NIJ Training Course for Forensic DNA Officers of the Court: Genotypes

The two chromosomes of any pair have the same  gene mapping and, in the case of coding regions, work together to determine how that  gene will manifest itself. Consider for example the gene for hair color; there are  two versions or alleles of any gene, in this case one allele tells the body to create brown hair  and one allele is programmed to cause blonde hair.  Each of the two chromosomes will possess its own  allele of the gene both found at the same locus one chromosome can carry one allele and  the other chromosome can carry the other or they can both carry the same allele.  The combination of alleles that is displayed by a pair of chromosomes at a  particular locus is called the genotype.