With people we cannot set up crosses, so geneticists read family trees called pedigrees. A square is a male, a circle a female, and a shaded shape means the trait shows. A horizontal line joins two parents; their children hang below. This topic uses test-cross reasoning and X-linked traits.
Key idea
Look for the telltale pattern. Two unaffected parents with an affected child: the trait is recessive. Two affected parents with an unaffected child: the trait is dominant. A dominant trait does not skip generations: every affected person has an affected parent.
Why it works
If unaffected parents have an affected child, the child got an allele for the trait from each parent, though neither parent showed it. Only a recessive allele can hide like that, so both parents are carriers (Aa). If two affected parents have an unaffected child, each parent carried a hidden "normal" allele. A hidden allele in an affected person means the trait's allele is dominant.
Worked example: Naming the pattern
In the pedigree above, is the trait dominant or recessive? What are the parents' genotypes?
- II-2 is affected, but neither parent is.
- So the allele hid in both parents: recessive.
- II-2 is aa and got a from each parent; the parents show the dominant look.
Answer: Recessive; both parents are Aa.
Worked example: A carrier chance
In the same family, II-1 is unaffected. What is the chance she is a carrier?
- Aa × Aa gives AA, Aa, Aa, aa.
- She is unaffected, so she is not aa. That leaves AA, Aa, Aa.
- 2 of those 3 are Aa.
Answer: $\frac{2}{3}$
X-linked recessive traits leave their own mark: many more affected males than females, passed from a carrier mother to some of her sons, and never from father to son. Each claim in a pedigree is a test: if a pattern breaks any rule, that pattern is ruled out.
Common mistake
It is tempting to call a trait dominant because many people in the family have it. How often a trait appears does not decide it; the pattern does. A recessive trait can be common in a family, and a dominant one can be rare. Look for the two telltale couples instead.