Introduction

DNA is the genetic material, and genes are formed of DNA. Therefore, genes must be present on chromosomes, which was not readily accepted. This idea was repeatedly tested before it was received. 

The Chromosomal Theory of Inheritance was proposed and tested to prove the idea that chromosomes constitute the genetic makeup of an organism. In easier words, it was suggested that genes are present on chromosomes.

The theory was first proposed by Thomas Hunt Morgan, an American biologist. He experimented on the fruit fly Drosophila melanogaster

Morgan’s Experiments

Drosophila has only four chromosomes, i.e. three pairs of autosomes and one pair of sex chromosomes. A sex chromosome is a chromosome that determines the gender of the organism. In Drosophila, red-eye is a dominant trait, while white-eye is a recessive trait. 

He performed the experiments in two steps. He crossed a red-eyed female with a white-eyed male in the first step. He reciprocally crossed a white-eyed female with a red-eyed male in the second step.

Cross between red female (Xw+Xw+)and white male (XY)

In the Parental 1 generation, he crossed a red-eyed female and a white-eyed male. The results Morgan got were as expected. The Filial 1, i.e. The F1 generation, contained all red-eyed males and females. 

The results were making sense. The gametes from red female fruit fly were homozygous, i.e. Xw+ and Xw+. The gametes from white males were heterozygous, i.e. Xw and Y.  If we arrange them in a Punnett square, Four different combinations are possible. All these are heterozygous red-eyed males and females. The results in the Punnett square are as follows:

Cross between red female (Xw+Xw+)and white male (XY)

Reciprocal Cross between red-male(Xw+Y) and white female (XwXw)

In the second phase of Morgan’s experiment, a reciprocal cross between a red maple and a white female was done. The P1, i.e. Parental 1, produces four gametes by meiosis. The white female produced two homozygous gametes, i.e. Xw and Xw. The red male-produced two heterozygous gametes, i.e. Xw+ and Y.

The Filial1 or F1 generation produced some results that were surprising for Morgan. Instead of having all the red-eyed progeny as observed in the experiment’s first phase, the F1 contained females with red eyes ( Xw+Xw+, Xw+Xw) and males with white eyes (XY). The Punnet’s square for this is as follows:

Reciprocal Cross between red-male(Xw+Y) and white female (XwXw) image

Observations Of The Experiment

The surprising fact was that the results violated Mendel’s Law of Independent Assortment. The only possible explanation of the situation was that the gene for determining eye colour was present on the X chromosome. This led to the discovery of Sex-linkage and Sex-linked genes.

Sex linkage is the phenomenon in which a gene is present on a sex chromosome. The genes present on a sex chromosome (X or Y) are called X-linked genes. In this case, the eye colour gene in Drosophila is considered an X-linked gene. 

How does the expression of X-linked genes vary in males and females?

Sex-linked genes have an exciting deviation from the Mendelian laws. The Y genes are present only in males. So Y-linked genes can never be expressed in females because there is no Y gene in females. 

The same thing occurs with X-linked genes: males carry only one X chromosome while a female contains two X chromosomes. If only one recessive allele shows up in a female, it will be masked by the dominant allele, so it will not be expressed. The case is different in males because they have only one X chromosome. If the recessive allele shows up in the male, it will be described. 

How do Sex-linked genes violate Mendelian Laws?

Mendel’s Law of independent assortment is violated in sex-linked, and sex-limited genes as the sex-limited traits can not be expressed independently. They are bound to either the X chromosome or the Y chromosome. 

Any gene on the Y chromosome can never be expressed in a female that does not contain Y chromosomes. 

Frequently Asked Questions

Q1. Why did he choose Drosophila for his experiments?

The chromosomal makeup of Drosophila is similar to man. The organism with an XX sex chromosomal pair is a female, and an organism with an XY sex chromosome pair is a male.

Also, Drosophila is easy to keep, easy to feed, and does not occupy too much space. 

Q2. What is Mendel’s Law of Independent Assortment?

Mendel’s Law of Independent Assortment states that the alleles of two or more than two different genes always get selected for gamete formation independently, i.e., the probability of choosing each allele for the gamete formation is equal.

Q3. What is the difference between homozygous and heterozygous?

A homozygous trait is a trait that contains two identical alleles of a particular gene. For example (XX). A heterozygous trait is a trait that has two different alleles of a specific gene. For instance (Xx)

References

  1. Microbiology: An Introduction by Tortora
  2. Molecular Biology by Robert F. Weaver
  3. Molecular Biology of the Cell by Bruce Alberts
  4. https://bioprinciples.biosci.gatech.edu/module-4-genes-and-genomes/4-4-linkage-sex-linkage-and-pedigree-analysis/
  5. https://www.sciencedirect.com/topics/immunology-and-microbiology/sex-linkage
  6. https://www.onlinebiologynotes.com/chromosomal-theory-of-inheritance/
  7. https://www.youtube.com/watch?v=KaxSDryqB6M

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