Question

When a sperm containing X chromosome fertilises the egg, the zygote would have two X chromosomes and develop into a female child. If the sperm contributes a Y chromosome to the egg (ovum) at fertilisation, the zygote would develop into a male child. Fig. 10.5 shows sex determination in humans.

Answer 1

The process of fusion of a sperm with an ovum is called fertilisation. During fertilisation, a sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane which blocks entry of additional sperms. This also induces the formation of the haploid secondary oocyte, by meiotic division, which fuses with sperm to form zygote. At this stage the sex of the baby is determined. Males have XY chromosomes so sperms who have haploid no. of chromosomes carry either X or Y. While females have XX chromosomes, so all ovum will carry X chromosome. So when a sperm containing X chromosome fertilises an ovum, a zygote with XX chromosomes is formed. Thus a female child is born.

Answer 2

Sex of child in humans is set by the sort of male haploid sex cell that fuses with the feminine egg. Throughout meiosis, all the gametes created by females contain solely the X chromosome (A + X), whereas males manufacture 2 varieties of gametes, 1/2 gametes with X- and 1/2 with Y-chromosome (A + X and A + Y). Fertilization of egg (A + X) with sperm cell carrying A+X chromosomes ends up in female descendant (AA + XX). Fertilization of egg (A+ X) with sperm cell carrying A+Y chromosomes ends up in teenager (AA + XY).