Haploid

For organisms, such as humans, which carry two copies of each chromosome in somatic (nonreproductive) cells, a haploid cell is one that has only one of each chromosome. The haploid number, N, is the number of chromosomes in a haploid cell. For humans, with 23 pairs of chromosomes in somatic cells, germ cells (sperm and eggs) are haploid cells with 23 unpaired chromosomes each.

The haploid origin of a chromosome is used by scientists to match samples of DNA to individuals in a variety of settings.

Given that a normal human typically has two alleles of each gene (one on each chromosome), it follows that at any given locus only one of the two alleles is represented in a given germ cell. Thus, we might imagine that each locus would determine two haploid germ cells. As reproduction requires the combination of germ cells from two different individuals, human reproduction represents the opportunity for up to four distinct combinations of haploid germ cells.

Once the particular combination of haploid cells is determined during fertilization, though, the maternal or paternal haploid origin of each chromosome will not, except in the case of a select few diseases such as Angelman and Prader-Willi syn-dromes—the uniparental disomies—be significant in the expression of genes. The relationship between particular loci and their haploid origin is not entirely without value, however. If individual chromosomes, or portions thereof, are sequenced, a genotype for a DNA sequence that came from a single parent can be identified: a haplotype. Shared haplotypes are indicative of relatedness. By comparing haplotypes of sufficient length or haplotypes that are sufficiently rare, it is possible to determine if two individuals share a common ancestor.