Question

How do meiosis I and II contribute to genetic variation? Use the diagram to explain your answer.

Answer 1

Meiosis increases genetic variation in two main ways. Referring to the typical diagram that shows homologous chromosome pairs (one maternal, one paternal) forming tetrads, undergoing crossing over, and then being segregated into four cells, you can see both mechanisms in action:

1. Crossing over in prophase I
- Homologous chromosomes pair up and exchange DNA at chiasmata.
- The diagram’s X-shaped crossing points show segments of maternal and paternal chromatids swapped.
- This produces recombinant chromosomes that carry new combinations of alleles along a chromosome, so sister chromatids are no longer identical.

2. Independent assortment in metaphase I → anaphase I
- The diagram usually shows different possible orientations of homologous pairs on the metaphase plate. Each pair lines up independently of the others.
- When homologs are pulled to opposite poles in anaphase I, you get different mixes of maternal and paternal chromosomes in the daughter cells. With n chromosome pairs, independent assortment alone can produce 2^n different gamete chromosome combinations.

3. Meiosis II (separation of sister chromatids)
- Meiosis II resembles mitosis but acts on the already reshuffled chromosomes. The diagram will show sister chromatids (now possibly recombined) aligned and then separated into four nuclei.
- Because crossing over made sister chromatids genetically different, their separation yields distinct haploid gametes rather than identical copies.

Putting it together: prophase I crossing over creates new allele combinations on chromosomes, metaphase I independent assortment shuffles whole chromosomes between daughter cells, and meiosis II separates the (now nonidentical) sister chromatids to produce four genetically distinct haploid gametes. Random fertilization further increases variation by combining different gametes.