Many genetic conditions are caused by alleles carried on the X chromosome โ these are called X-linked conditions. Because males have only one X chromosome, they are hemizygous for X-linked genes (they cannot be heterozygous or homozygous, just one copy). This means:
- Males with one X-linked recessive allele (X^a Y) will be affected.
- Females need two copies of the recessive allele (X^a X^a) to be affected โ much rarer.
- Female carriers (X^A X^a) are unaffected but can pass the allele to sons.
Examples of X-linked conditions:
- Colour blindness โ affects approximately 8% of males and 0.5% of females
- Haemophilia A โ lack of blood clotting factor VIII; much more common in males
- Duchenne muscular dystrophy โ progressive muscle weakness; almost exclusively males
Pedigree diagrams: Shaded = affected, circle = female, square = male, half-shaded = carrier. X-linked conditions show a characteristic pattern: affected males often have unaffected carrier mothers; daughters of affected fathers are often carriers.
This higher tier covers Higher X-Linked Inheritance and Pedigree Analysis within Sex Determination for GCSE Biology. Sex chromosomes and sex determination mechanisms It is section 8 of 11 in this topic. This section is most useful once the core foundation idea is secure, because it adds the detail that pushes answers higher.
Practice questions for Sex Determination
What determines the sex of a human baby?
Use a genetic diagram to show how sex is inherited in humans.