Sex determinationSpec 4.6.1.8
In short
Sex in humans is determined by one of the 23 pairs of chromosomes, the sex chromosomes. Females are XX and males are XY. All eggs contain an X chromosome, while half the sperm contain an X and half contain a Y, so each child has a 50% chance of being a boy or a girl.
Ordinary human body cells contain 23 pairs of chromosomes. 22 pairs control characteristics only, but one of the pairs carries the genes that determine sex.
- In females the sex chromosomes are the same: XX.
- In males the sex chromosomes are different: XY.
All eggs contain an X chromosome. Half of the sperm contain an X and half contain a Y. The sperm that fertilises the egg decides the sex of the baby.
Sex inheritance cross
Show the cross between a female (XX) and a male (XY) and state the ratio of female to male offspring.
- Gametes from the mother: X and X. Gametes from the father: X and Y.
- Punnett square: XX, XX, XY, XY.
- Two of the four boxes are XX (female) and two are XY (male).
Answer: Ratio of female : male = 1 : 1. The probability of each child being a girl, or a boy, is 50%.
Each fertilisation is independent. If a couple already have three girls, the chance of a boy next time is still 50%.
Quick check
Why are the offspring of asexual reproduction genetically identical to the parent?
Show answer
There is only one parent and no fusion of gametes, and only mitosis is involved.
How does meiosis halve the number of chromosomes?
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The cell divides twice to form four gametes, each with a single set of chromosomes.
Define the genome of an organism.
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The entire genetic material of that organism.
What is the difference between genotype and phenotype?
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Genotype is the alleles present; phenotype is how the characteristic is expressed.
What are the sex chromosomes of a human female and male?
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Female XX, male XY.
Written and checked against the AQA GCSE Combined Science (8464) specification · Updated October 2026