Mutations are changes to the DNA base sequence that result in changes to the amino-acid sequence of proteins.This may lead to different phenotypes. If a mutation occurs in the gametes it may be passed to the offspring.As a result of mutations, there are different versions of the same genes, called allelesDuring fertilisation one allele is inherited from each parent. The combination of alleles (the genotype) determines the phenotype or observable characteristics of the individual and the environment modifies these characteristics.Dominant alleles are always expressed even when the individual has different alleles (heterozygous) whereas a recessive allele is only expressed if there are two copies (homozygous). 