Overview
This lecture focuses on analyzing a hemophilia pedigree diagram, understanding sex-linked inheritance patterns, filling in genotypes, and answering exam questions related to genetic crosses.
Understanding Hemophilia and Pedigree Diagrams
- Hemophilia is a recessive, sex-linked disorder caused by an allele (x^h) on the X chromosome.
- The normal allele is represented as X^H; hemophilia is indicated by X^h.
- Pedigree diagrams show the inheritance of traits such as hemophilia through generations in a family.
- First, determine if the pedigree is sex-linked (X-linked) or autosomal.
- Assign sex chromosomes: males are XY, females are XX.
- For sex-linked traits, affected males have only one X^h since males have only one X chromosome.
- Affected females must have two X^h alleles (rare, but possible in hypothetical examples).
- Unaffected males must have X^H on their X chromosome.
- Females can be carriers (X^H X^h), meaning they do not have hemophilia but can pass the allele to offspring.
Filling in Alleles Using Pedigree Information
- Start with obvious cases: assign alleles to affected males and females first.
- Use the offspring's traits to infer parents' genotypes, especially for carrier status in females.
- For example, if a non-hemophiliac mother has hemophiliac sons, she must be a carrier (X^H X^h).
- Some individuals' genotypes cannot be determined without examining their offspring.
Answering Exam Questions on the Pedigree
- Phenotype refers to physical appearance; e.g., a normal female does not show hemophilia.
- Genotype involves the specific alleles an individual carries (e.g., X^H X^h for carriers).
- When explaining why females are less likely to have hemophilia, emphasize:
- Females have two X chromosomes.
- Both must carry the recessive allele for hemophilia to be expressed.
- A dominant allele (X^H) will mask the effect of a single recessive allele.
Genetic Crosses and Calculations
- Present genetic crosses with parental (P1) and first filial (F1) generations.
- Write out gametes, use Punnett squares, and show all possible genotype and phenotype outcomes.
- Calculate the percentage chance of specific offspring (e.g., hemophiliac son).
- Always convert fractions to percentages for final answers (e.g., 50% chance of hemophiliac son).
- For full marks, include all required steps, including P1/F1, gametes, meiosis, fertilization, and final percentage.
Key Terms & Definitions
- Hemophilia — a recessive, X-linked blood-clotting disorder.
- Pedigree Diagram — a chart showing genetic relationships and inheritance patterns in a family.
- Sex-linked — a trait carried on a sex chromosome, often the X chromosome.
- Genotype — the genetic makeup of an individual (alleles present).
- Phenotype — the observable traits of an individual.
- Carrier — a female with one normal and one hemophilia allele (X^H X^h).
- Punnett Square — a grid used to predict genetic cross outcomes.
Action Items / Next Steps
- Practice filling in all genotypes/phenotypes on pedigree diagrams.
- Review how to answer pedigree-based genetic exam questions for full marks.
- Prepare Punnett squares and calculate phenotype percentages for genetic crosses.