Free MCAT study guide — Biological and Biochemical Foundations of Living Systems
Genetics spans classical Mendelian inheritance, non-Mendelian patterns, and population genetics, all of which are tested on the MCAT. You must be able to solve monohybrid and dihybrid cross problems, calculate genotype and phenotype ratios, and construct pedigrees. Beyond simple dominance, you need to understand incomplete dominance, codominance, multiple alleles, epistasis, pleiotropy, polygenic inheritance, sex-linked traits, genomic imprinting, and mitochondrial inheritance. Linkage and recombination frequency, gene mapping, and the role of crossing over in generating genetic diversity are all testable. Population genetics requires fluency with the Hardy-Weinberg equilibrium (p^2 + 2pq + q^2 = 1 and p + q = 1), the five conditions required for equilibrium, and the evolutionary forces that disrupt it: natural selection, genetic drift, migration (gene flow), mutation, and nonrandom mating. The MCAT frequently presents genetic data in pedigrees, experimental crosses, or population studies and asks you to determine inheritance patterns or calculate allele frequencies.
Gregor Mendel established two fundamental laws. The Law of Segregation states that each organism carries two alleles for each gene, and these separate during gamete formation so each gamete carries only one allele. The Law of Independent Assortment states that alleles of different genes assort independently during gamete formation, provided the genes are on different chromosomes (unlinked). A monohybrid cross between two heterozygotes (Aa x Aa) yields a 3:1 phenotype ratio and 1:2:1 genotype ratio. A dihybrid cross (AaBb x AaBb) yields a 9:3:3:1 phenotype ratio. A test cross (crossing with a homozygous recessive individual) reveals the genotype of an organism with the dominant phenotype. Punnett squares are used for simple crosses; the branch/forked-line method is more efficient for complex multi-gene crosses. Always define alleles clearly (dominant/recessive) and determine parental genotypes before setting up any cross.
Many traits do not follow simple Mendelian dominance. In incomplete dominance, heterozygotes show an intermediate phenotype (e.g., red x white snapdragons produce pink offspring, with a 1:2:1 phenotype ratio in F2). In codominance, both alleles are fully expressed in heterozygotes (e.g., AB blood type shows both A and B antigens). Multiple alleles means more than two alleles exist in the population for a given gene (e.g., ABO blood groups with IA, IB, and i alleles). Epistasis occurs when one gene masks the expression of another (e.g., Labrador coat color where the E gene controls pigment deposition, modifying the 9:3:3:1 ratio to 9:3:4). Pleiotropy means one gene affects multiple phenotypes (e.g., sickle cell allele affects hemoglobin, RBC shape, spleen function, and malaria resistance). Polygenic traits are controlled by multiple genes, resulting in continuous variation (e.g., height, skin color). Penetrance is the percentage of individuals with a genotype who show the phenotype; expressivity is the degree to which a genotype is expressed.
X-linked recessive traits (e.g., hemophilia, color blindness, Duchenne muscular dystrophy, G6PD deficiency) are more common in males because males have only one X chromosome (hemizygous). Carrier females (heterozygous) are typically unaffected but can pass the allele to sons. In pedigrees, X-linked recessive traits show a pattern of affected males, carrier females, and no male-to-male transmission. X-linked dominant traits (rare) affect females more frequently and are seen in every generation. Y-linked traits are passed exclusively from father to son. X-inactivation (Lyonization) randomly silences one X chromosome in each female cell, creating a Barr body and resulting in mosaicism (e.g., calico cats). Chromosomal abnormalities include aneuploidy (e.g., trisomy 21/Down syndrome from nondisjunction), polyploidy, deletions, duplications, inversions, and translocations. Nondisjunction can occur in meiosis I (both homologs go to one cell) or meiosis II (sister chromatids fail to separate).
Genes located on the same chromosome are linked and tend to be inherited together, deviating from independent assortment. Recombination (crossing over) during prophase I of meiosis can separate linked genes. The recombination frequency between two genes is proportional to the physical distance between them: 1% recombination = 1 centimorgan (cM) = 1 map unit. Recombination frequency ranges from 0% (completely linked) to 50% (effectively unlinked, either on different chromosomes or very far apart on the same chromosome). To determine recombination frequency, perform a test cross and calculate: (number of recombinant offspring / total offspring) x 100%. Three-point crosses allow mapping of three genes simultaneously by identifying the gene order, distances between genes, and the coefficient of coincidence (observed double crossovers / expected double crossovers), which measures interference. Interference = 1 - coefficient of coincidence. High interference means fewer double crossovers than expected.
The Hardy-Weinberg principle states that allele and genotype frequencies remain constant across generations in a population under five conditions: no natural selection, no mutation, no migration (gene flow), random mating, and a large population size (no genetic drift). For a two-allele system, allele frequencies are p + q = 1, and genotype frequencies are p^2 (homozygous dominant) + 2pq (heterozygous) + q^2 (homozygous recessive) = 1. To use Hardy-Weinberg: if you know the frequency of the homozygous recessive phenotype, take the square root to find q, then p = 1 - q, and calculate carrier frequency (2pq). For example, if 1 in 10,000 people have a recessive disease, q^2 = 1/10,000, q = 1/100, p = 99/100, and carrier frequency = 2(99/100)(1/100) = approximately 1/50. Hardy-Weinberg is a null model -- deviations indicate that one or more evolutionary forces are acting on the population.
Natural selection changes allele frequencies based on fitness (reproductive success). Directional selection favors one extreme phenotype, stabilizing selection favors intermediate phenotypes, and disruptive selection favors both extremes. Fitness is relative: the fitness of an allele depends on the environment. Heterozygote advantage (e.g., sickle cell trait providing malaria resistance) maintains both alleles in a population (balancing selection). Genetic drift is random fluctuation in allele frequencies, most significant in small populations. The founder effect occurs when a small group establishes a new population with limited genetic diversity. The bottleneck effect occurs when a population is drastically reduced, losing alleles randomly. Gene flow (migration) introduces or removes alleles from a population, tending to reduce differences between populations. Mutation introduces new alleles but changes frequencies very slowly on its own. Nonrandom mating (assortative mating, inbreeding) changes genotype frequencies without changing allele frequencies -- inbreeding increases homozygosity.
Monohybrid cross (Aa x Aa): 3:1 phenotype ratio, 1:2:1 genotype ratio.
Dihybrid cross (AaBb x AaBb): 9:3:3:1 phenotype ratio (with complete dominance and no epistasis).
X-linked recessive: more males affected, no male-to-male transmission, carrier females.
Hardy-Weinberg: p + q = 1; p^2 + 2pq + q^2 = 1. Five conditions: no selection, mutation, migration, drift, or nonrandom mating.
Recombination frequency = (recombinant offspring / total) x 100%. Max is 50%.
1% recombination frequency = 1 centimorgan = 1 map unit.
Nondisjunction in meiosis I produces all aneuploid gametes; in meiosis II, only half are aneuploid.
Codominance: both alleles expressed (AB blood type). Incomplete dominance: blended phenotype (pink flowers).
Epistasis modifies the 9:3:3:1 ratio (e.g., 9:3:4, 9:7, 12:3:1, 15:1).
Sickle cell disease is autosomal recessive; heterozygotes have sickle cell trait with malaria resistance.
Genetic drift is strongest in small populations (founder effect, bottleneck effect).
Penetrance = % showing phenotype; expressivity = degree of phenotype shown.
Applying the Law of Independent Assortment to linked genes -- linked genes do NOT assort independently.
Confusing incomplete dominance (intermediate phenotype) with codominance (both phenotypes expressed).
Using Hardy-Weinberg when one of the five assumptions is clearly violated (the MCAT may test whether you recognize violations).
Forgetting to square root q^2 to find q before calculating carrier frequency (2pq).
Assuming recombination frequency greater than 50% -- maximum is 50%, genes appear unlinked beyond this distance.
Confusing the founder effect (small group leaves and starts new population) with the bottleneck effect (population is reduced by catastrophe).
Assuming mutations are always harmful -- mutations can be neutral, beneficial, or harmful depending on context.
Forgetting that inbreeding increases homozygosity but does NOT change allele frequencies.
Practice setting up and solving Punnett squares for monohybrid, dihybrid, and sex-linked crosses until they are automatic. For each cross, determine expected genotype and phenotype ratios and compare with observed ratios (chi-square analysis). The MCAT often gives you a pedigree and asks you to determine the most likely inheritance pattern -- practice distinguishing autosomal dominant, autosomal recessive, X-linked recessive, and X-linked dominant from pedigree patterns.
For population genetics, practice Hardy-Weinberg calculations starting from different given information (allele frequencies, genotype frequencies, or disease prevalence). Know when Hardy-Weinberg does and does not apply, and be able to identify which evolutionary force is responsible for observed deviations. Practice recombination frequency calculations from test cross data and use these to construct simple gene maps. The MCAT may present these as data tables in passage-based questions.
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