Higher X-Linked Inheritance and Pedigree Analysis

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.