O1 GENETIC VARIATION Key Notes Genes are pieces of DNA which contain the genetic code Genes and alleles necessary to produce a specific protein. A gene can have many alleles and individuals can have a homozy gous or heterozygous genotype. The phenotype expressed depends on whether alleles are dominant. recessive or codominant The total set of genes and alleles present in a population known as the gene poolGenes are pieces of DNA which contain the genetic code necessary to produce a specific protein. A gene can have many alleles and individuals can have a homozygous or heterozygous genotype. The phenotype expressed depends on whether alleles are dominant, recessive or codominant. The total set of genes and alleles present in a population is known as the gene pool. Key Notes Genes and alleles O1 GENETIC VARIATION