Heredity: Inheritance and Variation of Traits
What you must know
Heredity explains how traits pass from parents to offspring. Genes are segments of DNA on chromosomes; each parent contributes one allele for many traits. Distinguish genotype (the allele combination) from phenotype (the observable trait). Use dominant and recessive alleles and Punnett squares to predict offspring ratios.
Understand that asexual reproduction produces offspring genetically identical to the parent (barring mutations), while sexual reproduction mixes genetic material and produces variation. Variation is the raw material for natural selection: individuals with helpful traits survive and reproduce more, and those traits become more common over generations.
Know that mutations are changes in DNA; most are neutral or harmful, a few are helpful, and they are a source of new variation. Recognize that environmental factors can influence how traits are expressed, but inherited traits come from genes.
Traps to watch
- Dominant does not mean "more common"; it means expressed when present.
- A recessive trait can appear if both parents carry the recessive allele, even if neither shows it.
- Genotype and phenotype are not interchangeable.
- Sexual reproduction does not "create" traits on demand; it reshuffles existing alleles.
Rule of thumb
For a cross, write each parent's alleles, fill the square, then count phenotypes. Heterozygous × heterozygous gives a 3:1 phenotype ratio for a simple dominant trait.
Worked example
Cross Bb × Bb. Offspring are BB, Bb, Bb, bb - three show the dominant trait, one shows the recessive trait. The recessive phenotype appears even though both parents show the dominant trait.