Which parent decides
In many places a woman is still blamed for the sex of her children. The cross on the previous page shows that this cannot be right, and it is worth being able to say exactly why.
Look at where the two chromosomes came from
Each child has two sex chromosomes, one from each parent. Ask what each parent was able to contribute:
- The mother is XX, so every ovum she makes carries X. Whatever the child turns out to be, her contribution was the same.
- The father is XY, so his sperm carry either X or Y. Which of his two kinds of sperm fertilises the ovum is what makes the difference.
Deciding is not choosing
The father's gametes settle the outcome, but no one selects which sperm arrives first. It is the same chance process as any other fertilisation. The father determines the sex of the child in the sense that the deciding chromosome comes from him — not in the sense that anyone controls it.
Assignment 20.4
Work these out
- A couple has three daughters. What is the chance that a fourth child will be a son?
- Write down the sex chromosomes in a human ovum, in a sperm, and in the zygote formed when they fuse.
- Explain, using the cross, why a woman cannot be responsible for having only daughters.
A misconception with a one-line answer
Every ovum carries X. There is no ovum a woman could produce that would make a son more or less likely. The answer to a belief that has caused real harm is a single row of a Punnett square.