Laws of Inheritance

Core Concepts

The law of segregation

Mendel's first law states that the two alleles of each gene separate during gamete formation, and each gamete receives one allele. In modern terms, this is what meiosis accomplishes: homologous chromosomes (and their alleles) are pulled apart in anaphase I, so a gamete from a Pp individual carries either P or p — never both. At fertilization, gametes combine at random, restoring the pair. Segregation explains the monohybrid 3:1 ratio: two Pp heterozygotes produce P and p gametes in a 1:1 ratio from each parent, and random combination yields PP, Pp, pP, and pp in equal proportions — three dominant phenotypes for every recessive one. The law is really a restatement of chromosome behavior in meiosis, which is why the previous chapter matters here.

The law of independent assortment

Mendel's second law states that alleles of different genes assort independently during gamete formation — provided the genes are on different (nonhomologous) chromosomes. The mechanism again lives in meiosis: at metaphase I, each homologous pair orients independently, so one pair's separation has no bearing on another. A dihybrid individual (heterozygous for two genes, AaBb) therefore produces four kinds of gametes — AB, Ab, aB, ab — in equal proportions. When two dihybrids are crossed, the 16 gamete combinations produce the signature 9:3:3:1 phenotype ratio (9 both dominant, 3 each of the two mixed classes, 1 both recessive). The law holds only for genes on different chromosomes; genes on the same chromosome travel together and are linked, taken up next.

Punnett squares

A Punnett square is a grid that organizes gamete combinations. For a monohybrid cross (Pp × Pp), write each parent's two gamete types along the top and side of a 2×2 grid and fill the four boxes. For a dihybrid cross (AaBb × AaBb), each parent makes four gamete types, giving a 4×4 grid with 16 boxes; tallying phenotypes produces 9:3:3:1. For crosses with more genes, squares grow quickly (2ⁿ gamete types per parent for n heterozygous genes); probability rules become more practical than giant grids.

Probability rules: product rule and sum rule

Two probability rules handle most genetics problems. The product rule ("and") says the probability of two independent events both occurring is the product of their probabilities: pp from Pp × Pp is 1/2 × 1/2 = 1/4. The sum rule ("or") says the probability of either of two mutually exclusive events is their sum: a heterozygous offspring is 1/4 + 1/4 = 1/2.

Pedigree analysis

Because humans cannot be cross-bred experimentally, geneticists trace traits in families using pedigrees — family trees with standard symbols. Squares are males, circles females; a horizontal line connects parents, vertical lines connect children; filled = affected, half-filled = carrier (heterozygote for a recessive condition). Autosomal dominant traits (e.g., Huntington's disease) typically appear in every generation. Autosomal recessive traits (e.g., cystic fibrosis) often skip generations and can appear in children of two unaffected carriers. If two carriers have a child, the probability the child is affected is 1/4 — exactly Mendel's ratio, applied to a family.

How It Works / Step-by-Step Process

1. Write the parental genotypes (e.g., AaBb × AaBb).
2. List each parent's gametes: apply segregation (one allele per gene) and independent assortment — 2ⁿ types for n heterozygous genes.
3. Set up the Punnett square (or use probability rules): one parent's gametes across the top, the other's down the side; fill each cell.
4. Tally genotypes and phenotypes, grouping genotypes with the same phenotype.
5. Check against expected ratios: 3:1 for one trait, 9:3:3:1 for two.
6. For human traits, use a pedigree: map affected and carrier status across generations, then apply the same ratios to predict risk.

Common Confusions