Sex-Linked Diseases

Sex-linked diseases are caused by faulty alleles of genes on the sex chromosomes. Most are X-linked recessive. Well-known examples include haemophilia, red-green colour blindness and Duchenne muscular dystrophy.Because males have only one X chromosome, a male with one recessive allele (X^aY) is affected. A female needs two recessive alleles (X^aX^a) to be affected; a female with one (X^AX^a) is an unaffected carrier.Typical features of X-linked recessive inheritance:• far more males than females are affected
• affected males usually have unaffected parents; they inherit the allele from their carrier mother
• it is never passed from father to son
• all daughters of an affected male are carriers (or affected, if the mother also carries the allele)
• for a female to be affected, her father must be affected and her mother must be a carrier or affectedWorked example: an affected man (X^aY) and a carrier woman (X^AX^a) have children. Gametes: X^a, Y and X^A, X^a. Offspring: X^AX^a (carrier daughter), X^aX^a (affected daughter), X^AY (unaffected son), X^aY (affected son). So half the daughters and half the sons are expected to be affected.Genetic counsellors use these patterns and pedigree diagrams to tell families the probability of passing a sex-linked disease to their children.