noun, singular: homologous chromosome
In a cell, the chromosomes are of two sets: one of which is derived from the mother and the other is from the father. The chromosomes that have the same gene sequences are paired during meiosis. This pairing (called synapsis) happens between chromosomes that are homologous, i.e. chromosomes having the same genes at the same loci but possibly different alleles.
For example, two chromosomes have genes coding for eye color: one may code for brown eyes, the other for blue. One of them may be dominant and the other recessive. Hence, even if an individual has genes for brown eyes and blue eyes, only one eye color is expressed.
A human cell contains 23 pairs of homologous chromosomes: 22 of them are homologous non-sex chromosomes (or autosomes) and 1 homologous pair of sex chromosomes. In females, the homologous sex chromosomes are 2 X’s; in males the X and Y chromosomes.