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Linkage and Crossing over

BiologyPrinciples of Inheritance and VariationFor NEET aspirants

Linkage and crossing over explain why some genes do not assort independently. This page first covers why Mendel's work stayed unrecognised until 1900, the discovery of chromosomes, and how Sutton and Boveri used meiosis to explain Mendel's laws in the chromosomal theory of inheritance. It then covers Morgan's work on Drosophila, linkage, recombination and Sturtevant's gene maps, as in the NCERT Class 12 chapter Principles of Inheritance and Variation. NEET often asks Morgan's recombination values and the meaning of tight and loose linkage and crossing over.

On this page1Mendel ignored2Rediscovery3Chromosomal theory4Independent alignment5Morgan and Drosophila6Linkage and recombination7Gene maps8Exam essentials9Quick revision10Solved examples11Practice
Key Points at a Glance
  1. Mendel published his work in 1865; it stayed unrecognised till 1900.
  2. ★ Must learn 1900: de Vries, Correns and von Tschermak independently rediscovered Mendel's results.
  3. ★ Must learn Sutton and Boveri: chromosomes behave like genes; Sutton called the synthesis the chromosomal theory of inheritance.
  4. Chromosomes and genes both occur in pairs, segregate at gamete formation and assort independently.
  5. Morgan verified the theory using Drosophila melanogaster: life cycle about two weeks, many progeny, easily told sexes.
  6. ★ Must learn Linkage: physical association of genes on a chromosome. Recombination: generation of non-parental gene combinations.
  7. ★ Must learn White-yellow: 1.3% recombination (tight linkage). White-miniature wing: 37.2% (loose linkage).
  8. ★ Must learn Alfred Sturtevant used recombination frequency as a measure of gene distance to map genes.
  9. Genetic maps are the starting point for sequencing whole genomes, as in the Human Genome Project.

1. Why Mendel's Work Stayed Unrecognised

  • ★ Exam imp Mendel published his work on the inheritance of characters in 1865, but it remained unrecognised till 1900.
  1. Communication was not easy in those days, so his work could not be widely publicised.
  2. His concept of genes (factors) as stable, discrete units that did not blend was not accepted by his contemporaries as an explanation for the apparently continuous variation seen in nature.
  3. His approach of using mathematics to explain biological phenomena was totally new and unacceptable to many biologists of his time.
  4. Although his work suggested that factors were discrete units, he could not give any physical proof that factors existed or say what they were made of.
Memory Trick C-C-M-P: Communication was poor, Continuous variation seemed to contradict discrete factors, Mathematics in biology was unacceptable, and there was no Physical proof of factors.
Key idea
Mendel's ideas were ahead of his time: no publicity, no acceptance of maths in biology, and no physical proof of factors.

2. Rediscovery of Mendel and the Discovery of Chromosomes

  • ★ Exam imp In 1900, three scientists, de Vries, Correns and von Tschermak, independently rediscovered Mendel's results on the inheritance of characters.
  • By then, advances in microscopy allowed scientists to observe cell division carefully.
  • This led to the discovery of structures in the nucleus that appeared to double and divide just before each cell division.
  • These were called chromosomes (coloured bodies), as they were seen by staining.
  • By 1902, the movement of chromosomes during meiosis had been worked out.
Key idea
Better microscopes revealed chromosomes; by 1902 their movement in meiosis was known.

3. The Chromosomal Theory of Inheritance

  • ★ Exam imp Walter Sutton and Theodore Boveri noted that the behaviour of chromosomes was parallel to the behaviour of genes.
  • They used chromosome movement to explain Mendel's laws.
  • Recall: chromosomes behave differently in mitosis (equational division) and meiosis (reduction division).
  • Chromosomes as well as genes occur in pairs.
  • The two alleles of a gene pair are located at homologous sites on homologous chromosomes.
Meiosis and germ cell formation in a cell with four chromosomes A cell with two pairs of homologous chromosomes, a long pair coloured yellow and orange and a short pair coloured red and green, is followed through meiosis. In G1 each chromosome is single; in G2 each has two chromatids. In meiosis I anaphase the homologous chromosomes of each bivalent move to opposite poles. In meiosis II anaphase the chromatids separate. Four germ cells result, each with one long and one short chromosome. G1 G2 Meiosis I Meiosis II Germ cells anaphase anaphase Bivalent
Figure 1: Meiosis and germ cell formation in a cell with four chromosomes. Homologous chromosomes separate in meiosis I, so each germ cell gets only one member of each pair.

A comparison between the behaviour of chromosomes and genes

ChromosomesGenes
Occur in pairsOccur in pairs
Segregate at the time of gamete formation, so that only one of each pair passes to a gameteSegregate at gamete formation, and only one of each pair passes to a gamete
Independent pairs segregate independently of each otherOne pair segregates independently of another pair
Tips and Tricks The two columns are worded almost alike. Chromosomes are the visible structures; genes are the units they carry. Because the behaviour matches point for point, genes must lie on chromosomes.

3.1 Independent alignment of chromosome pairs

  • In meiosis I, the two chromosome pairs can align at the metaphase plate independently of each other, and so separate independently at anaphase.
  • Compare the four coloured chromosomes below: long orange and long yellow form one pair; short green and short red form the other.
  • Possibility I: the long orange and short green chromosomes go to the same pole, and the long yellow and short red to the other.
  • Possibility II: the long orange chromosome goes with the short red, and the long yellow with the short green.
  • Both are equally likely, so four kinds of germ cells form in equal numbers.
Independent assortment of chromosomes Two possible arrangements of two pairs of homologous chromosomes in meiosis I. In possibility I, the long orange and short green chromosomes move to one pole and the long yellow and short red to the other. In possibility II, the long orange goes with the short red and the long yellow with the short green. Meiosis II then separates the chromatids, giving germ cells with different combinations of chromosomes. Possibility I Meiosis I - anaphase Meiosis II - anaphase Germ cells Long orange and short green at one pole; long yellow and short red at the other Possibility II Meiosis I - anaphase Meiosis II - anaphase Germ cells Long orange and short red at one pole; long yellow and short green at the other
Figure 2: Independent assortment of chromosomes. The orange chromosome can go to a pole with either the green or the red chromosome, so the two pairs assort independently.
  • Sutton and Boveri argued that the pairing and separation of a pair of chromosomes would lead to the segregation of the pair of factors they carried.
  • ★ Exam imp Sutton united the knowledge of chromosomal segregation with Mendelian principles and called it the chromosomal theory of inheritance.
★ Very important Chromosomal theory of inheritance: genes are carried on chromosomes, and the segregation and independent assortment of chromosomes during meiosis explain Mendel's laws. It was proposed by Sutton, who, with Boveri, noted the parallel behaviour of chromosomes and genes.
Memory Trick Sutton Synthesised. Sutton and Boveri saw the parallel; Sutton made the Synthesis and named it the chromosomal theory of inheritance.
Quick Recall: tap to check
Which three scientists rediscovered Mendel's work in 1900?
de Vries, Correns and von Tschermak.
Who proposed the chromosomal theory of inheritance?
Walter Sutton, together with Theodore Boveri, who noted the parallel behaviour; Sutton named the synthesis.
Where are the two alleles of a gene pair located?
At homologous sites on homologous chromosomes.
In a table comparing chromosomes and genes, how can you tell which column describes chromosomes?
Chromosomes are visible structures that segregate during meiosis; genes are the units they carry. Since both columns describe the same behaviour, the comparison itself shows that genes lie on chromosomes.
Key idea
Genes behave like chromosomes in meiosis, so genes must be carried on chromosomes.

4. Morgan and the Fruit Fly

  • The chromosomal theory was verified experimentally by Thomas Hunt Morgan and his colleagues.
  • This also revealed the basis for the variation produced by sexual reproduction.
  • ★ Exam imp Morgan worked with the tiny fruit fly, Drosophila melanogaster, which was very suitable for such studies.
Drosophila melanogaster: male and female Two fruit flies seen from above, side by side, labelled (a) and (b). (a) The male is smaller; its abdomen is shorter, with a rounded tip that is dark at the end. (b) The female is larger; her abdomen is longer, banded and pointed at the tip. Both have red eyes, a brownish body, six legs and a pair of clear wings. (a) (b) Male Female
Figure 3: Drosophila melanogaster: (a) male, (b) female. The two sexes are easy to tell apart, one reason Morgan chose this fly.

Why Drosophila suited genetic studies

  1. It can be grown on a simple synthetic medium in the laboratory.
  2. It completes its life cycle in about two weeks.
  3. A single mating produces a large number of progeny flies.
  4. The sexes are clearly different: male and female flies are easily distinguishable.
  5. It has many types of hereditary variations that can be seen with low power microscopes.
Memory Trick Fly in five: Food, Fast, Family, Females, Features. Simple synthetic food; fast two-week life cycle; large family from one mating; females easily told from males; many visible features (variations).
Key idea
Drosophila breeds fast and cheaply, gives many progeny and shows visible variations: ideal for testing the chromosomal theory.

5. Linkage and Recombination

  • Morgan carried out several dihybrid crosses in Drosophila to study genes that were sex-linked; they were similar to Mendel's dihybrid crosses in peas.
  • Example: he crossed yellow-bodied, white-eyed females with brown-bodied, red-eyed males, and intercrossed their F1 progeny.
  • ★ Exam imp The two genes did not segregate independently of each other: the F2 ratio deviated very significantly from 9 : 3 : 3 : 1, the ratio expected for independent genes.
  • Morgan's group knew these genes were on the X chromosome.
  • When the two genes of a dihybrid cross were on the same chromosome, the proportion of parental gene combinations was much higher than that of the non-parental type.
  • Morgan attributed this to the physical association, or linkage, of the two genes.
★ Very important Linkage: the term Morgan coined for the physical association of genes on a chromosome. Recombination: the term he coined for the generation of non-parental gene combinations.
  • Morgan's group also found that genes grouped on the same chromosome can be linked to different degrees.
  • Some genes were very tightly linked and showed very low recombination (Cross A below).
  • Others were loosely linked and showed higher recombination (Cross B below).
  • ★ Exam imp The genes white and yellow were very tightly linked and showed only 1.3 per cent recombination.
  • ★ Exam imp The genes white and miniature wing showed 37.2 per cent recombination.
  • In the figure, dominant wild-type alleles carry a + sign as a superscript (y+, w+, m+).
Linkage: results of two dihybrid crosses conducted by Morgan Two crosses in Drosophila for genes on the X chromosome. Cross A uses genes y (yellow body) and w (white eye), which lie close together: the F1 female's eggs carry the parental combinations in 98.7 per cent and the recombinant combinations in 1.3 per cent. Cross B uses genes w (white eye) and m (miniature wing), which lie far apart: 62.8 per cent parental and 37.2 per cent recombinant. Wild-type alleles are marked with a plus sign. The F2 phenotypes are wild type, yellow white, white and yellow in cross A, and wild type, white miniature, miniature and white in cross B. Cross A: genes y and w Parental y w y w y+ w+ Y F1 y w y+ w+ y w Y yellow, white wild type wild type yellow, white Parental types Recombinant types y+ w+ y w y+ w y w+ 98.7% 1.3% wild type yellow, white white yellow Cross B: genes w and m Parental w m w m w+ m+ Y F1 w m w+ m+ w m Y white, miniature wild type wild type white, miniature Parental types Recombinant types w+ m+ w m w+ m w m+ 62.8% 37.2% wild type white, miniature miniature white Gametes (X chromosomes) of the F1 female Each F2 fly gets one of these X chromosomes from its mother; the name under it is the F2 phenotype.
Figure 4: Linkage: results of two dihybrid crosses by Morgan. Genes y and w (Cross A) give only 1.3% recombinants, while w and m (Cross B) give 37.2%, so the linkage between y and w is stronger than between w and m.
FeatureCross A (y and w)Cross B (w and m)
GenesYellow body, white eyeWhite eye, miniature wing
Parental types98.7%62.8%
Recombinant types1.3%37.2%
Strength of linkageVery tight (genes close together)Loose (genes far apart)
NEET Focus Low recombination means tight linkage, and high recombination means loose linkage. Linked genes give many more parental than recombinant combinations, so the dihybrid F2 departs from 9 : 3 : 3 : 1. Remember the pairs: white-yellow 1.3%, white-miniature 37.2%.
Extra Depth: Recombinants arise by crossing over: the exchange of segments between non-sister chromatids of homologous chromosomes during prophase I of meiosis. The farther apart two genes lie, the more often a crossover falls between them, which is why recombination frequency rises with distance.
Quick Recall: tap to check
Who coined the terms linkage and recombination?
Thomas Hunt Morgan.
How much recombination did white and miniature wing show?
37.2 per cent, so they are loosely linked.
Which F2 ratio did Morgan's linked genes fail to give?
9 : 3 : 3 : 1; parental combinations were much more frequent.
Key idea
Genes on the same chromosome tend to stay together; the closer they are, the rarer the recombinants.

6. Gene Maps

  • ★ Exam imp Morgan's student Alfred Sturtevant used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between the genes.
  • He used it to map the positions of genes on the chromosome.
  • Today, genetic maps are widely used as a starting point in sequencing whole genomes, as in the Human Genome Sequencing Project.
  • Closely located genes assort together; distantly located genes, because of recombination, assort almost independently.
  • Linkage maps therefore correspond to the arrangement of genes on a chromosome.
  • So the Law of Independent Assortment does not hold true for genes located on the same chromosome (linked genes).
Memory Trick Morgan Linked, Sturtevant Mapped. Morgan coined linkage and recombination; his student Sturtevant turned recombination frequency into distance on a map.
Key idea
Recombination frequency measures the distance between genes, so it can be used to map them.

7. Exam Essentials

Pairs to Match

Scientist or termMatches with
de Vries, Correns, von TschermakRediscovered Mendel's results in 1900
Walter SuttonChromosomal theory of inheritance
Sutton and BoveriParallel behaviour of chromosomes and genes
Thomas Hunt MorganLinkage and recombination in Drosophila
Alfred SturtevantGene mapping by recombination frequency
LinkagePhysical association of genes on a chromosome
RecombinationGeneration of non-parental gene combinations
White and yellow1.3% recombination; tight linkage
White and miniature wing37.2% recombination; loose linkage
ChromosomesColoured bodies seen by staining
MitosisEquational division
MeiosisReduction division
Genetic mapsStarting point for whole-genome sequencing
Exceptions
  • Linked genes do not follow the Law of Independent Assortment.
  • Linked genes give a F2 that is not 9 : 3 : 3 : 1; parental types are in excess.
  • Morgan's crosses studied sex-linked genes on the X chromosome, unlike Mendel's genes in peas.
  • Sutton and Boveri gave the idea, but only Sutton named the chromosomal theory; Morgan verified it experimentally.
  • Low recombination does not mean loose linkage; it means tight linkage.

Numbers to Remember

  • 1865: Mendel published his work; 1900: rediscovery.
  • 1902: chromosome movement during meiosis worked out.
  • Drosophila life cycle: about two weeks.
  • White-yellow: 1.3% recombination (98.7% parental).
  • White-miniature wing: 37.2% recombination (62.8% parental).

8. Quick Revision

  • Mendel published in 1865; ignored till 1900 for four reasons: communication, continuous variation, mathematics, no physical proof.
  • 1900: de Vries, Correns and von Tschermak rediscovered his results independently.
  • Chromosomes: double and divide before cell division; coloured bodies seen by staining.
  • 1902: chromosome movement in meiosis worked out.
  • Sutton and Boveri: chromosomes behave like genes; alleles sit at homologous sites on homologous chromosomes.
  • Both occur in pairs, segregate at gamete formation and assort independently.
  • Chromosome pairs align independently at the metaphase plate in meiosis I.
  • Sutton named the chromosomal theory of inheritance.
  • Morgan verified it using Drosophila melanogaster.
  • Drosophila: synthetic medium, two-week life cycle, many progeny, distinct sexes, visible variations.
  • Yellow-bodied, white-eyed females × brown-bodied, red-eyed males: F2 far from 9 : 3 : 3 : 1.
  • Linkage: physical association of genes; recombination: non-parental combinations.
  • White-yellow 1.3% (tight); white-miniature 37.2% (loose).
  • Sturtevant: recombination frequency measures gene distance; gene maps.
  • Genetic maps guide whole-genome sequencing, as in the Human Genome Project.

9. Solved Examples

Solved Example 1
Match List I with List II.
List I: A. Sutton, B. Morgan, C. Sturtevant, D. de Vries
List II: I. Mapped genes using recombination frequency, II. Rediscovered Mendel's results, III. Chromosomal theory of inheritance, IV. Coined the terms linkage and recombination
Choose the correct answer:
(A) A-III, B-IV, C-I, D-II
(B) A-IV, B-III, C-I, D-II
(C) A-III, B-I, C-IV, D-II
(D) A-II, B-IV, C-I, D-III
Solution:

Answer: (A). Sutton named the chromosomal theory (III), Morgan coined linkage and recombination (IV), Sturtevant mapped genes (I), and de Vries was one of the three who rediscovered Mendel (II).

Solved Example 2
Read the statements about Morgan's work.
A. He worked with Drosophila melanogaster.
B. The genes he studied in these crosses were on the X chromosome.
C. White and yellow showed 37.2 per cent recombination.
D. Parental gene combinations were much more frequent than non-parental ones.
E. His dihybrid F2 gave a 9 : 3 : 3 : 1 ratio.
Choose the correct answer:
(A) A, B and D only
(B) A, C and D only
(C) B, C and E only
(D) A, B, D and E only
Solution:

Answer: (A). C is wrong: white and yellow showed 1.3 per cent; 37.2 per cent was for white and miniature wing. E is wrong: the ratio deviated very significantly from 9 : 3 : 3 : 1.

Solved Example 3
Arrange the events in chronological order.
A. Rediscovery of Mendel's results
B. Mendel publishes his work
C. Chromosome movement during meiosis worked out
D. Mendel begins his hybridisation experiments
Choose the correct answer:
(A) D, B, A, C
(B) B, D, A, C
(C) D, B, C, A
(D) D, A, B, C
Solution:

Answer: (A). Experiments began in 1856 (D), publication in 1865 (B), rediscovery in 1900 (A), and chromosome movement in meiosis by 1902 (C).

Solved Example 4
Two genes on the same chromosome show 2 per cent recombination, and two others show 30 per cent. Which statement is correct?
(A) The first pair is loosely linked
(B) The first pair lies closer together
(C) The second pair assorts in a 9 : 3 : 3 : 1 ratio
(D) Both pairs are unlinked
Solution:

Answer: (B). Recombination frequency measures distance, so 2 per cent means the genes are close and tightly linked; 30 per cent means they are farther apart and loosely linked.

Solved Example 5
Which of the following is NOT a reason why Drosophila suited Morgan's studies?
(A) It completes its life cycle in about two weeks
(B) A single mating gives many progeny
(C) Its sexes look identical
(D) It shows many hereditary variations visible under low power microscopes
Solution:

Answer: (C). The sexes of Drosophila are clearly different and easy to tell apart.

10. Practice Questions

Practice Questions
  1. Match List I with List II.
    List I: A. Linkage, B. Recombination, C. Chromosomes, D. Meiosis
    List II: I. Reduction division, II. Coloured bodies, III. Non-parental gene combinations, IV. Physical association of genes on a chromosome
    (A) A-IV, B-III, C-II, D-I (B) A-III, B-IV, C-II, D-I (C) A-IV, B-III, C-I, D-II (D) A-II, B-III, C-IV, D-IAnswer: (A). Linkage-IV, recombination-III, chromosomes-II, meiosis-I.
  2. Read the statements.
    A. Chromosomes and genes both occur in pairs.
    B. Alleles of a gene pair lie on non-homologous chromosomes.
    C. Chromosome pairs align independently at the metaphase plate in meiosis I.
    D. Mitosis is a reduction division.
    Choose the correct answer: (A) A and C only (B) A, B and C only (C) B and D only (D) A, C and D onlyAnswer: (A). B is wrong (homologous sites on homologous chromosomes); D is wrong (mitosis is equational).
  3. Which of the following was NOT a reason for Mendel's work remaining unrecognised? (A) Poor communication (B) Use of mathematics in biology (C) Lack of physical proof of factors (D) His experiments used too few plantsAnswer: (D). His experiments had a large sampling size.
  4. Statement I: Tightly linked genes show high recombination.
    Statement II: The genes white and yellow in Drosophila are tightly linked.
    (A) Both correct (B) I correct, II incorrect (C) I incorrect, II correct (D) Both incorrectAnswer: (C). Tightly linked genes show low recombination; white and yellow showed only 1.3 per cent.
  5. In Cross B of Morgan (white and miniature wing), what percentage of the gametes of the F1 female carried parental combinations?Answer: 62.8 per cent (100 - 37.2).
  6. Two heterozygous parents are crossed. If the two loci are linked, what would be the distribution of phenotypic features in the F1 generation for a dihybrid cross?Answer: With linked loci, independent assortment fails. Parental combinations appear in much higher proportion than recombinant ones, so the progeny depart from 9 : 3 : 3 : 1. With complete linkage, the result depends on how the alleles sit: if both dominant alleles are on the same chromosome (AB/ab × AB/ab), the progeny are 3 dominant : 1 recessive, as for a single gene; if they are on opposite homologues (Ab/aB), they are 1 : 2 : 1. With loose linkage, some recombinants appear, in proportion to the distance between the genes.
  7. Briefly mention the contribution of T.H. Morgan in genetics.Answer: He experimentally verified the chromosomal theory of inheritance using Drosophila. His dihybrid crosses on X-linked genes showed that genes on the same chromosome do not assort independently. He coined the terms linkage and recombination, and showed that genes differ in the strength of their linkage (white-yellow 1.3%, white-miniature 37.2%).
  8. Who proposed the chromosomal theory of inheritance?Answer: Walter Sutton, who, with Theodore Boveri, noted the parallel behaviour of chromosomes and genes and united chromosomal segregation with Mendelian principles.

Common Mistakes to Avoid

Watch out
  • Saying Morgan proposed the chromosomal theory. Correct: Sutton proposed it; Morgan verified it experimentally.
  • Writing 37.2% for white and yellow. Correct: white-yellow is 1.3%; white-miniature wing is 37.2%.
  • Linking high recombination with tight linkage. Correct: tight linkage gives low recombination.
  • Saying Mendel's work was ignored because of small samples. Correct: his samples were large; the reasons were communication, continuous variation, mathematics and no physical proof.
  • Placing alleles on non-homologous chromosomes. Correct: alleles lie at homologous sites on homologous chromosomes.
  • Expecting 9 : 3 : 3 : 1 from linked genes. Correct: linked genes give an excess of parental combinations.
  • Calling mitosis a reduction division. Correct: mitosis is equational; meiosis is the reduction division.
  • Crediting Morgan with gene mapping. Correct: his student Alfred Sturtevant mapped genes.

Frequently Asked Questions

Why did Mendel's work remain unrecognised for 35 years?

Communication was poor, so the work was not widely known. His idea of discrete, non-blending factors did not seem to explain the continuous variation seen in nature. Using mathematics in biology was unacceptable to many biologists, and he could not give physical proof of what factors were.

What is the chromosomal theory of inheritance?

It states that genes are carried on chromosomes, and that the pairing, segregation and independent assortment of chromosomes during meiosis explain Mendel's laws. Sutton and Boveri noted the parallel behaviour of chromosomes and genes, and Sutton named the theory. Morgan later verified it experimentally.

How does chromosome behaviour in meiosis explain Mendel's laws?

Homologous chromosomes, which carry the two alleles of a gene, separate in meiosis I, so each gamete gets one allele: this is segregation. Different chromosome pairs align at the metaphase plate independently, so genes on different chromosomes assort independently.

Why was Drosophila melanogaster suitable for Morgan's experiments?

It grows on a simple synthetic medium, completes its life cycle in about two weeks, and a single mating gives many progeny. Males and females are easy to tell apart, and it shows many hereditary variations that can be seen with low power microscopes.

What is the difference between linkage and recombination?

Linkage is the physical association of genes on the same chromosome, which keeps parental combinations together. Recombination is the generation of non-parental gene combinations. Morgan coined both terms. Tightly linked genes show little recombination, while loosely linked genes show more.

What did Morgan find for the genes white, yellow and miniature wing?

White and yellow were very tightly linked, with only 1.3 per cent recombination, so 98.7 per cent of combinations stayed parental. White and miniature wing were loosely linked, with 37.2 per cent recombination. So the linkage between yellow and white is stronger than between white and miniature.

How did Sturtevant make a genetic map?

Alfred Sturtevant, Morgan's student, used the frequency of recombination between pairs of genes on the same chromosome as a measure of the distance between them. From these distances he mapped the order and positions of genes on the chromosome.

Why do linked genes not follow the Law of Independent Assortment?

Genes on the same chromosome are physically associated, so they tend to pass together into the same gamete. Parental combinations therefore greatly outnumber recombinant ones, and a dihybrid cross does not give the 9 : 3 : 3 : 1 ratio expected for independent genes.

Previous year questions on Linkage and Crossing over

3 questions from past papers, each with a step-by-step solution.

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