Sex Determination in Humans and Birds

Sex determination is the general term for the genetic mechanism by which the sex of a developing individual — male or female — is fixed at the very moment of fertilisation, and it varies considerably from one group of organisms to another. In humans, and indeed in most mammals generally, sex is determined by a single, specialised pair of chromosomes called the sex chromosomes, conventionally labelled X and Y, following what is known as the XX-XY system. A human female carries two X chromosomes in every one of her diploid body cells (genotype XX) and is therefore described as the HOMOGAMETIC sex, because meiosis in her ovaries can only ever produce ONE kind of egg with respect to the sex chromosomes — every single egg she produces carries exactly one X chromosome, with no exceptions. A human male, by contrast, carries one X chromosome and one (structurally quite different, and much gene-poorer) Y chromosome in his body cells (genotype XY), and is therefore described as the HETEROGAMETIC sex, because meiosis in his testes produces TWO distinct kinds of sperm cell in exactly equal numbers: half of all sperm produced carry an X chromosome, and the other half carry a Y chromosome instead.

Because every single egg a woman produces contributes an X chromosome to the zygote regardless of which particular sperm happens to fertilise it, the sex of the resulting offspring is, in a strict genetic sense, decided entirely by which TYPE of sperm succeeds in fertilising that egg — not by anything about the egg or the mother at all. If an X-bearing sperm happens to fertilise the egg, the resulting zygote carries the genotype XX and develops as a female; if, instead, a Y-bearing sperm happens to fertilise the egg, the resulting zygote carries the genotype XY and develops as a male. It is therefore the FATHER's own gamete — specifically, which one of his two equally numerous types of sperm wins the race to fertilise the egg — that is genetically responsible for determining a child's sex, a purely chance (1:1 probability) outcome that has nothing whatsoever to do with the mother, and one worth stating explicitly because the mistaken belief that a mother somehow "determines," or is in some way responsible for, the sex of her children remains a persistent social misconception in many communities even though it directly contradicts basic Mendelian genetics. …