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Before DNA was identified as the blueprint of life, Central European monk Gregor Mendel used pea plants to uncover the principles that govern inheritance. By observing the offspring of carefully designed crosses, he established the concepts of dominance and recessiveness that still underpin modern genetics.

Genes and Phenotypes

In Mendelian genetics, each observable trait—such as flower color, stem length, or seed shape—is controlled by a pair of genes, one inherited from each parent. Variations in these traits arise when individuals carry different versions of the same gene, known as alleles. For example, Mendel’s peas displayed either round or wrinkled seeds. Plants that were true‑breeding produced offspring that all shared the same seed shape, confirming that they carried identical alleles.

Masking the Recessive

Mendel noticed that some round‑seed plants, when self‑pollinated, produced a mix of round and wrinkled seeds. In contrast, self‑pollinated wrinkled plants never produced round seeds. He deduced that the round plants were either homozygous (two dominant alleles) or heterozygous (one dominant and one recessive allele). The recessive wrinkled allele was hidden, or “masked,” by the dominant round allele. True‑breeding wrinkled plants were therefore homozygous recessive. This observation led Mendel to label round as a dominant trait and wrinkled as recessive.

Performing a Test Cross

To determine whether an unknown round‑seed plant was homozygous or heterozygous, Mendel devised the test cross. He crossed the unknown plant with a known homozygous recessive plant (wrinkled). Because every progeny inherits one allele from each parent, all offspring were guaranteed to receive a recessive allele from the wrinkled parent.

Genotypic Ratios and Interpretation

Two scenarios emerged from the cross:

• If the unknown plant was homozygous dominant, every offspring received a dominant allele and all seeds appeared round.
• If the unknown plant was heterozygous, half of the offspring received the recessive allele from the unknown parent, producing a 1:1 ratio of round to wrinkled seeds.

These observable ratios allowed Mendel to infer the hidden genotype of the unknown plant and revealed the underlying mechanics of inheritance.

Through this elegant experiment, Mendel laid the groundwork for genetics, showing that traits follow predictable patterns that can be decoded by simple cross‑breeding tests.