Make and defend a claim, based on evidence, that meiosis produces genetic variation by forming new combinations of alleles in gametes (Louisiana Student Standards for Science, High School Biology, HS-LS3-2).

What this topic is asking

Louisiana's LS3 standards (HS-LS3-2) ask you to make and defend a claim that meiosis is a source of genetic variation. For LEAP 2025 Biology you should know that meiosis halves the chromosome number to make gametes, and that crossing over and independent assortment shuffle the alleles so that no two gametes are alike. Because this is an evidence-and-argument standard, the test often asks you to explain how a process creates variation, not just to name it.

Meiosis halves the chromosome number

Body (somatic) cells are diploid: humans have 46 chromosomes, arranged as 23 pairs. Meiosis reduces this to the haploid number, 23, in each gamete. The reason this matters is fertilization: when an egg (23) fuses with a sperm (23), the offspring receives 46 again. If meiosis did not halve the number first, the chromosome count would double every generation, which would be fatal.

Two divisions, four gametes

Meiosis involves two rounds of division (meiosis I and meiosis II) after a single DNA replication. The first division separates the matching pairs (so the number halves); the second separates the copies of each chromosome (like mitosis). The end result is four haploid cells from one diploid parent cell, in contrast to mitosis, which makes only two diploid cells.

How meiosis creates variation

A third source of variation comes after meiosis: random fertilization, the chance combination of one particular egg with one particular sperm. All three mechanisms (crossing over, independent assortment, random fertilization) make offspring genetically unique, which is the LS3 claim the standard asks you to defend.

Meiosis versus mitosis

It is worth holding the contrast clearly, because the test relies on it:

• Mitosis makes two cells, diploid (full number), genetically identical, used for growth and repair.
• Meiosis makes four cells, haploid (half number), genetically varied, used to make gametes.

So "two identical cells, full number" is mitosis; "four varied cells, half number" is meiosis.

Why variation matters

The variation meiosis creates is the raw material for natural selection. Because offspring differ from one another and from their parents, some will by chance carry traits better suited to the environment. This links the LS3 idea of variation directly to the LS4 idea of evolution: without genetic variation, populations could not adapt over time.

Try this

Q1. State how many chromosomes a gamete has compared with a body cell, and explain why this halving is necessary. [2]

• Cue. A gamete has half the number; halving is necessary so that fertilization (joining two gametes) restores the full number in the offspring instead of doubling it.

Q2. Name and briefly describe one way meiosis increases genetic variation. [2]

• Cue. Crossing over (matching chromosomes swap DNA sections, making new allele combinations) or independent assortment (chromosome pairs separate randomly, so gametes get different mixes).