Question

A woman has only daughters. Analyse the situation generically and provide a suitable explanation.

Answer 1

Sex Determination:

1. Human cells have 23 pairs of chromosomes of which 22 pairs are autosomes and one pair is a sex chromosome.
2. Sex chromosomes of women have two X chromosomes and men have one X and one Y chromosome.
3. Therefore, a female produces gametes which are all of the same kind and only contribute one chromosome to the zygote.
4. The other chromosome comes from the male and it can be either the X or Y chromosome.
5. Sex determination in humans depends on the Y chromosome of the male.

Case 1- If an X chromosome arrives from the male then the zygote will have XX chromosomes on fusion with the X chromosome of the female and the resulting offspring will be a female (daughter).
Case 2- If a Y chromosome arrives from the male then the zygote will have XY chromosomes on fusion with the X chromosome of the female and the resulting offspring will be a male (son).

In this case, during each fertilization, the sperm having X chromosome of the male was fused with the ovum having X chromosome of female, thus, this woman has only daughters.