Study guide
Follows AAMC Content Category 1C. Educational content only. Ranges from discrete calculation items (e.g., Hardy-Weinberg) to passage-based inheritance-pattern or linkage inference from cross data.
Mendelian Inheritance and Pedigree Analysis
Covers autosomal dominant/recessive crosses, testcrosses, X-linked recessive inheritance patterns, and pedigree analysis logic.
Linkage and Recombination Mapping
Covers crossing over, linked-gene testcross ratios (parental vs. recombinant classes), recombination frequency calculation, and map units/centimorgans.
Hardy-Weinberg Population Genetics
Covers p+q=1 and p^2+2pq+q^2=1, solving for allele/genotype frequencies from recessive phenotype frequency, and the five equilibrium assumptions and their violations.
Meiosis and Sources of Genetic Variation
Covers meiosis I/II mechanics and the three sources of variation: crossing over, independent assortment (2^n), and random fertilization.
Key terms
- Testcross
- — Cross with a homozygous recessive individual to reveal an unknown genotype.
- X-linked recessive inheritance
- — A recessive X allele is expressed in any hemizygous male who carries it (no second X allele to mask it), while females typically must be homozygous recessive to show the phenotype; heterozygous females are unaffected carriers.
- Linkage
- — Genes near each other on a chromosome inherited together more often than independent assortment predicts.
- Recombination frequency
- — Proportion of recombinant offspring, used to estimate genetic map distance.
- Map unit (centimorgan)
- — 1% recombination frequency equals 1 map unit.
- Hardy-Weinberg principle
- — p^2+2pq+q^2=1 with p+q=1 models a non-evolving population's genotype frequencies.
- Genetic drift
- — Random allele frequency change, pronounced in small populations.
- Independent assortment
- — Random homologous pair orientation in meiosis I producing 2^n gamete combinations.
- Crossing over
- — Exchange of chromosomal segments between homologs during prophase I.
Exam tips
- Check for father-to-son transmission first to test/rule out X-linkage in pedigrees.
- Identify most frequent offspring classes as parental and least frequent as recombinant before calculating recombination frequency.
- Remember recombination frequency caps near 50% even for unlinked or very distant genes.
- Solve for q from recessive phenotype frequency first in Hardy-Weinberg problems.
- Consider genetic drift first for small/founder populations deviating from equilibrium.
- Distinguish crossing over (new combinations within a chromosome) from independent assortment/random fertilization (new combinations of whole chromosomes/gametes).