The scientists discovered the sex chromosome for the first time while studying the insects. In the year 1950, Clarence McClung, Nettie Stevens, and Edmund Wilson worked on the insect sex chromosomes. They discovered certain accessory chromosomes in the grasshoppers. These accessory chromosomes determined the sex of the insects. They also determined the presence of an even number of chromosomes in the Orthoptera females and the odd number of chromosomes in the males. The Orthoptera females had an extra copy of a chromosome (meaning, two copies of an accessory chromosome). Thus they named it as X chromosome. Each of the egg consisted of X chromosomes. However, half of the sperms had X chromosomes. The remaining half of the sperms did not have X chromosomes. Those sperms had some other type of accessory chromosome. Later on, the scientists named it as Y chromosome. Insects having two X chromosomes got identified as females. Those having X and Y chromosomes got identified as males.

Another study involved common mealworms. The scientists found a partner chromosome for X. Stevens called it the Y chromosome. It was smaller in size consisting of a lesser number of genes. Similarly, human sex chromosomes also got identified.





Image: Human karyotype (showing X and Y chromosomes)





The human X chromosome:

The human cells possess two kinds of sex chromosomes. The X chromosome consists of 155 million base pairs and occurs in each cell. The homogametic sexes consist of two copies of this chromosome (females). The heterogametic sex (males) consists of one copy of this chromosome. During the early embryonic stage in females, one of the two X chromosomes gets inactivated. The process of X-inactivation plays a crucial role in avoiding the occurrence of lethality. The process usually depends on the X-controlling element and the XIST gene (X-inactivation specific transcript. It gets transcribed into a large RNA molecule. It, later on, does not undergo translation. Instead, it coats the X chromosome and makes it inactive. Hence, the genes on the X chromosome get silenced. Although, the females possess two X chromosomes, one of them occurs in a silenced state. The process is known as Lyonization.

The active X chromosome achieves its state randomly. Meaning, the X chromosome derived from any of the parents achieves an active state. The active chromosome gets derived either from the maternal or the paternal cells. Some of the genes situated at the end of the X chromosome do not get covered by the X-inactivation process. These genes are present in the pseudoautosomal regions. They purposely get skipped from the process of X-inactivation since they play a crucial role in the normal developmental process. Also, if all the genes on one of the X chromosome get inactivated for the entire life, it would be useless to keep such a chromosome in the cell. Thus, the X-inactivation of one X chromosome does not include all the genes on that particular X chromosome.





The human Y chromosome:

It is very short in size. It consists of 59 million base pairs of DNA. The genes occurring in the Y chromosome play a crucial role in sex determination. The inheritance of the Y chromosome depends on the father to son transmission. It is also known as the holandric mode of inheritance. The mutant genes get transmitted from father to the son. Hence, it follows a male to male transmission pattern. It does not involve dominance or recessiveness. The transmission pattern is straightforward. The affected father has all his sons affected. The genes present on the Y chromosome are known as Y-linked genes. Although we know only a few genes on the Y chromosome, every gene encodes for a particular product. For example, SRY region of the Y chromosome is also known as sex region determining factor. It is a testis-determining factor. It is adjacent to the pseudo-autosomal region. It plays a crucial role in normal male development. These regions only exhibit a capacity of recombination. Not all genes on the Y chromosome undergo genetic recombination.





Sex chromosomal disorders in humans:

Certain conditions arise due to an abnormality in the sex chromosome number or the structure. Numerical abnormalities arise due to trisomies, monosomies, and mosaicism. The structural abnormalities arise due to deletions, translocations, inversions, and duplications. The disorders associated with the X chromosome involve different features as compared with the disorders associated with the Y chromosome. The former ones are known as X-linked disorders. The latter ones are known as Y-linked disorders. Their patterns of inheritance also differ. The X-linked inheritance follows dominance or recessive types. The Y-linked inheritance does not follow any such types.





X-linked dominant inheritance X-linked recessive inheritance The expression of the trait depends only on the presence of one copy of the mutated gene on the X chromosome. The expression of the trait depends on the presence of two copies of the mutated genes on the X chromosome. Affects both males and females. Highly affects males. Examples include Fragile X syndrome, Rett syndrome, Alport’s syndrome, and X-linked hypophosphatemia. Examples include Duchenne muscular dystrophy, hemophilia, and color blindness. Y-linked inheritance It gets transmitted through the Y chromosome. It mainly affects the males. Examples include abnormal testicular development, gonadal dysgenesis, and retinitis pigmentosa.

Table: Sex-linked inheritance in humans

The Z and W chromosomes in birds:

They show the presence of Z and W chromosomes. The W chromosomes determine the femininity of the bird. Hence, females are known as heteromorphic sexes in the birds. They show the presence of one Z and one W chromosome.





References:

[1] Chromosomes: Organization and Function, Adrian T. Sumner

[2] Medical genetics, G.P. Pal