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X-Linked Disorders and Reading a Pedigree

Hemophilia and Duchenne muscular dystrophy fall mostly on boys — because the gene lives on the X chromosome. Learn why sons of carrier mothers are at risk, and how to read a family pedigree like a detective.

Why boys, mostly

Females have two X chromosomes; males have one X and one Y. For a gene on the X chromosome, that asymmetry changes everything. In an X-linked recessive disorder, a boy with a single faulty allele on his only X has no backup copy — so he is affected. A girl would need the faulty allele on both X chromosomes, which is far rarer. That is why hemophilia and Duchenne muscular dystrophy are seen mostly in boys.

A woman with one faulty X and one healthy X is a carrier of this sex-linked trait: usually healthy, because her second X compensates. (Thanks to X-inactivation, one X is silenced in each cell, so carriers can occasionally show mild signs — but typically they are unaffected.) She passes the faulty X to half her children on average.

A carrier mother's cross

Notation: X = healthy allele,  Xh = faulty allele,  Y = Y chromosome

Mother is a carrier (X Xh)  x  Father unaffected (X Y)

              mother
            |  X   |  Xh
      ------+------+------
  f    X    | X X  | X Xh     <- daughters
  a         |      |
  t  ------+------+------
  h    Y    | X Y  | Xh Y     <- sons
  e         |      |
  r

Daughters:  1/2 X X  (unaffected)  : 1/2 X Xh (CARRIER, healthy)
Sons:       1/2 X Y  (unaffected)  : 1/2 Xh Y (AFFECTED)

=> Of the SONS, half are affected; of the DAUGHTERS, half are carriers.
   No son ever inherits his father's X, so an affected boy gets
   his Xh from his mother. Fathers cannot pass X-linked traits to sons.
Carrier mother x unaffected father: half of sons affected, half of daughters carriers; no father-to-son transmission.

Reading a pedigree

A pedigree is a family tree drawn with symbols: squares for males, circles for females, filled in if affected, a horizontal line joining a couple, and a vertical line dropping to their children. Reading one is detective work — you infer hidden genotypes from who is affected and who is not.

  1. Count the sexes of affected people. Mostly males, with healthy mothers linking affected boys? Suspect X-linked recessive.
  2. Check for father-to-son transmission. If you find it, the trait is NOT X-linked — rule that mode out.
  3. Look for skipped generations. Affected children of two unaffected parents points to recessive (the parents are carriers).
  4. Look for every-generation, both-sex transmission. A trait passing parent to child in each generation, in both sexes, points to autosomal dominant.