Punnett 2 alelos heterozigotos

Punnett square — heterozygous cross

The Punnett square tells you how often each genotype and phenotype should show up in the offspring of a cross. Take a monohybrid cross between two heterozygotes, Aa × Aa. The genotypes come out at 1 AA : 2 Aa : 1 aa, while the visible traits split 3 dominant : 1 recessive. Example: cross tall (T) and dwarf (t) pea plants as Tt × Tt and you get 25% TT, 50% Tt, 25% tt, which works out to 75% tall and 25% dwarf. Reginald Punnett proposed the layout, building on the laws of segregation that Gregor Mendel published in 1866. Add a second gene for a dihybrid cross AaBb × AaBb and the phenotypes fall into a 9:3:3:1 pattern, assuming the genes assort independently.

Applications

It shows up everywhere in Mendelian genetics, from ENEM and entrance-exam questions to genetic counselling, where it estimates the risk of a recessive disease appearing in a couple's children. Breeders lean on it too: cattle breeding uses it to pick crosses for productive traits, and pedigree dog breeding uses it to anticipate coat colour, eye colour and hereditary defects.

FAQ

What is the difference between genotype and phenotype? The genotype is the actual allele combination an organism carries (AA, Aa, aa), and the phenotype is the trait you can see. Since AA and Aa both look dominant, they get counted together, which is where the 3:1 ratio comes from.

Does it work for any number of genes? In principle yes, but the grid balloons fast: 1 gene needs 4 boxes, 2 genes need 16, 3 genes need 64. Once you have a handful of genes, the product rule of probabilities is far less painful than drawing the square.

And in codominance or incomplete dominance? The genotypes still come out 1:2:1, but now the phenotypes match them 1:2:1 as well, because the heterozygote shows its own intermediate or mixed appearance. Think MN blood type, or the pink flowers you get from crossing red and white snapdragons.