Overview
This lecture provided a revision of Genetics 101, focusing on patterns of dominance, sex-linked disorders, and how to explain genetic outcomes using Punnett squares, ratios, and key terminology.
Types of Dominance
- Complete dominance: One trait (allele) is completely dominant over the other.
- In complete dominance, use same letter with different sizes (e.g., T = dominant, t = recessive).
- Homozygous: Two identical alleles (can be dominant or recessive).
- Heterozygous: Two different alleles.
- Incomplete dominance: Neither allele is dominant, resulting in an intermediate phenotype (e.g., red + white flower = pink).
- For incomplete dominance, use two different capital letters representing each allele (e.g., R for red, W for white; RW = pink).
- Codominance: Both alleles are equally and simultaneously expressed (e.g., red + white flower = red and white flower).
- For codominance, use a common base letter with superscripts for each allele (e.g., Cᴿ, Cᵂ).
Proving Dominance with Punnett Squares
- Pure breeding (homozygous) parents can be used to determine which trait is dominant.
- Crossing homozygous dominant with homozygous recessive results in all heterozygous offspring showing the dominant trait.
Sex-Linked Disorders
- Sex-linked traits are carried on sex chromosomes, mainly the X chromosome.
- Females: XX; Males: XY.
- Only X-linked disorders covered (no Y-linked).
- Most sex-linked disorders are recessive; for a female to show it, she must have two recessive alleles.
- A carrier female has one dominant and one recessive allele (not affected but can pass it on).
- Males with a single X carrying the disorder allele will express the disorder.
- Males cannot be heterozygous because they have only one X chromosome.
Examples: Hemophilia & Red-Green Color Blindness
- Hemophilia: Use H (capital for normal, lowercase for hemophilia); Xᴴ or Xʰ.
- Red-green color blindness: Use R (capital for normal, lowercase for colorblindness); Xᴿ or Xʳ.
- Know these specific letters for exams; new disorders will provide the necessary information.
Explaining Genetic Outcomes
- Use ratios (e.g., 3:1 or 1:1:1:1) from Punnett squares to support answers.
- Explaining inheritance: State each parent's genotype and what each contributes to the offspring.
- Explaining sex-linkage prevalence: Males (XY) are more likely to express X-linked disorders as they have only one X.
Key Terms & Definitions
- Allele — Different forms of a gene.
- Homozygous — Two identical alleles for a trait.
- Heterozygous — Two different alleles for a trait.
- Dominant allele — Expressed when present.
- Recessive allele — Expressed only when both alleles are recessive.
- Phenotype — Observable trait.
- Genotype — Genetic makeup.
- Carrier — Individual with one normal and one disorder allele (for sex-linked disorders).
- Punnett square — Chart predicting genetic cross outcomes.
- Codominance — Both alleles expressed equally.
- Incomplete dominance — Blending of alleles.
- Sex-linked disorder — Disorder related to sex chromosomes.
Action Items / Next Steps
- Review the cheat sheet study guide for codominance and incomplete dominance.
- Practice Punnett square problems, especially for sex-linked traits.
- Prepare for upcoming lessons on pedigree diagrams and dihybrid crosses.
- Watch for notifications about additional live revision sessions.