Consider the phylogenetic trees below pertaining to three related species (A, B, C) that share a common ancestor (last common ancestor, or LCA). The lineage leading to species A diverges before the divergence of species B and C.

For gene X, no gene duplications have occurred in any lineage, and each gene X is derived from the ancestral gene X via speciation events. Are genes AX, BX, and CX orthologous, paralogous, or homologous? <>

Key Concepts

Orthologous Genes

Orthologous genes are genes in different species that evolved from a common ancestral gene through speciation. They typically retain the same function across species. In the context of the question, genes AX, BX, and CX are orthologous because they originated from the same ancestral gene X after the divergence of species A, B, and C.

Paralogous Genes

Paralogous genes are genes that arise from a duplication event within a genome, leading to multiple copies that may evolve new functions. Since the question specifies that no gene duplications have occurred in any lineage, the genes in question cannot be classified as paralogous.

Homologous Genes

Homologous genes refer to genes that share a common ancestry, which includes both orthologs and paralogs. In this case, while AX, BX, and CX are homologous because they derive from the same ancestral gene X, they are specifically orthologous due to their evolutionary relationship through speciation rather than duplication.