturtevant named the different factors B, C, O, P, R, and M. In developing flies, those factors affected the flies’ eye color, body color, and wing shape. Previously, scientists had observed that C and O never appeared to recombine, meaning that they always appeared together in the same fly. Sturtevant treated those factors as if they shared the same location on a chromosome, and called them CO throughout his research report.

Sturtevant theorized that if genetic factors were arranged linearly across the chromosome, he could deduce the order of those factors by determining all of the distances between pairs of factors. Sturtevant described the logic in his article. He stated that if there were three hypothetical genetic factors arranged in order, A, B, and C, the distance between A- C would be the sum of the distances between A-B and B-C.

With that logic, Sturtevant developed a method to determine the relative distances between the six individual genetic factors. According to Morgan’s hypothesis, the distance between two factors was related to the frequency with which those factors recombined. In other words, the more likely two factors appeared together in a fly, the more likely they were located close to each other on the chromosome. Therefore, Sturtevant sought to determine how often each of the factor pairs recombined, a measure called the recombination frequency, in order to find the respective distances between those pairs. He mated two flies that had chromosomes with differences in two of their genetic factors and observed the number of recombinants that were found in later generations of the offspring. Sturtevant used that method for all pairs of factors. For instance, he observed how many times there was recombination between B and CO, B and P, B and M, B and O, CO and P, etc. He then calculated the recombination frequency between each of those pairs of factors by dividing the respective number of recombinant offspring by the total number of offspring.

After calculating the recombination frequencies, Sturtevant used the data to determine the arrangement of the factors on the chromosome. By assuming that factors with the largest recombination frequencies were located the farthest apart, Sturtevant determined which factors were on opposite ends of the chromosome. He further analyzed the recombination data to determine the positioning of the factors between the two outermost factors. The data supported Sturtevant’s theory that the factors were arranged linearly on the chromosome. Furthermore, Sturtevant found that the specific order of the factors was B, CO, P, R, and M.

Sturtevant recognized that the distances between the factors may not be entirely accurate due to a process he called double recombination. Double recombination occurs when paired chromosomes recombine two times, which can cause the resulting offspring to appear as if recombination never occurred. Sturtevant and other members of Morgan’s lab later studied double recombination to determine how to make more accurate genetic maps. Despite not accounting for double recombination, Sturtevant’s arrangement was validated by later research.

Throughout the twentieth century, other researchers modified Sturtevant’s method to develop more accurate forms of gene mapping that were eventually used to map human genomes. In 1984, an international collaboration of researchers called the Human Genome Project began mapping the human genome. Over the following decades, researchers working on the Human Genome Project identified portions of the genome. In 2001, they published a complete first draft of the entire human genome.

The development of gene mapping techniques also enabled scientists to create a method, called gene therapy, of treating and preventing diseases. Gene therapy often relied on identifying genes that cause or promote disease, finding their location, removing them, and then replacing them with different genes. In 1990, scientists at the National Institute of Health headquartered in Bethesda, Maryland, conducted the first human application of gene therapy to cure disease. They successfully treated a four-year-old girl that had adenosine deaminase deficiency, a disease that causes infants to be born with severe immunodeficiency. Scientist’s later developed treatments for other genetic diseases, including cancer, cystic fibrosis, acquired immune deficiency syndrome, and Alzheimer’s disease.

Sturtevant’s 1913 experiments provided scientists with an early method of gene mapping that was later used to treat diseases.