Overview
This lecture explains how to read and analyze pedigrees, with a focus on autosomal and sex-linked recessive traits using attached earlobes as an example.
Pedigree Basics
- A pedigree is a diagram that shows inheritance of traits across generations, similar to a family tree.
- Circles represent females; squares represent males in pedigrees.
- Roman numerals indicate generations; horizontal marriage lines connect parents, while vertical lines connect to children.
- Shaded shapes track individuals expressing a specific trait.
Autosomal Recessive Traits
- Autosomal traits are on chromosomes not involved in determining sex (chromosomes 1-22).
- A recessive trait only appears if both alleles are recessive (e.g., "ee" for attached earlobes).
- Dominant alleles (e.g., "E" for free earlobes) mask recessive ones.
- Carriers have one dominant and one recessive allele (Ee); they do not express the recessive trait but can pass it on.
- When assigning genotypes, ensure children can inherit one allele from each parent.
Sex-Linked Recessive Traits
- Sex-linked traits are usually associated with genes on the X chromosome.
- Females have two X chromosomes (XX); males have one X and one Y (XY).
- For a female to express a recessive X-linked trait, she must have both recessive alleles (XĘłXĘł).
- Males only need one recessive allele on the X chromosome (XĘłY) to show the trait.
- Carrier females have one dominant and one recessive allele (Xá´żXĘł).
- Always check that children's genotypes are possible given parental genotypes.
Solving Pedigrees
- Label shaded individuals with the homozygous genotype for the recessive trait.
- Unshaded individuals' genotypes may require considering all possible allele combinations based on offspring.
- Half-shaded symbols may indicate carrier status.
Key Terms & Definitions
- Pedigree — Family diagram showing inheritance of traits.
- Autosome — Chromosome not involved in sex determination.
- Recessive Trait — Expressed only when both alleles are recessive.
- Carrier — Individual with one dominant and one recessive allele; does not express the recessive trait but can pass it on.
- Sex-linked Trait — Trait associated with a gene on a sex chromosome (usually X).
Action Items / Next Steps
- Complete the handout on autosomal dominant pedigrees.
- Practice labeling genotypes in sample pedigrees, checking allele inheritance from both parents.