01 · Foundations
Mendel's laws and Punnett squares
Gregor Mendel's experiments with Pisum sativum in the 1860s established two laws that remain central to genetics today:
Law I
Segregation
Each organism carries two alleles for each gene. During gamete formation, these alleles separate so each gamete carries exactly one. The allele a gamete receives is random.
Law II
Independent assortment
Alleles for different genes assort independently during gamete formation — provided the genes are on different chromosomes (not linked). This is the basis of dihybrid cross ratios.
A Punnett square maps all possible allele combinations from a cross. For a monohybrid cross between two heterozygotes (Aa × Aa):
| A (♂) | a (♂) | |
|---|---|---|
| A (♀) | AA | Aa |
| a (♀) | Aa | aa |
Genotype ratio: 1 AA : 2 Aa : 1 aa. Phenotype ratio: 3 dominant : 1 recessive.
Common cross ratios
| Cross type | Cross | Genotype ratio | Phenotype ratio |
|---|---|---|---|
| Monohybrid (het × het) | Aa × Aa | 1 AA : 2 Aa : 1 aa | 3 dom : 1 rec |
| Test cross | Aa × aa | 1 Aa : 1 aa | 1 dom : 1 rec |
| Complete dominance | AA × aa | all Aa | all dominant |
| Dihybrid (het × het) | AaBb × AaBb | 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb | 9:3:3:1 |
Mendel's laws apply to genes that assort independently. When two genes are located close together on the same chromosome, they tend to be inherited together (linkage). Crossing over during meiosis can separate linked genes — the frequency of recombination estimates the distance between loci. See the Mitosis and meiosis guide for how crossing over occurs.
02 · Beyond Mendel
Codominance, sex linkage, and epistasis
Codominance and incomplete dominance
Not all alleles show simple dominance. In codominance, both alleles are expressed simultaneously — the AB blood type is the clearest human example. In incomplete dominance, the heterozygote shows an intermediate phenotype: red × white snapdragons produce pink offspring. Both patterns produce a 1 : 2 : 1 genotype ratio but the phenotype ratio matches it (instead of 3 : 1).
Sex-linked inheritance
Genes on the X chromosome show sex-linked (X-linked) inheritance. Because males carry only one X (XY), they express any allele on that X — there is no second copy to mask a recessive. This is why X-linked recessive conditions such as haemophilia and red-green colour blindness are more common in males. Key rules:
- Carrier females (XAXa) are unaffected but pass the recessive to half their sons.
- Affected males (XaY) pass the allele to all daughters (who become carriers) but to no sons.
- An affected female must have an affected father and a carrier (or affected) mother.
Epistasis
Epistasis occurs when one gene masks the expression of a second. The result is a modified dihybrid ratio. A classic example: Labrador coat colour. Two loci (B/b for pigment type, E/e for pigment deposition) interact: genotype eeB_ and eebb both produce yellow dogs, giving a 9 black : 3 chocolate : 4 yellow ratio rather than the standard 9:3:3:1.
03 · Molecular genetics
From alleles to molecules
Classical genetics describes patterns of inheritance. Molecular genetics explains the mechanism. Each allele is a version of a DNA sequence at a specific chromosomal locus. A mutation — a change in that sequence — creates new alleles. Most mutations are neutral; some change the amino acid sequence of the encoded protein and alter its function; a few have large phenotypic effects.
The sickle-cell allele (HbS) is a single-nucleotide change: A→T in codon 6 of the beta-globin gene. This converts glutamic acid (hydrophilic) to valine (hydrophobic). Valine causes deoxyhaemoglobin molecules to polymerise under low oxygen, distorting red blood cells into a sickle shape that blocks capillaries and triggers haemolytic anaemia.
For the protein-level view — transcription, translation, codon table — see the Protein synthesis guide. For the molecular basis of the DNA code, see DNA, genes and heredity.
Gene editing tools such as CRISPR now allow deliberate, targeted changes to the DNA sequence. The CRISPR base editing article explains how single-letter corrections can fix disease-causing mutations without cutting both DNA strands.
04 · Population genetics
Hardy–Weinberg equilibrium
Individual inheritance explains what happens in a single cross. Hardy–Weinberg equilibrium predicts allele frequencies across an entire population over many generations.
p + q = 1
p² + 2pq + q² = 1
Equilibrium holds only under five conditions: no mutation, no migration, random mating, an infinitely large population, and no selection. In reality, these are never all met simultaneously. Deviations from H–W expectations indicate that one or more evolutionary forces — natural selection, genetic drift, gene flow, non-random mating, or mutation — are acting on the locus.
Worked example: if cystic fibrosis (recessive) affects 1 in 2 500 individuals, then q² = 1/2500 = 0.0004, q = 0.02, p = 0.98, and the carrier frequency 2pq ≈ 0.039 (roughly 1 in 26).
For how selection changes allele frequencies over time, see the Natural selection guide and the Evolution guide.
Genetics and Inheritance — common questions
- The genotype is the actual alleles an organism carries. The phenotype is the observable characteristic that results from genotype plus environment. See the Allele glossary entry for more on how alleles combine.
- A dominant allele produces its phenotype even when only one copy is present. A recessive allele only shows when two copies are present. Codominant alleles both contribute to the phenotype in the heterozygote — as in AB blood type.
- A dihybrid cross examines two genes at once. If both parents are AaBb, the expected phenotype ratio is 9:3:3:1, assuming the genes assort independently (Mendel's second law). Genes on the same chromosome violate this due to linkage.
- Hardy–Weinberg equilibrium states that for alleles p and q (where p + q = 1), genotype frequencies are p² (AA) + 2pq (Aa) + q² (aa) = 1. Deviations from these frequencies reveal which evolutionary forces are acting. Linked to natural selection and evolution.
- Epistasis occurs when one gene masks or modifies another. It distorts the expected 9:3:3:1 dihybrid ratio into patterns like 12:3:1 (dominant epistasis) or 9:7 (duplicate recessive). It is common in coat colour genetics and metabolic pathways.