IGCSE Biology / Genes and inheritance
X and Y chromosomes revision notes
Edexcel International GCSE Biology (4BI1), chapter 18: Genes and inheritance, specification point 3.26, 3.27. Free lesson.
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In humans, the sex chromosomes in the fertilising sperm determine whether a child is XX or XY. This gives a 1:1 ratio, and each fertilisation is independent.
How the sex chromosomes determine sex
The 23rd pair of chromosomes determines sex. A female has XX and a male has XY. The mother’s eggs all carry an X chromosome. The father produces sperm carrying either X or Y, so the sperm that fertilises the egg determines the child’s sex. X from the father and X from the mother gives XX, female. Y from the father and X from the mother gives XY, male.
Why the chance is 1:1
A Punnett square shows the possible combinations. Each type of sperm is equally likely to fertilise the egg. Two of the four combinations are XX and two are XY, giving a 1:1 ratio and a 50% chance of either sex.
How X and Y chromosomes differ
X and Y are the chromosome pair that are not truly homologous, meaning they do not carry matching versions of most genes. The X chromosome is relatively large and carries many genes unrelated to sex. The Y is much smaller, carries fewer genes, and includes a gene that triggers male development. A female (XX) has two copies of every gene on the X chromosome. A male (XY) has only one copy of each gene on the X because the Y has no matching version of most of them. A recessive allele on the X can therefore be expressed in a male without a second copy to mask it. This is why some…
Why previous children do not change the chance
Each fertilisation is an independent event. The children a couple have already had do not affect which sperm fertilises the next egg. The father still produces sperm carrying X or Y, so the chance of a boy remains 50% each time. After three daughters, the probability that the fourth child is a boy is still 50%.