Using a pedigree to determine the genotype of an individual for a particular trait, based on the individual’s family history can be done via the following steps. Please note that these steps are for basic genetics. They are not for sex-link, polygenetic traits, autosomal disorders, or codominant traits.
1. Determine if the trait of interest is dominant or recessive.
2. If the trait of interest is dominant, then individuals on the pedigree that have trait will have their shapes shaded. However, if the trait is recessive, then individuals with the trait will have shapes that are unshaded. Some pedigrees use half-shaded shapes to represent carriers of a trait.
3. The genotype of recessive individuals will be represented by to lower case alleles. The lowercase letter that is used is usually the first letter of the dominant allele that is being represented.
4. If a pedigree does not use half-shaded shapes to represent carriers of a trait, then it can be assumed that all individuals with the dominate phenotype have at least one dominant allele. Dominant alleles are indicated by a capital letter. The capital letter used is often the first letter of the dominant trait that is being represented.
(a) To determine the second allele of an individual on a pedigree that has the dominant phenotype of a trait that is on a pedigree that does not use half-shaded shapes to represent carriers, the genotypes of the parents or offspring of that individual must be examined.
I. If one parent is homozygous recessive, then it can be determined that the individual has a heterozygous genotype. A heterozygous genotype has one dominant and one recessive allele.
II. If both parents are homozygous dominant, then the individual will also have a homozygous dominant genotype and contain two dominant alleles for the trait.
5. If the offspring of the two parents has a recessive trait, but the parents do not, then it can be assumed that the parents each have a heterozygous genotype for the trait. A heterozygous genotype has one dominant and one recessive allele.