Common Misconceptions

Misconception: "The mother determines the sex of the baby."

Reality: The father determines the sex. The mother always contributes an X chromosome (because she is XX). The father contributes either X or Y. If the father's sperm carries Y, the child is male (XY); if it carries X, the child is female (XX). Throughout history, women have been blamed for not producing male heirs, but genetically it is always the father's contribution that decides sex.

Misconception: "X-linked disorders affect males and females equally."

Reality: Because males only have one X chromosome (XY), a single copy of an X-linked recessive allele is sufficient to cause the condition — there is no second X with a dominant allele to mask it. Females (XX) need two copies of the recessive allele to be affected. This is why conditions such as colour blindness and haemophilia are far more common in males than females.

Misconception: "The probability of a boy is greater than 50% because more boys are born."

Reality: Theoretically, the probability is exactly 50:50, because X- and Y-bearing sperm are produced in equal numbers. Slight differences in the observed birth ratio (approximately 105 males : 100 females) are due to other biological factors, not a difference in the probability of fertilisation.

Misconception: "Since the 23rd pair determines sex, all other 22 pairs are identical in males and females."

Reality: The 22 autosome pairs are indeed the same in males and females in terms of the genes they carry. However, there are small individual differences in alleles between any two people — the point is that the sex chromosome pair is the key difference that determines biological sex.