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Genetics & Heredity
Chapter: Genetics and Heredity
Updated 2026-10-19
Mendelian ratios
- Monohybrid Aa × Aa: genotypes 1 AA : 2 Aa : 1 aa; phenotypes 3 : 1.
- Dihybrid AaBb × AaBb (unlinked): 9 : 3 : 3 : 1.
- Test cross (unknown × homozygous recessive): Aa × aa gives 1 : 1; AaBb × aabb gives 1 : 1 : 1 : 1.
- Product rule (independent events, 'and'): multiply. Sum rule (mutually exclusive, 'or'): add.
- For multi-gene crosses, solve each gene separately and multiply: P(aabb) from AaBb × AaBb = ¼ × ¼ = 1/16.
Beyond simple dominance
| Pattern | Feature | Example |
|---|
| Incomplete dominance | Heterozygote is intermediate; phenotype ratio 1 : 2 : 1 | Pink snapdragons |
|---|
| Codominance | Both alleles fully expressed | AB blood type |
|---|
| Multiple alleles | More than two alleles in the population | ABO (Iᴬ, Iᴮ, i) |
|---|
| Epistasis | One gene masks another; modified dihybrid ratios such as 9 : 3 : 4 | Labrador coat color |
|---|
| Polygenic | Many genes add up; continuous variation | Height, skin color |
|---|
| Pleiotropy | One gene affects many traits | Marfan syndrome |
|---|
| Mitochondrial | Passed from mother to all children | Some myopathies |
|---|
Pedigree clues
| Inheritance | Clues |
|---|
| Autosomal dominant | Every generation; affected child has an affected parent; males and females equal |
|---|
| Autosomal recessive | Skips generations; unaffected carrier parents; consanguinity raises risk |
|---|
| X-linked recessive | Mostly males; passed through carrier mothers; never father → son |
|---|
| X-linked dominant | Affected father → ALL daughters and no sons |
|---|
| Mitochondrial | Affected mother → all children; affected father → none |
|---|
Linkage and populations
- Recombination frequency = recombinant offspring ÷ total offspring; 1% = 1 map unit (centimorgan). Maximum is 50% (looks unlinked).
- Hardy–Weinberg: p + q = 1 and p² + 2pq + q² = 1. Requires a large population, random mating, no mutation, no migration (gene flow) and no selection.
- Start from the recessive phenotype frequency: q = √(q²). Carriers = 2pq.
- Common aneuploidies: Down syndrome (trisomy 21), Turner (45,X), Klinefelter (47,XXY).
⚠️ Common traps
- Using the 9 : 3 : 3 : 1 ratio for linked genes.
- Taking q directly from the affected fraction without the square root.
- Allowing father-to-son transmission in X-linked traits.
- Adding probabilities for independent events instead of multiplying.
Mini-quiz
Pick an answer to see right away whether it is correct and why.
Two AaBb individuals (unlinked genes) are crossed. What fraction of offspring is aabb?
In a population at Hardy–Weinberg equilibrium, 9% show a recessive trait. What fraction are carriers?
An affected father has only affected daughters and only unaffected sons, across several families. Which inheritance fits best?
Two genes show 12% recombinant offspring. How far apart are they?
Parents with blood types A (Iᴬi) and B (Iᴮi) have a child. What is the probability the child is type O?
0 of 5 answered
Answer key: 1 C · 2 C · 3 C · 4 B · 5 B
