Short notes on Chromosomal Theory of Inheritance and Linkage & Recombination for CBSE & NEET

The Chromosomal Theory of Inheritance is possibly one of the most world-changing discoveries ever made and its also a very very important topic for NEET as well as CBSE class 12th examination.  Read the following short notes on the Chromosomal Theory of Inheritance to understand why it is so significant!

Chromosomal Theory of Inheritance: Also called as Sutton and Boveri Theory

Key points:

  • Boveri and Sutton’s chromosome theory of inheritance states that genes are found at specific locations on chromosomes, and that the behavior of chromosomes during meiosis can explain Mendel’s laws of inheritance.
  • Thomas Hunt Morgan, who studied fruit flies (Drosophila melanogaster), provided the first strong confirmation of the chromosome theory.
  • Morgan discovered a mutation that affected fly eye color. He observed that the mutation was inherited differently by male and female flies.
  • Based on the inheritance pattern, Morgan concluded that the eye color gene must be located on the X chromosome.

Morgan’s experiments began when he found a mutation in a gene that made a fly’s eyes white, rather than their normal red.  He found that the eye color gene was inherited in different patterns by male and female flies. Male flies have an X and a Y chromosome (XY), while female flies have two X chromosomes (XX).  Morgan realized that the eye color gene was being inherited in the same pattern as the X chromosome.

The first white-eyed fly he found was male, and when this fly was cross with normal, red-eyed female flies, the F1 offspring were all red-eyed. Therefore, the white allele was concluded to be recessive.

But when the F2 flies were crossed to each other, all of the female F2 flies were red-eyed, while about half of the male F2 flies were white-eyed. Clearly, the male and female flies were inheriting the trait in different patterns. In fact, they were inheriting it in the same pattern as the chromosome X.

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Earlier, we said that female flies have an XX genotype and male flies have an XY genotype. If we stick the eye color gene on the X chromosome (writing it as a little subscript, w+ for red and w for white), we can use a Punnett square to show Morgan’s first cross:

The predictions match the F1 phenotypes, but this set of phenotypes could also be explained by a gene that is not on the X chromosome, since all the flies were red-eyed (regardless of sex).

So the real test comes when the F1 flies are mated to make the F2 generation:

Here is where the X makes the difference.

Our Punnett square correctly predicts that all of the female flies will have red eyes, while half of the male flies will have white eyes. The male flies get their only X chromosome from their mother, who is heterozygous (Xw+,Xw) , leading to the fifty-fifty split of phenotypes.

A strong confirmation of this theory came later, from Morgan’s student Calvin Bridges. Bridges showed that rare male or female flies with unexpected eye colors were produced through nondisjunction (failure to separate) of sex chromosomes during meiosis—basically, the exception that proved the rule.

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Linkage 

Linkage is inheritance of traits in a pattern that violates Mendel’s principle of independent assortment. Genetic linkage occurs when the genes controlling two different traits are located near each other on the same chromosome.

The tendency of two or more genes of the same chromosome to remain together in the process of inheritance is called linkage.

The inheritance of genes that are on the sex chromosome can also be called sex linkage. Sex-linked traits show interesting inheritance patterns in part because females have two copies of each X chromosome, but males only have one. 

This inheritance pattern means that a male with the recessive allele will always show the recessive trait, because he only has one copy of the allele.  In contrast, most genes are located on the autosomes, or non sex chromosomes, where both males and females have two copies of each gene.

Recombination

During the process of passing of genetic material to progeny cells, sometimes, chromosomes come together and exchange geneteic material. This is called a crossing over event.

A crossover event between the locations of two genes on a chromosome results in genetic recombination, or new combinations of alleles on a chromosome.

(Crossing over between genes A and B results in recombinant chromosomes with new allele combinations a, b and A, B, in addition to the original parental combinations A, b and a, B.)

Because crossing over occurs randomly along the chromosome, the closer two genes are physically located to each other on a chromosome, the less likely that a crossover will occur between them.

Conversely, the farther apart two genes are located from each other along the chromosome, the more likely they are to be swapped with the alleles on the homologous chromosome. The image below illustrates this idea:

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