Mendelian Genetics (Punnett Square)
The science of genetics, born from peas
Summary
Combine the parents' alleles and predict the offspring's genotypes. A is dominant, a recessive. The Punnett square shows the 4 possible combinations; the classic 3:1 ratio of dominant to recessive is the basic rule of inheritance that Mendel discovered.
In depth
Between 1856 and 1863 Gregor Mendel discovered the basic rules of genetics through experiments with 28,000 pea plants in his monastery garden. Each trait (colour, shape, height) is passed on by two hereditary units. In Mendel's day these were called "factors"; today we call them alleles, the different versions of a gene. Every individual gets one allele of each gene from its mother and one from its father. In pea seed colour, yellow (A) is dominant and green (a) recessive. If an individual carries at least one A allele, its phenotype is yellow; only aa is green. Mendel set out two basic laws: the law of segregation (each individual passes one allele of each gene to a gamete) and the law of independent assortment (the alleles of different genes are passed on independently). The Punnett square was developed in the early 1900s by the English geneticist Reginald Punnett to visualise these rules easily.
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