The genes crossing over occurs during?

1. Analyze the question The question asks when crossing over of genes happens. Crossing over is the exchange of genetic material between chromosomes.

2. Explain the concept Crossing over is the exchange of segments between non-sister chromatids of homologous chromosomes. It happens in Prophase I of meiosis, when homologous chromosomes pair up (synapsis) to form bivalents. The points where the chromatids cross and exchange segments are called chiasmata (singular: chiasma).

The result is new combinations of alleles, called genetic recombination. This is one reason offspring from the same parents show variation.

3. Step-by-step solution

• A. Fertilization: This is the fusion of male and female gametes to form a zygote. No chromatid exchange takes place. ❌
• B. Mutation: This is a change in the DNA or chromosome structure. It is not the same as the normal exchange of segments between homologous chromosomes. ❌
• C. Mitosis: Homologous chromosomes do not pair up in mitosis, so there is no normal crossing over. The daughter cells are identical to the parent cell. ❌
• D. Meiosis: Homologous chromosomes pair in Prophase I and exchange segments. ✅

4. Answer The correct option is D. meiosis.

5. Exam tip Remember: "Meiosis = Mixing." Crossing over in meiosis mixes up genes and creates variation. Mitosis makes identical copies, so there is no mixing.

Also remember that crossing over occurs in Prophase I, and the structures seen are chiasmata. JAMB often asks about these.