Sex Linkage

Reproduction is essential for the continuity of generation. In different organisms, there are different means of reproduction and just as reproduction, there are different means of sex determination in different animals. At the early stages of life, the process of sex determination holds much importance as it initiates the further process of development of the body. Sex determination is governed by environmental factors such as temperature. In humans and insects, sex determination takes place with the help of genetic factors and sex linkage traits.

Chromosomal Basis of Sex Determination

After understanding the sex linkage meaning, we will now understand sex linkage traits and the chromosomal basis of sex determination. Henking was the one who formulated this theory. He traced the specific nuclear structure in some insects with the help of spermatogenesis. He made an observation that the structures were received by only 50% of the sperms. These structures were then called “X body” by him. After a few years of studies and research, some other scientists concluded that this structure was a chromosome and they named it an X chromosome. The linkage associated with the X chromosome is called X linkage. The further Y chromosome was also discovered and these two together were known as the sex chromosomes. We will learn about different types of sex linkage inheritance below. 

Male Heterogamety

In this condition, two different types of gametes are produced by males. So we can say that the sperm helps in determining the sex of the offspring. XO and XY type of methods are there in it.

XO Type: It is also known as the XX-XO type. The insects show this type of sex determination. The males and females have different numbers of chromosomes. The eggs of the female here have an extra X-chromosome besides the number of autosomes. And only 50% of the sperms have the X-chromosomes. The eggs of the grasshopper are fertilized by (A+X) type of sperm and they become females and the males are fertilized by (A+O) type of sperm. So we can conclude that the sperm can helps in determining the sex of the offspring. As the X-chromosome helps in determining the sex, so because of this we have named them as sex chromosomes. 

XY Type: This type of sex determination method is present in drosophila and mammals. It is also present in human beings. X and Y are the two types of sex chromosomes that are present in the body. The length of the Y chromosome is shorter than the X chromosome. The number of chromosomes in both males and females is the same. 

Sex Determination in Humans

This is also called sex linkage in human. There are 22 pairs of autosomes that are present in human beings. They also have one pair of sex chromosomes. The females are homogametic in nature. This means that all the ova that are formed by the female have similar chromosomes in them that is X chromosomes. The male has an additional Y chromosome and this is present in a 50% ratio and the rest 50% is by the X chromosome. So we can say that males produce two types of chromosomes. And at the time of fertilization, there is an equal probability of either the X or the Y chromosome fusing with the egg that has only the X chromosome. So a girl child is born when both the X chromosomes come together and a boy child is born when the Y chromosome fuses with the X chromosome. So we can say that there is a 50% probability of either a girl child or a boy child. On the Y chromosome, there is a gene named Sry. This is the sex-determining region that helps in coding the testes determining factors. This helps in developing the male characters and it is not present then females would develop. This is how sex linkage in man works. 

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Female Heterogamety

In this type, the females produce two different types of gametes. Here the egg helps in determining the sex of the offspring. ZW type and ZO type are the two types of sex-determining mechanisms. 

ZW Type: It has two types of chromosomes that are ZZ and ZW type. This type of determination is present in birds. The males have homomorphic sex chromosomes and the females have heteromorphic chromosomes. So, the females are heterogametic in nature. 

Sex Determination in Honey Bee

In the sex linkage in honey bees, the sex is determined by the number of sets of chromosomes that an individual receives. When egg and sperms fuse together, they form a female and if the egg is left unfertilized then it develops into a male. So, from this, we can conclude that the males have half the number of chromosomes as that of the females. This type of sex determination is known as haplodiploid sex determination. Now we will study sex linkage in drosophila. 

Sex-Linked Inheritance

Here we will study sex linkage in drosophila. In drosophila, sex-linked inheritance was seen. Morgan worked on the inheritance patterns of the eye colour of drosophila. Three types of crosses were made by him. 

Cross 1: In the first case he crossed a white-eyed male with red-eyed females. When they reproduced it was found that the first generation has all the flies having red eyes. Then these flies were allowed to self breed and it was seen that both types of traits were seen in the second generation. 

Cross 2: Here the white-eyed male was crossed with the red-eyed female. This was almost similar to the test cross. Here he obtained both red and white-eyed females and male and they all were present in equal proportions. 

Cross 3: Red-eyed males were crossed with white-eyed females and a reciprocal cross was taken off the first cross. But here we obtained all red-eyed females and white-eyed males which was surprising as it should have obtained a similar result as that of Mendel. 

So after all these experiments, it was concluded by Morgan that the eye colour gene is linked to sex and is present on the X-chromosome. So it was also concluded that these X chromosomes are present in a zig-zag formation and not passed in a direct pattern. This is how sex linkage in drosophila was studied.