IGCSE Biology glossary
Sex linkage
Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Definition
A pattern of inheritance where a gene is located on the X chromosome and therefore the trait is more commonly expressed in males. Males have only one X chromosome (XY), so a single recessive allele on the X chromosome will be expressed since there is no second X to carry a dominant allele.
What to include in an exam answer
Show alleles as superscripts on the X chromosome: e.g. XH (normal) and Xh (haemophilia). Males are XHY or XhY. Females can be carriers (XHXh). Colour blindness and haemophilia are common IGCSE examples.
Example in context
Explain why colour blindness is more common in males than females (3 marks): The gene for colour vision is on the X chromosome. Males have only one X chromosome (XY), so a single recessive allele (XbY) causes colour blindness. Females have two X chromosomes, so they need two recessive alleles (XbXb) to be colour blind. A female with one recessive allele (XBXb) is a carrier with normal vision.
Related syllabus topic
Inheritance