Making comparisons between samples in a DNA database and identifying any identical chromosomal segments shared between two samples is an essential step in relative finding. Consider that the data obtained from a SNP analysis are individual snapshots of genotypes at positions along each chromosome pair organized in relation to the reference human genome sequence. Each site is individually determined as having the same DNA basepair on both chromosomes (i.e., homozygous), or as having different DNA basepairs (i.e., different alleles) on the two chromosomes (i.e., heterozygous). Each site is reported as one representative base of each basepair, thus possible genotypes at a site known to have variation for an A-T basepair versus an G-C basepair are AA, GG, or AG. In the case of a heterozygous site, it is not known which member of a chromosome pair contains one or the other allele.
Comparing the genotypes of two individuals, the genotypes