LabBench Activity

Crossing Over

Each parent cell has pairs of homologous chromosomes, one homolog from the father and one from the mother. In meiosis, the maternal and paternal chromosomes can be shuffled into the daughter cells in many different combinations (in humans there are 223 possible combinations!). This ensures genetic variation in sexually reproducing organisms. Further genetic variation comes from crossing over, which may occur during prophase I of meiosis.

In prophase I of meiosis, the replicated homologous pair of chromosomes comes together in the process called synapsis, and sections of the chromosomes are exchanged. You can see that after crossing over, the resultant chromosomes are neither entirely maternal nor entirely paternal, but contain genes from both parents. Synapsis and crossing over occur only in meiosis.