Homologous chromosomes

Homologous chromosomes are pairs of chromosomes that are the same length and have the same centromere position. They carry genes for the same characteristics, situated at the same loci (positions) on each chromosome, although the alleles of these genes may either be identical or different on each chromosome (homozygosity or heterozygosity, respectively).

Humans have 46 chromosomes, arranged into 23 homologous pairs. In each pair, one chromosome is donated from the mother, while the other is donated from the father. Of these 23 pairs, 22 contain the non-sex chromosomes (autosomes) and the 23rd pair are the sex chromosomes. In the case of human males, with the sexual genotype XY, the X and Y chromosomes are not of similar length (the Y chromosome being much smaller) and there are fewer regions of homology than in normal homologous chromosomes.

Homologous chromosomes pair together (synapse) to form a bivalent during Prophase I of meiosis, in the formation of gametes. During Metaphase I of meiosis, each member of each pair is independently assorted into gametes. This transforms diploid somatic cells, carrying two full sets of chromosomes, into haploid gametes, carrying only one full set (see also: ploidy).

The independent assortment of homologous chromosomes means that the orientation of one bivalent (maternal vs paternal) on the metaphase plate before chromosome separation bears no effect on the orientation of every other bivalent. This means that, in theory, a gamete could contain exclusively maternal chromosomes, exclusively paternal chromosomes, or any mixture in between.

The complete set of maternal, or paternal, chromosomes in a cell is called a non-homologous set. The number of chromosomes in one of these complete sets is the monoploid number (x) of that organism.