Transmission of heritable information and genetic diversity

A gene sits at a locus; its variants are alleles. An organism with two identical alleles is homozygous, with two different ones heterozygous. The dominant allele's phenotype shows in heterozygotes; the recessive phenotype appears only when homozygous recessive. Genotype is the allele combination; phenotype is what you observe (set by genotype plus environment).

Segregation falls out of meiosis (homologs separate, so each gamete gets one allele). Independent assortment holds for genes on different chromosomes (or far apart on one). A Punnett square enumerates gamete combinations:

Bb × Bb → 1 BB : 2 Bb : 1 bb (genotype), i.e. 3 dominant : 1 recessive phenotype. A test cross (organism × homozygous recessive) reveals whether a dominant-phenotype individual is homozygous or heterozygous.

Incomplete dominance gives an intermediate phenotype (the heterozygote looks "in between"). Codominance shows both phenotypes simultaneously, not a blend — the AB blood group (both A and B antigens) and sickle-cell trait are the examples. The ABO system also illustrates multiple alleles (Iᴬ, Iᴮ, i): a population carries more than two allele versions even though each person has only two.

Males (XY) have only one copy of X-linked genes, so a single recessive allele is expressed — that's why X-linked recessive disorders are more common in males, and why an affected father passes the allele to all daughters (carriers) but no sons. Pedigree clues: a trait that skips generations and affects mostly males suggests X-linked recessive; a trait in every generation affecting both sexes suggests autosomal dominant.

Real inheritance is messier than one-gene-one-trait. Epistasis is one gene's product masking another gene's expression (e.g., coat-color genes). Pleiotropy is the reverse — a single gene affecting many traits (sickle-cell affects blood, spleen, pain, infection resistance). Polygenic traits (height, skin color) are controlled by many genes and show continuous variation. Incomplete penetrance explains why some carriers of a "dominant" disease allele never show it.

Because mitochondria are maternally (cytoplasmically) inherited, mitochondrial-DNA disorders (e.g., Leber's hereditary optic neuropathy, MERRF) ignore the nuclear rules entirely: every child of an affected woman is at risk regardless of sex, and the line dies out through affected males. Expression is variable (heteroplasmy — cells carry a mix of normal and mutant mitochondria), so severity differs even among siblings. This is the fourth pedigree pattern to recognize alongside autosomal and X-linked inheritance (see sex-linked inheritance & pedigrees).

In prophase I, homologous chromosomes pair (synapsis) into tetrads/bivalents and exchange segments at chiasmata (crossing over). In metaphase I tetrads line up and assort independently; in anaphase I the homologs separate (sister chromatids stay together) — this is the reduction to haploid. Meiosis II then separates the sister chromatids, like a mitotic division, yielding four haploid cells.

Crossing over swaps segments between homologs, creating new allele combinations on a chromosome. Independent assortment means each homolog pair orients randomly at metaphase I, giving 2ⁿ possible gamete combinations (n = haploid number; 2²³ for humans). Random fertilization multiplies that by the variation in the other gamete. These are the engines of diversity meiosis provides.

The discriminators AAMC tests: mitosis has no synapsis and no crossing over; meiosis I pairs homologs and recombines them. Mitosis preserves chromosome number; meiosis halves it. Mitosis yields genetic clones; meiosis yields variation. (Mitosis itself is covered in cell division.)

If homologs fail to separate in meiosis I, or sister chromatids in meiosis II, a gamete ends up with an extra or missing chromosome. Fertilization then gives trisomy (three copies) or monosomy (one). Examples: trisomy 21 (Down), XXY (Klinefelter), X0 (Turner). Maternal age raises the risk.

Because crossovers occur roughly at random along a chromosome, the chance of a crossover between two loci rises with the distance between them. So recombination frequency is a proxy for distance: 1% recombinants = 1 centimorgan. The maximum is 50% — at that point the genes behave as if unlinked (on different chromosomes or very far apart).

These differ from the point mutations of 1B by acting on whole chromosome segments. They often arise from errors in crossing over or repair, and can disrupt many genes at once.

Hardy–Weinberg lets you compute genotype frequencies from allele frequencies (and back). The classic move: a recessive disease frequency gives q², so q = √(q²), then p = 1 − q, and carriers = 2pq. The five assumptions (the conditions for no evolution): no mutation, no gene flow (migration), no natural selection, random mating, and a large population (no genetic drift). Violate any one and allele frequencies shift.

Natural selection is non-random and adaptive — it favors the fittest (those with the most reproductive success). Genetic drift is random and matters most in small populations: a bottleneck (population crash) or founder effect (a few individuals start a new population) can fix or lose alleles by chance. Gene flow mixes alleles between populations; mutation introduces brand-new alleles. Only mutation creates variation; the others redistribute it.

Picture the trait's bell curve: directional selection pushes it toward one tail (e.g., antibiotic resistance), stabilizing selection trims both tails toward the average (e.g., human birth weight), and disruptive selection hollows out the middle, favoring the two extremes.

Evolutionary fitness is measured in offspring, not strength. Speciation happens when gene flow stops and populations diverge until they can no longer interbreed (reproductive isolation). Allopatric speciation follows a physical barrier; sympatric speciation occurs within the same area (e.g., via polyploidy in plants or niche divergence).

Homologous structures (e.g., the bat wing, human arm, and whale flipper — same bones rearranged) are evidence of common ancestry and arise by divergent evolution. Analogous structures (e.g., a bat wing vs. a bird wing, or insect vs. bird wings) do the same job but evolved separately — the signature of convergent evolution. Parallel evolution is the in-between case: related lineages independently evolve similar traits after diverging. Vestigial structures (the human appendix, tailbone) are reduced remnants of ancestral organs — more evidence of descent with modification.