Sex-Linked Traits, Pedigrees, and Human Genetics

Humans have two sex chromosomes: XX produces a female, XY produces a male. A gene located on the X chromosome is called X-linked, and because a male has only one X chromosome, he is hemizygous for every X-linked gene -- a single recessive allele is enough to produce the trait, since there is no second X copy to mask it. A female needs the recessive allele on both of her X chromosomes to show the same trait, which is exactly why X-linked recessive conditions such as red-green color blindness and hemophilia show up far more often in males than in females.

A pedigree chart traces a trait through a family across generations using a standard set of symbols: a square for a male, a circle for a female, a filled shape for someone affected by the trait, a horizontal line connecting a couple, and a vertical line leading down to their offspring. Reading a pedigree is often a process of elimination -- for example, if two unaffected parents have an affected daughter, the condition cannot be X-linked recessive, because her father would have had to be affected himself (he is hemizygous, so a recessive allele on his only X would show up) in order to pass it to her; since he is unaffected, an autosomal recessive pattern is more likely. A karyotype -- a picture of all of a person's chromosomes arranged in matching homologous pairs by size -- is a real diagnostic tool that can reveal chromosome number abnormalities, such as the extra copy of chromosome 21 that causes Down syndrome, and genetic counselors use exactly this week's pedigree-and-probability skills professionally to help families understand inheritance risk.