Intro to Pedigree Analysis

Pedigree Analysis

• Circles represent females and squares represent males in the chart below.
• Colored shapes represent individuals affected by a genetic trait (e.g. color blindness).
• The roman numeral stands for which generation the individuals belong to.
• The digit identifies the individual within each generation.

• The founding parents are the male and female in the first generation.
• The founding family consists of the founding parents and their children.

Use of Pedigree Charts and Rules

• Pedigrees can be used to study the genetics of inherited diseases – to determine the mode of transmission:
• Dominance: whether a disease is dominant or recessive.
• Linkage: whether a disease is X-linked (on the X chromosome) or autosomal (not on sex chromosomes).

4 key things to remember when analyzing pedigrees:

1. An unaffected person cannot carry any alleles of a dominant trait.
2. Why? If the trait is dominant, having the allele means the person would present disease.
3. An unaffected person can be carrying the trait if it is recessive.
4. Why? If the trait is recessive, someone requires two alleles in order to show disease.
5. If the trait is X-linked, a recessive allele is sufficient to affect a male.
6. Why? Males only have one X chromosome. Let's say the mutation is on a protein that is on chromosome X. Since they only have one X, the protein produced is only the mutated one. There's no second X to compensate.
7. Fathers transmit X-linked genes to their daughters only, while mothers transmit X-linked genes to both their sons and daughters.
8. Why? Fathers (males) only have one X chromosome (and one Y). If a man has a male son, they must have given their Y chromosome to their son. If a man has a daughter, they must have given their X chromosome to them.