pedigree analysis

Meaning: A pedigree charts a trait across generations. Squares are males, circles are females, shaded symbols are affected, a horizontal line joins mates, and vertical lines lead to offspring. Typical patterns: Autosomal recessive: the trait can skip generations, and unaffected carrier parents can have affected children. Autosomal dominant: affected people usually have an affected parent, and the trait rarely skips generations. X-linked recessive: more males are affected, and affected sons typically get the allele from carrier mothers. Mitochondrial: passed from mother to all children.

On the MCAT: You are given a pedigree and asked for the likely mode of inheritance, the genotype of a marked person, or the probability that a future child is affected. Rule out modes one at a time. Then multiply probabilities. Two carriers (Aa × Aa) have a 1/4 chance of an affected child. An unaffected child of two carriers has a 2/3 chance of being a carrier, because the aa outcome is excluded.

Common confusion: Students often say a trait seen mostly in males must be X-linked. Many more affected males is only a clue. Father-to-son transmission settles it, because a son gets his father's Y, not his X, so an X-linked trait cannot pass that way. Also, carriers are usually not shaded, so an unshaded person may still carry a recessive allele.