Higher X-Linked Inheritance and Pedigree Analysis
Part of Sex Determination — GCSE Biology
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.
Topic position
Section 8 of 11
Practice
26 questions
Recall
20 flashcards
Higher X-Linked Inheritance and Pedigree Analysis
Many genetic conditions are caused by alleles carried on the X chromosome — 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 — much rarer.
- Female carriers (X^A X^a) are unaffected but can pass the allele to sons.
Examples of X-linked conditions:
- Colour blindness — affects approximately 8% of males and 0.5% of females
- Haemophilia A — lack of blood clotting factor VIII; much more common in males
- Duchenne muscular dystrophy — 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.