Inheritance and variation

Genes and alleles

A gene is a section of DNA that codes for a characteristic. Each gene comes in versions called alleles, and you inherit one from each parent. A dominant allele shows whenever it is present; a recessive allele only shows when both alleles are recessive.

Genotype and phenotype

Genotype is the pair of alleles ($BB$, $Bb$, $bb$); phenotype is what you actually see. $BB$ and $Bb$ look the same, which is why two brown-eyed parents can have a blue-eyed child.

Inherited or environmental

Eye colour and blood group are inherited. A scar or a suntan is environmental. Height and mass are both: genes set a range, and diet decides where within it you land.