Overview
This lecture covers how to read and interpret pedigree diagrams, focusing on the inheritance patterns of autosomal dominant, autosomal recessive, and X-linked (sex-linked) recessive disorders.
Understanding Pedigree Diagrams
- Pedigree diagrams are family trees showing relationships and trait inheritance.
- Squares represent males; circles represent females.
- Shaded shapes indicate individuals with the trait/disorder.
- Roman numerals mark generations.
- Key skill: deducing each individual’s genotype.
Autosomal Dominant Inheritance
- Autosomal means the gene is on chromosomes 1–22 (not sex chromosomes).
- Dominant means one dominant allele (e.g., D) causes the disorder.
- Affected individuals have at least one dominant allele (e.g., Dd or DD).
- Unaffected individuals have two recessive alleles (e.g., dd).
- Work backward from children to parents to deduce genotypes.
Autosomal Recessive Inheritance
- Recessive traits require two recessive alleles (e.g., rr) to express the disorder.
- Carriers (Rr) do not show symptoms but can pass the gene.
- Unaffected individuals may be carriers or homozygous dominant (RR).
- Analyze offspring genotypes to infer parent genotypes.
X-Linked (Sex-Linked) Recessive Inheritance
- Genes are located on the X chromosome.
- Example: hemophilia or red-green color blindness.
- Females: two X chromosomes; can be normal, carrier, or affected.
- Males: one X chromosome; if it carries the recessive allele, the disorder is present (no carrier status).
- Males are more frequently affected because they lack a second X chromosome to mask recessive alleles.
Interpreting Pedigrees (Examples)
- Assign genotypes using pattern rules and clues from affected/unaffected offspring.
- For X-linked disorders, fill in X and Y chromosomes, then superscript alleles.
- For dominant conditions, shaded = at least one dominant allele; unshaded = two recessive alleles.
- For sex-linked recessive, affected females must have two recessive alleles; affected males have one on their single X.
Key Terms & Definitions
- Pedigree Diagram — Chart showing family relationships and trait inheritance.
- Autosomal — Refers to non-sex chromosomes (chromosomes 1–22).
- Dominant — Only one allele is needed to express the trait.
- Recessive — Two alleles are needed to express the trait.
- Carrier — Heterozygous for a recessive trait; does not show symptoms.
- X-linked — Gene located on the X chromosome.
- Phenotype — Observable traits or characteristics.
- Genotype — The genetic makeup (allele combination) of an individual.
Action Items / Next Steps
- Practice filling out genotypes and phenotypes on sample pedigree diagrams.
- Review key vocabulary and inheritance patterns.
- Read textbook sections on Mendelian inheritance and sex-linked genes.