The exchange of genes between homologous chromosomes is called

1. Analyze the question The question asks for the term for the exchange of genes (genetic material) between homologous chromosomes. The key phrases are "exchange of genes" and "homologous chromosomes."

2. Explain the concept During Prophase I of meiosis, homologous chromosomes (one from each parent, carrying the same genes in the same order) pair up in a process called synapsis. At this stage, non-sister chromatids of the homologous pair can break and swap segments. This exchange is called crossing over. The points where the chromatids cross are called chiasmata.

Crossing over produces new combinations of alleles, called genetic recombination, which is a major source of variation in offspring.

3. Evaluate each option

• A. Crossing over: Correct. It is the exchange of genetic material between non-sister chromatids of homologous chromosomes.
• B. Back cross: A breeding method where an offspring is crossed with one of its parents (or a genetically identical organism). It has nothing to do with chromosome events.
• C. Test cross: A cross between an individual showing the dominant trait and a homozygous recessive individual, used to find out if the dominant one is homozygous or heterozygous. It is also a breeding method, not a chromosome event.
• D. Mutation: A sudden change in the structure or amount of genetic material. It creates new alleles, rather than exchanging existing genes between homologous chromosomes.

4. Answer The correct option is A. crossing over.

5. Exam tip Remember: "Crossing over = Chromosomes Cross." Whenever you see "homologous chromosomes" together with "exchange," think crossing over in Prophase I of meiosis. Also, "cross" in back cross and test cross refers to crossing organisms, not chromosomes. Don't let the similar word confuse you!