Genetics & Heredity

DNA, genes, Punnett squares, and trait inheritance

Heredity is the passing of traits from parents to offspring through genes. Genes are segments of DNA that code for proteins determining traits. Each organism has two copies of each gene (alleles) — one from each parent. Dominant alleles (uppercase, A) mask recessive alleles (lowercase, a) when both are present. Genotype is the actual gene combination (AA, Aa, aa); phenotype is the visible trait. Punnett squares predict offspring genotype probabilities. An Aa × Aa cross gives 25% AA, 50% Aa, 25% aa — 3:1 ratio showing dominant trait. Some traits are sex-linked (on the X chromosome), giving different patterns in males vs females. Mutations are changes in DNA sequence that can alter traits and are the source of genetic variation for evolution.