Sex Determination and Zygote Formation — NEET Biology
✅ Asked in NEET 2020This induces the completion of the meiotic division of the secondary oocyte. The second meiotic division is also unequal and results in the formation of a second polar body and a haploid ovum (ootid). Soon the haploid nucleus of the sperms and that of the ovum fuse together to form a diploid zygote. How many chromosomes will be there in the zygote? One has to remember that the sex of the baby has been decided at this stage itself. Let us see how? As you know the chromosome pattern in the human female is XX and that in the male is XY. Therefore, all the haploid gametes (ova) produced by the female have the sex chromosome X whereas in the male gametes (sperms) the sex chromosome could be either X or Y, hence, 50 per cent of sperms carry the X chromosome while the other 50 per cent carry the Y. After fusion of the male and female gametes the zygote would carry either XX or XY depending on whether the sperm carrying X or Y fertilised the ovum. The zygote carrying XX would develop into a female baby and XY would form a male (you will learn more about the chromosomal patterns in Chapter 5). That is why, scientifically it is correct to say that the sex of the baby is determined by the father and not by the mother!
NTA tests whether students understand that sperm entry triggers completion of meiosis II in the secondary oocyte, forming a mature ovum that fuses with the sperm nucleus to create a diploid zygote. The key concept is that sex determination depends on which sperm fertilizes the egg—an X-carrying sperm produces XX (female), while a Y-carrying sperm produces XY (male). Students commonly mistake that mothers determine the baby's sex, but scientifically the father does since he produces both X and Y sperm. Remember: the secondary oocyte completes meiosis only after sperm entry, and the resulting zygote always has 46 chromosomes (23 pairs), with sex determined by the male gamete contribution.
Meiotic division of the secondary oocyte is completed:
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