Dominant and recessive
A dominant allele shows its effect already in a single copy; a recessive one needs both copies. The Punnett square shows how two parents' alleles combine in the children. Many traits, however, are not simply dominant or recessive but arise from many small contributions.
Dominant or recessive
Dominant means: a single allele is enough for the trait to show, even if the second allele differs. Recessive means: the trait appears only when both alleles carry it. Capital letters usually stand for the dominant, lower case for the recessive allele. A heterozygous Aa person therefore shows the dominant trait but carries a hidden along.
The Punnett square
The Punnett square is a small table that lists all combinations of the parental alleles. With two carriers (Aa × Aa) four equally likely cells arise: AA, Aa, Aa and aa. From this follows the ratio 1 : 2 : 1. For a recessive trait on average one in four children is affected (aa), two are healthy carriers, one does not carry the allele at all. This holds per child, not as a fixed order.
Beyond dominant and recessive
The simple scheme that goes back to Mendel describes only part of reality. There are codominant alleles where both show, as in blood groups, and incomplete dominance with a blended form. Most common traits are polygenic: they arise from many genes each with a small contribution and follow no simple square.
What Genome measures. For a recessive trait a heterozygous genotype means carrier, not affected. This distinction is decisive for interpreting a Genome result.
Related topics
Sources
- 1Mendel, 1866 Versuche über Pflanzen-Hybriden. Verhandlungen des naturforschenden Vereines in Brünn 4:3–47
- 2Amberger et al., 2019 OMIM.org: leveraging knowledge across phenotype-gene relationships. Nucleic Acids Research 47:D1038–D1043. doi.org/10.1093/nar/gky1151