Overview
Genetics rewards careful bookkeeping. Most mistakes come from rushing: not writing out genotypes, forgetting that a phenotype can come from more than one genotype, or mixing up recombination frequency with map distance over long intervals.
Practice the calculations until they are routine. Then spend your time on the reasoning: which inheritance pattern fits this pedigree, and what cross would tell two hypotheses apart?
Core concepts
3.1Mendelian inheritance and probability
Segregation: each gamete gets one allele of each gene. Independent assortment: genes on different chromosomes (or far apart on one chromosome) assort independently. Use the product rule for independent events and the sum rule for mutually exclusive outcomes.
For multi-gene crosses, work one gene at a time and multiply. For example, the chance of an aabbCc offspring from AaBbCc × AaBbCc is ¼ × ¼ × ½ = 1/32.
Key terms: segregation · independent assortment · product rule · test cross · chi-square
3.2Linkage and mapping
Genes close together on a chromosome are inherited together more often than chance predicts. Recombination frequency = recombinant offspring ÷ total offspring. 1% recombination ≈ 1 map unit (centimorgan) for short distances. Recombination frequency never exceeds 50%.
In a three-point test cross, the rarest classes are the double crossovers. They identify the middle gene: it is the one that switches relative to the parental arrangement.
Key terms: recombination frequency · centimorgan · three-point cross · coefficient of coincidence · interference
3.3Pedigrees and inheritance patterns
Autosomal recessive traits can skip generations and appear in children of unaffected parents. Autosomal dominant traits usually appear in every generation, and an affected child usually has an affected parent. X-linked recessive traits affect mostly males and are never passed father to son. Mitochondrial traits pass from an affected mother to all of her children.
Rule out patterns rather than trying to prove one. A single affected daughter of an unaffected father rules out X-linked recessive inheritance.
Key terms: carrier · penetrance · expressivity · X-linked · maternal inheritance
3.4Beyond simple dominance
Incomplete dominance, codominance, multiple alleles, epistasis (for example 9:3:4 and 9:7 ratios), polygenic traits, pleiotropy, genomic imprinting, and sex-linked inheritance all change the expected ratios.
A modified 9:3:3:1 ratio (any set of numbers adding to 16) is a strong hint that two genes interact.
Key terms: epistasis · complementation · imprinting · polygenic · lethal alleles
3.5Population genetics
Hardy–Weinberg: p + q = 1 and p² + 2pq + q² = 1, assuming no selection, mutation, migration, or drift, and random mating. If 1 in 10,000 people has a recessive condition, q² = 0.0001, so q = 0.01 and the carrier frequency 2pq ≈ 0.0198 (about 1 in 50).
Departures from these expectations are evidence that one of the assumptions is violated. For example, an excess of homozygotes suggests inbreeding or population structure.
Key terms: allele frequency · Hardy–Weinberg equilibrium · genetic drift · inbreeding · gene flow
Practice
Review the concepts above, then complete the practice set. Missed a question? Read the explanation and try a similar problem.
Common mistakes
| Mistake | Instead |
|---|---|
| Taking the square root of the dominant phenotype frequency to get p. | Start from the homozygous recessive frequency (q²). The dominant phenotype includes heterozygotes. |
| Adding map distances across a long interval and getting more than 50 cM from a single cross. | Observed recombination tops out at 50%. Long distances are estimated by adding shorter intervals. |
| Assuming a trait is dominant because it appears often. | Frequency and dominance are unrelated. Use the pedigree structure. |
Recommended resources
- OpenStax Biology 2e, Chapters 12–13 Textbook (free)
- Khan Academy: Classical and molecular genetics Videos & practice
- Scitable: Genetics topics Website
Further reading
- Griffiths et al., Introduction to Genetic Analysis. This is the standard reference for olympiad-level genetics problems.
- Campbell Biology, chapters on Mendel and the chromosomal basis of inheritance.