16.1.2—Homologous pairs of chromosomes

A homologous pair contains two chromosomes with the same genes in the same order at corresponding loci. They have the same characteristic shape, length and centromere position, but the allele at a locus may differ between the two homologues.

• In a diploid zygote, one chromosome of each homologous pair came from the female gamete and the other came from the male gamete.
• The homologues therefore carry corresponding genetic information, but they are not necessarily genetically identical because their alleles may differ.
• A locus is the position of a gene on a chromosome; the allele is the particular form found at that locus.

Matching genes in the same order allows homologous chromosomes to line up alongside one another during meiosis. This pairing relationship is different from sister chromatids: sister chromatids are replicated copies of one chromosome joined at a centromere, whereas homologues are the maternal and paternal members of a pair.

Homologous chromosomes are not the same as sister chromatids and do not have to carry the same alleles. This card stops at homologue identity and pairing; crossing over and independent assortment are covered separately.