Sex Chromosome Mechanisms - Heterogamesis

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Sex Chromosome Mechanisms - Heterogamesis

Sex Chromosome Mechanisms (Heterogamesis)
In sexually dimorphic dioecious organisms besides morphological and behavioural difference between both sexes, the sexual diversity also occurs at the level of chromosomes. The chromosomal differences between the sexes of several dioecious species were found earlier in the course of cytological investigations. A German biologist, Henking in 1891 while studying spermatogenesis of the squash bug, Pyrrhocoris, noted that half of the spermatozoa contained an extra chromosome which was called by him as "X body'. The significance of X body was not immediately understood, but in 1902 an American geneticists McCJung who had made extensive observations of gemetogenesis in grasshoppers, suggested that the X body was involved in some way with the determination of sex.

He reported that the somatic cells of the female grasshopper (Xiphidium fasciatum) contained 24 chromosomes whereas those of the male had only 23. Three years later, Miss Steven and Wilson

Chromosomes of Protenor
1. X Chromosome
2. Autosomes
3. X Chromosome
4. Autosomes
5. Protenor

Chromosomes of Millkweed Bug Lygaeus Turcicus
1. Autosomes
2. X Chromosome
3. Autosomes
4. Lygaeus (Bug)

succeeded in following both oogensis and spermatogenesis in certain bugs of' Protenor genus. In these insects, different number of chromosomes were observed in the cells of two sexes, the diploid number in the female being 14 while that of the male was 13. By following oogenesis, these investigators found that all the eggs contained 7 chromosomes.

A similar study of spermatogenesis showed that two types of sperms were formed, those with 7 chromosomes and those with 6 chromosomes. During these studies it was also realized that the X body was a chromosome, so the X body became known as X chromosome. Because the X chromosome has been found to have relation with determination of sex of individual by its presence or absence so it is called sex chromosome. Stevens and Wilson (1905) while working with the milkweed bug, Lygaeus turcicus found same number of chromosome in both the sexes, (i.e., 14). In female all chromosomes were paired and the homologues were equal in size.

In the male, all the chromosomes were paired, but the chromosome identified as the homologue to the X chromosome was distinctly smaller and was called the Y chromosome. In dioecious organisms thus two types of chromosome were recognised which are as follows:
1. Autosomes: The chromosomes which have no relation with the sex and contain the genes which determine the somatic characters of the individuals are known as autosomes (A).
2. Sex chromosomes: The chromosomes which are responsible for the determination of sex are known as sex chromosome, e.g., X and Y chromosomes.
Types of Sex Chromosomal Mechanism of Sex Determination

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