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Inheritance

Inheritance is how genetic information passes from parents to offspring. Working with garden peas in the 1850s and 1860s, Gregor Mendel showed that traits are passed on as discrete units — what we now call alleles of genes — rather than blending together, and that a cross between two heterozygotes gives a predictable 1:2:1 genotype ratio and, under complete dominance, a 3:1 phenotype ratio. A Punnett square turns those rules into expected offspring frequencies.

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Segregation, assortment and linkage

The law of segregation has a physical basis in meiosis I, when homologous chromosomes — each carrying one allele — separate. The law of independent assortment states that alleles of different genes sort into gametes independently; for a dihybrid cross of double heterozygotes this predicts a 9:3:3:1 phenotype ratio, which is the product of two independent 3:1 ratios.

(34+14)2=916+316+316+116\left(\tfrac{3}{4} + \tfrac{1}{4}\right)^2 = \tfrac{9}{16} + \tfrac{3}{16} + \tfrac{3}{16} + \tfrac{1}{16}

Two independent 3:1 ratios combine into the 9:3:3:1 dihybrid ratio.

Interactive simulation

DNA Inheritance

Explore Mendelian inheritance interactively with a real monohybrid Punnett square — choose each parent's alleles and see the offspring genotype and phenotype ratios recompute live.

The simulation covers the single-gene (monohybrid) case under complete dominance — the building block of the dihybrid ratio.

Independent assortment fails for linked genes that sit close together on one chromosome: they tend to be inherited together, although crossing over during meiosis can separate them.

Common misconception: Blending inheritance — the pre-Mendelian idea that parental traits merge irreversibly — was refuted by the reappearance of recessive traits in the F2 generation. Apparent blending in traits such as height reflects many genes acting together, not the loss of discrete alleles.
Full explanation — the complete reference version every reading depth is based on

What inheritance is

Children resemble their parents because they inherit copies of their parents' genes. For most genes a person carries two copies, one from each parent, and those copies can be different versions — alleles. Inheritance describes the rules that decide which alleles an offspring receives and how they show up as visible traits.

  • Genotype: the alleles an organism carries for a gene, written with letters such as AA, Aa or aa.
  • Phenotype: the trait that is actually observed, such as violet or white flowers.
  • Homozygous: both copies are the same allele (AA or aa). Heterozygous: the two copies differ (Aa).
  • Dominant allele (capital letter): shows its trait even when only one copy is present. Recessive allele (small letter): shows its trait only when both copies are recessive.

The laws of inheritance

  1. Law of segregation: the two copies of a gene separate equally into gametes (eggs or sperm), so each offspring is equally likely to receive either copy. The physical basis is the separation of homologous chromosomes in the first division of meiosis.
  2. Law of independent assortment: alleles of different genes are sorted into gametes independently, giving a 9:3:3:1 phenotype ratio in a cross of two double heterozygotes — except for linked genes that sit close together on the same chromosome.

Worked example: a Punnett square

Cross two heterozygous parents, Aa × Aa. Each parent's gametes carry A or a with equal probability, so the four boxes of the square are AA, Aa, aA and aa, each with a 1 in 4 chance. That gives genotypes in a 1 AA : 2 Aa : 1 aa ratio. Because AA and Aa both show the dominant trait, the phenotype ratio is 3 dominant : 1 recessive — exactly the pattern Mendel counted. The chance of an aa offspring is 1/2 × 1/2 = 1/4.

P(aa)=P(a from parent 1)×P(a from parent 2)=12×12=14\begin{aligned} P(aa) &= P(a \text{ from parent 1}) \\ &\quad \times P(a \text{ from parent 2}) \\ &= \tfrac{1}{2} \times \tfrac{1}{2} = \tfrac{1}{4} \end{aligned}

The product rule of probability applied to an Aa × Aa cross.

A test cross uses the same logic to find an unknown genotype. Cross a dominant-looking organism with a homozygous recessive one (aa). If the unknown is AA, every offspring is Aa and shows the dominant trait; if it is Aa, the offspring come out about half Aa and half aa — a 1:1 ratio.

Beyond simple dominance

  • Incomplete dominance: the heterozygote is intermediate. Pink heterozygous snapdragons self-crossed give red, pink and white flowers in a 1:2:1 ratio.
  • Codominance: both alleles show at once, as in the human MN blood group.
  • Many traits involve several genes, so they do not follow a single neat Punnett square.
Common misconception: A common misconception is that eye colour is controlled by one gene with brown simply dominant over blue, so two blue-eyed parents could never have a brown-eyed child. Eye colour involves several genes, chiefly OCA2 and HERC2 on chromosome 15; it is uncommon, but blue-eyed parents can have a brown-eyed child.
Common misconception: Another misconception is that a 3:1 ratio means a family with four children will have exactly three with the dominant trait. The ratio is a probability for each child; Mendel saw 3:1 because he counted hundreds of plants.

Where this connects

Inheritance depends on genes and DNA, and it supplies the heritable variation that natural selection works on. Joining Mendel's genetics with Darwin's natural selection was a central step in modern evolutionary biology.

How we know: Mendel's peas

Gregor Mendel began studying inheritance in 1856 and settled on the garden pea, Pisum sativum. He presented results from nearly 30,000 pea plants in 1865 and published them in 1866. When he crossed true-breeding violet-flowered plants with white-flowered ones, every plant in the first (F1) generation had violet flowers — no pale violet blend. When those F1 plants self-fertilised, the white flowers came back: 705 violet to 224 white, about 3.15 to 1. He saw the same roughly 3:1 return of the hidden trait in all seven characteristics he studied.

Info: Mendel's work went virtually unnoticed at the time and was rediscovered in 1900. He had no knowledge of DNA or genes; he called the heritable factors 'elementen'.

Assumptions and limits

  • Punnett-square ratios are expectations for each offspring, built on equal segregation and random fertilisation; small families scatter widely around them.
  • The 3:1 phenotype ratio needs complete dominance; incomplete dominance and codominance keep the 1:2:1 genotype ratio but change what is seen.
  • The 9:3:3:1 ratio assumes the two genes assort independently, which fails for linked genes close together on one chromosome.
  • The ScienceVerse inheritance simulation models one gene with complete dominance only, so it shows the law of segregation, not linkage or multi-gene traits.

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Content status: published 1 October 2026.

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