Explain how meiosis produces gametes with half the chromosome number and how meiosis and fertilization, together with mutation, create genetic variation among offspring (MA STE HS-LS3-2, HS-LS3-3).

What this topic is asking

The Massachusetts STE framework (HS-LS3-2 and HS-LS3-3) asks you to explain how sexual reproduction creates genetic variation, which is the raw material that natural selection acts on. On the High School Biology MCAS, meiosis is tested by contrast with mitosis, by reasoning about chromosome number, and by explaining the sources of variation. The crosscutting concept is cause and effect: meiosis and fertilization cause the variation seen among offspring.

What meiosis does

Unlike mitosis, which makes two identical body cells, meiosis makes four cells that are not identical and that each carry only half the chromosomes. In humans, a body cell has 46 chromosomes (diploid), and a gamete has 23 (haploid). Meiosis happens only in the reproductive organs, to make sperm and eggs.

Why gametes have half the chromosomes

This is the point the MCAS checks most. If a gamete kept the full 46 chromosomes, then at fertilization, when a sperm joins an egg, the offspring would have 92, and the number would double every generation. Halving the chromosome number in gametes prevents this:

• Each gamete carries 23 chromosomes (half).
• At fertilization, sperm (23) plus egg (23) gives 46, the full number.
• So the chromosome number stays constant from one generation to the next.

This balance between halving (meiosis) and rejoining (fertilization) is what keeps sexual reproduction stable. Meiosis sets up the alleles that are then tracked in patterns of inheritance.

How sexual reproduction creates variation

The MCAS wants you to name the sources of genetic variation, and there are three:

1. Shuffling and exchange during meiosis. As gametes form, the chromosomes are randomly sorted (independent assortment) and can swap sections (crossing over). This means each gamete receives a different combination of genes, so no two gametes from the same person are identical.
2. Random fertilization. Any sperm can fertilize any egg, so the offspring receives a new, unpredictable mix of genes from two parents.
3. Mutation. A change in the DNA base sequence creates a new version of a gene (a new allele). Mutation is the ultimate source of brand-new variation, while meiosis and fertilization shuffle existing variation. Mutation is covered in mutations and biotechnology.

Together, these sources explain why offspring of the same parents differ from one another and from their parents. This variation is essential for natural selection.

Try this

Q1. State how many cells meiosis produces and how their chromosome number compares with the parent cell. [2]

• Cue. Four cells, each with half the number of chromosomes of the parent cell.

Q2. Explain why it is important that gametes have half the chromosome number. [2]

• Cue. So that fertilization restores the full number, keeping the chromosome number constant from one generation to the next.