Topic 5.2 Meiosis and Genetic Diversity: explain how crossing over, independent assortment and random fertilization produce genetic variation.

What this topic is asking

The College Board (Topic 5.2) wants you to explain how meiosis and fertilization generate genetic variation, through three processes: crossing over, independent assortment, and random fertilization. You should be able to describe each and calculate the number of possible gamete combinations.

Crossing over

Because the exchange points vary, crossing over makes a huge number of unique chromosome combinations possible.

Independent assortment

For humans ($n = 23$), independent assortment alone produces $2^{23}$ (over 8 million) chromosome combinations, before crossing over is even considered.

Random fertilization

This is why sexual reproduction generates so much more variation than asexual reproduction, which simply copies the parent. The variation produced by these three processes is the raw material that natural selection acts on, linking meiosis directly to the evolution topics in Unit 7.

Try this

Q1. Identify the stage at which independent assortment occurs and state its effect. [2 points]

• Cue. Metaphase I; homologous pairs orient independently, randomly mixing maternal and paternal chromosomes into gametes.

Q2. Explain why genetic variation is important for a population. [2 points]

• Cue. Variation provides the differences in heritable traits that natural selection acts on, so a varied population is more likely to contain individuals that survive environmental change.