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Genetics Linkage, Mapping, Population Genetics Cheat Sheet
A printable reference covering linkage, recombination frequency, map distance, Hardy-Weinberg equilibrium, selection, mutation, inbreeding, FST, and population structure for college.
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This cheat sheet covers the core tools geneticists use to connect inheritance patterns with evolutionary change in populations. Linkage and mapping explain how genes on the same chromosome are tracked through crosses and pedigrees. Population genetics explains how allele frequencies change through mating patterns, selection, mutation, migration, drift, and structure. College students need these formulas because they appear repeatedly in genetics problems, lab analysis, and evolutionary biology models. The most important ideas are recombination frequency, map distance, Hardy-Weinberg genotype expectations, and forces that shift allele frequencies. Recombination frequency estimates how far apart loci are, while Hardy-Weinberg equations provide a null model for random mating without evolution. Selection and mutation change allele frequencies in predictable ways when fitness values and mutation rates are known. Inbreeding and population subdivision alter genotype frequencies, especially heterozygosity, even when allele frequencies stay similar.
Key Facts
- Recombination frequency is RF = recombinant offspring / total offspring, and 1 percent recombination is approximately 1 map unit or 1 centimorgan.
- Linked genes are on the same chromosome and tend to be inherited together unless crossing over separates them during meiosis.
- For a two-point testcross, map distance in cM = 100 x recombinant progeny / total progeny.
- Hardy-Weinberg genotype frequencies for two alleles are p^2 + 2pq + q^2 = 1, where p + q = 1.
- Allele frequency can be calculated from genotype counts as p = (2AA + Aa) / (2N) and q = (2aa + Aa) / (2N).
- Relative fitness is often written as wAA, wAa, and waa, and the mean fitness is w-bar = p^2wAA + 2pqwAa + q^2waa.
- With inbreeding coefficient F, expected heterozygosity becomes H = 2pq(1 - F), while homozygote frequencies increase.
- Population differentiation is commonly measured by FST = (HT - HS) / HT, where HT is total expected heterozygosity and HS is average within-population heterozygosity.
Vocabulary
- Linkage
- Linkage is the tendency of genes located near each other on the same chromosome to be inherited together.
- Recombination frequency
- Recombination frequency is the proportion of offspring with allele combinations different from the parental combinations.
- Centimorgan
- A centimorgan is a genetic map unit equal to about 1 percent recombination between two loci.
- Hardy-Weinberg equilibrium
- Hardy-Weinberg equilibrium is the condition in which allele and genotype frequencies remain constant under random mating and no evolutionary forces.
- Selection coefficient
- The selection coefficient s measures the fitness disadvantage of a genotype, often written as fitness w = 1 - s.
- FST
- FST is a statistic that measures genetic differentiation among populations based on the reduction of heterozygosity within subpopulations.
Common Mistakes to Avoid
- Treating recombination frequency as unlimited is wrong because observed recombination frequency cannot exceed 50 percent, even for loci far apart.
- Assuming map distance is always exactly physical distance is wrong because recombination rates vary across chromosomes, sexes, and genomic regions.
- Using p^2, 2pq, and q^2 without checking assumptions is wrong because Hardy-Weinberg expectations require random mating, no selection, no mutation, no migration, and very large population size.
- Forgetting to count two alleles per diploid individual is wrong because allele frequency calculations use 2N total allele copies, not N individuals.
- Interpreting high homozygosity as proof of selection is wrong because inbreeding, drift, population subdivision, or nonrandom mating can also reduce heterozygosity.
Practice Questions
- 1 In a testcross, 84 offspring show parental types and 16 show recombinant types. What is the recombination frequency and map distance between the two loci?
- 2 A population has 49 AA individuals, 42 Aa individuals, and 9 aa individuals. Calculate p and q for the A and a alleles.
- 3 If p = 0.7 and q = 0.3 in a Hardy-Weinberg population, what are the expected frequencies of AA, Aa, and aa?
- 4 A population is split into small isolated subpopulations, and each subpopulation shows fewer heterozygotes than expected. Explain why population structure can produce this pattern without requiring natural selection.