Pedigrees

Punnett squares are used to predict how traits might be passed from parents to potential offspring. Pedigrees show how a trait has been passed down through a family. We can determine genotypes from phenotypes by following a few simple rules. We must also understand some assumptions and limitations:

Traits are heritable - they can be passed down in DNA
Traits are determined by single genes - not polygenic traits(which many traits are).
All members of a family are biologically related

Pedigrees also use certain symbols to show information about family members:

Shapes show the sex of the individual people:
- Circles - female
- Squares - male
The trait is shown by darkening the shape
- A fully dark shape means the person is showing the trait
- A half-shaded shape means the person is a carrier - they have a gene, but don't show the trait (heterozygous for a recessive trait)

Same parents, different kid

The first step in figuring out a pedigree is to determine whether the trait is dominant or recessive. This has nothing to do with how common a trait is. For example, having 5 fingers is due to a recessive trait; having 6 on each hand is dominant.

Look for parents that are the same (for the trait) with at least one child that is different.
- The only way this can happen is for parents to be heterozygous (Bb) and the different child is homozygous recessive (bb).
- If the parents are shaded, they are expressing a dominant trait.
- If the different child is shaded, they are expressing a recessive trait.
Next, look at other members of the family. Genes don't switch back and forth being dominant or recessive, so what is true in one family member is true for others as well.
- If they are like the parents, they are dominant, and have at least one dominant allele. (B)
- If they are like the different kid, they are recessive and must have two recessive alleles. (bb)
Finally, figure out whether dominant individuals are definitely heterozygous (Bb) or possibly homozygous (B_).
- If a child is homozygous recessive, they must get one recessive allele from each parent.
- If a parent is homozygous recessive, they must pass a recessive allele to each of their children.
- If a child is dominant and their parents are also both dominant, you may not know their full genotype (B_).