Sex Determination and Inherited Disorders [H]

This lesson covers how biological sex is determined by the sex chromosomes and how some disorders are sex-linked, meaning the gene responsible is located on the X chromosome. The sex-linked inheritance section is Higher Tier only content for AQA GCSE Biology.

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Sex Determination

In humans, biological sex is determined by the 23rd pair of chromosomes, known as the sex chromosomes.

Sex	Chromosome combination	Notation
Female	Two X chromosomes	XX
Male	One X and one Y chromosome	XY

The Y chromosome carries a gene called SRY (Sex-determining Region Y) that triggers the development of male characteristics. Without this gene, the embryo develops as female.

Inheritance of Sex

• A mother is always XX, so she can only pass on an X chromosome
• A father is XY, so he can pass on either an X or a Y chromosome
• The sex of the offspring depends on which chromosome the father contributes