XY – MECHANISM
This pattern of sex determim tion is round in Drosophila, man and many other organisms.
(a) Male: Male is XY. Mite is heterogametic. It produces two types of sex-determining sperms. Half the sperms carry X-chromosome and the other half carry Y — chromosome. Chances for both types of sperms are equal.
) Female: Female is XX. Female is homogametic. So it produces only one type of eggs. lacit egg has an X chromosome.
The type of sperm determines sex of the offspring. If an X — carrying sperm fertilizes the egg. the –zygote will be XX. –films a female offspring is produced. If a Y— carry ing sperm fertilizes the egg. the zygote will be XY. I hus a male offspring will be produced. –The sex-ratio between male and female offspring is I :I. Sex ratio indicates chances of the sex in the offspring. Chances for a son or daughter in human birth are equal.
Human: In human, SRY is the male determining gene. SRY stands for Sex determining regions of Y. It is located at the tip of short arm of Y-chromosome. It is comparatively a small gene. It encodes for a protein with only 204 amino acids. This protein stimulates the somatic cells of the developing gonad to become Sertoli cells. The sertoli cells secrete Mullerian inhibiting substance (MIS). MIS eliminates the Mullerian duct system (the part that would produce major female reproductive organs). The gonad is now a testis. Certain cells of gonads produce testosterone hormone. This hormone develops the major male reproductive organs. If no MIS is produced,
further development of the Mullerian duct structures take place and normal female structures are produced. Embryos lacking SRI or having mutated forms of SRY become females even if they are XY. This system of sex determination is called 1st-dominant. It is present
in almost all mammals.
Drosophila: Drosophila also has XY mechanism of sex determination. But V does not play much role in Drosophila. It does not have SRY gene. There is a close genic balance between genes of different chromosomes. Drosophila has an X chromosomeautosome balance system. Its Y chromosome has very little influence on sex. In Drosophila, the X chromosome determines the female and the autosomes determine the male. Sex depends on the number of X chromosomes relative to the number of sets of autosomes. An X :A ratio of 1.00 or higher produces female. But an
X: A ratio of 0.5 or lower produces males.