Biology · Ch 4 — Principles of Inheritance and Variation
Sex Determination in Humans
Sex Determination in Humans
The sex of a human child is determined by the chromosomes carried by the sperm that fertilises the egg. This is a clear example of the XY sex-determination system.
In humans, there are 23 pairs of chromosomes in total. Of these, 22 pairs are identical in both males and females — these are called autosomes. The 23rd pair is the sex chromosome pair. A female has two X-chromosomes (XX), while a male has one X and one Y chromosome (XY). The presence of the Y chromosome is what makes an individual male.
During sperm formation (spermatogenesis), a male produces two types of sperm in equal numbers. 50 per cent of the sperm carry an X-chromosome along with the autosomes, and the other 50 per cent carry a Y-chromosome. Females, on the other hand, produce only one type of ovum — every egg carries a single X-chromosome.
When an egg is fertilised, there is an equal chance that the sperm will carry an X or a Y chromosome. If an X-carrying sperm fertilises the egg, the resulting zygote is XX — a female. If a Y-carrying sperm fertilises the egg, the zygote is XY — a male. This means that the genetic makeup of the sperm, not the egg, determines the sex of the child.
In every pregnancy, there is always a 50 per cent probability of having a male child and a 50 per cent probability of having a female child. This is a fixed, equal chance each time. …