Genes occupy permanently fixed positions in chromosomes and cannot move around the genome.

Classical genetics treated chromosomes like ordered maps. Genes occupied particular positions, or loci, along a chromosome. Crossing over could reshuffle which chromosome segments were inherited together, but the positions themselves were expected to remain stable.

Barbara McClintock found an exception while studying maize in the 1940s.

Some genetic elements appeared to move to new chromosome positions. When they inserted near or inside other genes, they could change whether those genes worked, producing shifting patterns of color in maize kernels.

McClintock called them controlling elements. The idea was difficult to fit into the prevailing picture of a stable chromosome map and received limited acceptance for years.

Later research showed that mobile DNA is widespread. These sequences are now called transposable elements. Some can move directly from one DNA location to another. Others, called retrotransposons, are first copied into RNA and then converted back into DNA before a new copy inserts elsewhere.

Most ordinary genes do not constantly jump around the genome. The important correction is that chromosome positions are not universally fixed: genomes contain mobile sequences that can generate mutations, alter gene regulation, and help reshape genomes over evolutionary time.