While the experimental method is the main approach to studying genetics in
nonhuman organisms, the study of human genetics is largely based on pedigree
analysis. A pedigree shows the distribution of particular traits within a
specific family group. From this information, the mode of inheritance can
usually be determined. As we saw in Exercise 2-1, knowing the phenotype of
the individual often enables us to reconstruct an individual’s genotype as
well.
Reading a pedigree: In a pedigree, a male is indicated by a square, and a
female by a circle. A horizontal line between a male and female indicates a
mating and a vertical line indicates descent (i.e., offspring). Individuals
carrying the trait being studied are indicated by blackened symbols.
From a pedigree, we may be able to see whether the trait is inherited as
dominant or as recessive. If the trait was inherited as dominant, we would
see that every individual with the trait must have at least one parent who
also has the trait. Therefore, the trait will be seen in all generations
that precede any individual with the trait. Of course, the individual could
be either homozygous dominant or heterozygous for that trait. An individual
without the trait would be homozygous recessive. On the other hand, if the
trait is inherited as recessive, any affected individual must be homozygous
recessive. That individual could have two parents who phenotypically lack
the trait, but are carriers (heterozygous).
