XO MECHANISM

The sex determining mechanism in which sex is determined by In X chromosome only is called X() mechanism. This pattern of sex determination is found in grasshopper and protenor bug.

(a) Male: Male protenor has 13 chromosomes in diploid cells. It has

6 pairs of autosomes but only one X chromosome. So it is XO. The other sex chromosome is missing entirely. Thus male is hetemgametic and it limns two types of sperms. Half of the sperms have 6 autosomes and one X chromosome. The other half have 6 autosomes and w ithout any sex chromosome. A gamete without any sex chromosome is called nullo gamete.

Female: The female of protenor bug has 14 chromosomes in diploid cells. The female has 6 pairs of autosomes and two X chromosomes. Thus it is XX. It forms only one type of eggs. So it is homogametie. Every egg carries an X chromosome.

Sex of the offspring depends on the kind of sperm. If an X-carrying sperm fertilizes the egg. an XX female offspring is produced. If the nullo sperm fertilizes the egg. an X0 male offspring is produced. Sex ratio between male and female offspring is 1:1.


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