Overview
This lecture reviews a medium-difficulty genetics exam question about sex-linked inheritance of color blindness, focusing on pedigree analysis, genotype determination, and explanation of inheritance patterns.
Understanding the Question
- The question involves a pedigree diagram showing the inheritance of color blindness in a family.
- Color blindness is sex-linked (on the X chromosome) and caused by a recessive allele (d); normal vision is dominant (D).
- Important to recognize that males are XY and females are XX when filling in genotypes.
Analyzing the Pedigree and Answering Questions
- To count normal male offspring, identify male squares from parents 1 and 2 with normal vision (answer: 2).
- Always fill in X and Y chromosomes plus appropriate D/d alleles for relevant individuals before assigning genotypes.
- For individual 2 (female): genotype is XᴰXᵈ (carrier, normal vision).
- For individual 5 (female): genotype is XᵈXᵈ (colorblind).
Explaining Sex-Linked Inheritance
- The color blindness allele is on the X chromosome.
- Males have only one X chromosome, so a single recessive allele (d) results in color blindness.
- Females need two copies of the allele (d) to be colorblind; carriers have one D and one d.
Predicting Offspring Genotypes
- When a colorblind female (XᵈXᵈ) marries a normal male (XᴰY):
- All daughters are XᴰXᵈ (carriers with normal vision).
- All sons are XᵈY (colorblind).
- Therefore, 100% of the daughters inherit the allele for color blindness but are not colorblind themselves.
Key Terms & Definitions
- Pedigree diagram — a family tree showing inheritance of traits.
- Sex-linked — a gene located on a sex chromosome (usually X).
- Allele — different forms of a gene (e.g., D or d).
- Genotype — genetic makeup of an individual (e.g., XᴰXᵈ).
- Carrier — individual with one normal and one disease allele, not showing symptoms.
Action Items / Next Steps
- Review the memo for model answers, focusing on how explanation questions are marked.
- Practice filling in alleles and genotypes on pedigree diagrams for similar sex-linked inheritance problems.