Use a model of meiosis to explain how sexual reproduction halves the chromosome number and creates genetic variation through crossing over, independent assortment, and random fertilization (Tennessee Academic Standards for Science, Biology I, BIO1.LS3).

What this topic is asking

The Tennessee LS3 standards (heredity) ask you to model how meiosis produces gametes and why sexual reproduction creates genetic variation. For the Biology I EOC that means knowing that meiosis halves the chromosome number to make haploid gametes, how that contrasts with mitosis, and the three sources of variation: crossing over, independent assortment, and random fertilization. Because the standards are three-dimensional, items often ask you to explain why variation matters, linking this topic forward to inheritance and evolution.

Why halve the chromosome number?

So the logic is a cycle: meiosis halves, fertilization restores. An EOC item that gives a body-cell chromosome number is usually testing whether you can halve it for the gamete or double it for the fertilized egg.

Meiosis versus mitosis

The two divisions are easy to confuse, so the EOC tests the contrast directly.

• Mitosis makes two cells, each diploid ($2n$) and genetically identical to the parent. It is used for growth and repair.
• Meiosis makes four cells, each haploid ($n$) and genetically different. It is used to make gametes for sexual reproduction.

Meiosis involves two rounds of division (meiosis I and meiosis II) after a single DNA replication, which is why it produces four cells and halves the number. The headline difference to remember: mitosis gives identical diploid cells; meiosis gives variable haploid cells.

The three sources of genetic variation

Sexual reproduction is powerful because it shuffles genes. Three mechanisms create the variation:

• Crossing over. Early in meiosis, homologous chromosomes (the matching pair, one from each parent) pair up and exchange segments. This creates new combinations of alleles on a single chromosome that did not exist in either parent.
• Independent assortment. When the homologous pairs line up before separating, each pair orients randomly, independently of the others. So a gamete can receive any mix of maternal and paternal chromosomes. With $23$ pairs, this alone gives over eight million combinations ($2^{23}$).
• Random fertilization. Which particular sperm fertilizes which particular egg is a matter of chance, multiplying the variation again.

Crossing over and independent assortment happen during meiosis; random fertilization happens at fertilization. Together they explain why siblings (other than identical twins) are genetically different.

Why variation matters

The reason the standard cares about variation is its role in evolution and survival. A population with more genetic variation has a better chance that some individuals carry traits suited to a changing environment, so variation is the raw material on which natural selection acts (a link forward to the LS4 standards). On the EOC, "increases genetic variation" is the expected reason for naming these mechanisms.

Try this

Q1. A plant has $20$ chromosomes in its body cells. State the number of chromosomes in (a) one of its gametes and (b) a cell produced by mitosis. [2]

• Cue. (a) $10$ (meiosis halves the number); (b) $20$ (mitosis keeps the number the same).

Q2. Explain why meiosis is necessary for sexual reproduction. [2]

• Cue. It halves the chromosome number to make haploid gametes, so that fertilization restores the diploid number rather than doubling it each generation, and it creates genetic variation.