homologous

Meaning: Homologous means sharing a common origin. In genetics, homologous chromosomes are the two chromosomes of a pair in a diploid cell, one maternal and one paternal. They match in length, centromere position and gene order, but they can carry different alleles of a gene. In evolution, homologous structures (a human arm, bat wing and whale flipper) share an ancestral origin and basic design, even when their functions differ. (In organic chemistry, a homologous series is a set of compounds differing by CH₂ units.)

On the MCAT: Meiosis passages ask where homologs act. They synapse in prophase I to form tetrads, where crossing over occurs. They separate in anaphase I, which halves the chromosome number (diploid to haploid). Sister chromatids separate in anaphase II. Nondisjunction questions ask which division failed. Evolution passages use homologous structures as evidence of divergent evolution from a common ancestor, and contrast them with analogous structures such as bird and insect wings, which have similar function but arose by convergent evolution.

Common confusion: Homologous chromosomes are not sister chromatids. Sister chromatids are identical copies made by DNA replication and joined at the centromere. Homologs come from different parents and may differ in alleles. Homologs are present in mitosis too, but they pair only in meiosis I. For structures, homologous means shared ancestry, not shared function.