Skip to main content
North CarolinaBiologySyllabus dot point

How does meiosis produce gametes and create genetic variation?

Explain how meiosis produces gametes with half the chromosome number and generates genetic variation (North Carolina Standard Course of Study, Biology, LS.Bio.6).

A standard-level answer on meiosis for the North Carolina Biology EOC: how meiosis halves the chromosome number, the role of crossing over and independent assortment, and why sexual reproduction creates variation.

Generated by Claude Opus 4.813 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

Jump to a section
  1. What this topic is asking
  2. What meiosis does
  3. Why halving matters: fertilization
  4. Meiosis versus mitosis
  5. How meiosis creates variation
  6. Try this

What this topic is asking

North Carolina LS.Bio.6 asks how meiosis produces gametes with half the chromosome number and generates variation. For the Biology EOC you need to know that meiosis makes four cells with half the chromosomes, how fertilization restores the full number, and the three sources of variation (crossing over, independent assortment, and the random combination of gametes). Meiosis is the contrast partner for mitosis.

What meiosis does

Body cells have chromosomes in pairs (one of each pair from each parent); this is the diploid number, written 2n2n. Meiosis separates the pairs, so each gamete ends up with only one of each pair, the haploid number, written nn. In humans, a body cell has 46 chromosomes (23 pairs), so each gamete has 23. The EOC commonly asks you to halve a given chromosome number.

Why halving matters: fertilization

Meiosis versus mitosis

It helps to set meiosis against mitosis, which you met in the molecular genetics module.

Feature Mitosis Meiosis
Number of daughter cells 2 4
Chromosome number Full (diploid) Half (haploid)
Genetically Identical to parent Different from parent and each other
Purpose Growth, repair, replacing cells Making gametes for sexual reproduction

The two key differences the EOC tests are the number of cells (two versus four) and the chromosome number (full versus half), plus the fact that meiosis produces variation while mitosis produces identical cells.

How meiosis creates variation

Sexual reproduction is a major source of variation, and meiosis is where most of it arises. There are three sources:

  • Crossing over. Early in meiosis, homologous chromosomes (the matching pairs) line up and exchange segments, mixing the alleles between them. This produces new combinations of alleles on a chromosome.
  • Independent assortment. When the chromosome pairs line up to be separated, the orientation of each pair is random and independent of the others, so gametes get a random mix of the parents' chromosomes.
  • Random fertilization. Which sperm fertilizes which egg is also random, so the combination of two already-varied gametes adds yet more variation.

Together these make each gamete, and each offspring, genetically unique (except identical twins). This variation is exactly what natural selection acts on, linking meiosis to evolution.

Try this

Q1. A body cell has 20 chromosomes. State the number in its gametes and explain why. [2]

  • Cue. 10 chromosomes, because meiosis halves the number so that fertilization can restore the full 20.

Q2. Name two processes during meiosis that increase genetic variation. [2]

  • Cue. Crossing over and independent assortment (random fertilization also adds variation).

Exam-style practice questions

Practice questions written in the style of NCDPI exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

NC Biology EOC (style)1 marksA body cell of an organism has 46 chromosomes. How many chromosomes will its gametes have after meiosis? (A) 92. (B) 46. (C) 23. (D) 12.
Show worked answer →

A 1-point item on the chromosome number after meiosis.

The correct answer is C. Meiosis halves the chromosome number, so gametes from a cell with 46 chromosomes have 23. This haploid number lets fertilization restore 46 in the offspring. A is doubling, B is no change, and D is unrelated.

Meiosis halves the chromosome number; fertilization restores it.

NC Biology EOC (style)2 marksMeiosis and sexual reproduction create variation in offspring. (a) Name two processes during meiosis that create variation. (b) Explain why variation is important for a population.
Show worked answer →

A 2-point item on the sources and value of variation.

(a) 1 point: crossing over (exchange of segments between homologous chromosomes) and independent assortment (random arrangement of chromosomes into gametes); the random combination of gametes at fertilization also adds variation.
(b) 1 point: variation means individuals differ, so some are more likely to survive environmental change, which is the raw material for natural selection.

Markers reward two correct sources of variation and a link to survival and natural selection.

Related dot points

Sources & how we know this