How does meiosis halve the chromosome number and create genetic variation in sexual reproduction?
Explain how meiosis produces gametes with half the chromosome number and generates genetic variation through crossing over and independent assortment, and how fertilization restores the chromosome number (NYSSLS LS3, patterns; cause and effect).
A NYSSLS-level answer on meiosis for the New York Life Science: Biology Regents: how meiosis halves the chromosome number to make gametes, how crossing over and independent assortment create variation, and how fertilization restores the chromosome number.
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What this topic is asking
NYSSLS LS3 wants you to explain how meiosis produces gametes (sex cells) with half the chromosome number, and how sexual reproduction creates genetic variation. On the Life Science: Biology Regents this is tested through chromosome-number reasoning and questions about where variation comes from. The crosscutting concepts are patterns (the halving and restoring) and cause and effect (variation leading to differences among offspring).
Halving the chromosome number
The reason for the halving is fertilization. When two gametes join, their chromosomes combine. If each gamete carried the full number, the offspring would have double, and the number would double again every generation. By halving the number in the gametes, meiosis ensures that fertilization restores exactly the correct number. This is a clean pattern: halve, then combine to restore.
Meiosis versus mitosis
It is worth keeping the contrast sharp, because the exam tests it:
- Mitosis makes two cells, genetically identical, with the same chromosome number (for growth and repair).
- Meiosis makes four cells (gametes), genetically varied, with half the chromosome number (for sexual reproduction).
How meiosis creates variation
Sexual reproduction produces offspring that differ genetically from their parents and from one another. Two events during meiosis are responsible:
- Crossing over. When chromosomes pair up, they may exchange matching segments, producing new combinations of alleles on each chromosome.
- Independent assortment. The pairs of chromosomes line up and separate randomly, so each gamete gets a different mixture of the chromosomes the individual inherited from each parent.
Fertilization restores the number
At fertilization, a sperm (23 chromosomes) fuses with an egg (23 chromosomes) to form a zygote with the full 46. The zygote then divides by mitosis to grow into the new organism, so every body cell again has 46. Because the sperm and egg came from two different individuals and were themselves varied, the offspring is genetically unique. This variation is why sexual reproduction matters for evolution (see natural selection and adaptation).
Try this
Q1. State how many chromosomes a human gamete contains and the type of division that produces it. [2]
- Cue. 23 chromosomes; produced by meiosis.
Q2. Explain why meiosis must halve the chromosome number. [2]
- Cue. So that when two gametes combine at fertilization, the full chromosome number is restored and does not double each generation.
Exam-style practice questions
Practice questions written in the style of NYSED exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Regents (Life Science sample, 2024)3 marksIn humans, body cells contain 46 chromosomes. (a) State the number of chromosomes in a human gamete (egg or sperm). (b) Name the type of cell division that produces gametes. (c) Explain why it is important that gametes have half the normal chromosome number.Show worked answer →
A 3-point constructed-response item assessing patterns and cause and effect.
(a) 1 point: 23 chromosomes.
(b) 1 point: meiosis.
(c) 1 point: at fertilization two gametes join, so each must carry half the number (23) for the offspring to have the full, correct number (46); otherwise the number would double each generation.
Markers reward "23, meiosis" and the reason that halving keeps the chromosome number constant across generations.
Regents (Life Science CR, 2025)2 marksOffspring produced by sexual reproduction are genetically varied. (a) Identify one process during meiosis that produces genetic variation. (b) Explain how fertilization adds further variation.Show worked answer →
A 2-point item on the sources of genetic variation.
(a) 1 point: either crossing over (exchange of segments between paired chromosomes) or independent assortment (the random way chromosome pairs line up and separate). Either is acceptable.
(b) 1 point: fertilization randomly combines one gamete from each parent, so the offspring receives a new, unique mix of alleles from two different individuals.
Markers reward naming a meiosis source of variation and the random combination of gametes at fertilization.
Related dot points
- Explain how mitosis and the cell cycle produce two genetically identical cells, describe its role in growth, repair and asexual reproduction, and explain how uncontrolled division leads to cancer (NYSSLS LS1 and LS3, stability and change; cause and effect).
A NYSSLS-level answer on mitosis for the New York Life Science: Biology Regents: the cell cycle, how mitosis produces two identical cells, its role in growth, repair and asexual reproduction, and what happens when division is not controlled.
- Use the rules of inheritance (dominant and recessive alleles, genotype and phenotype) and Punnett squares to predict the outcomes of genetic crosses, and interpret pedigrees (NYSSLS LS3, patterns; using mathematics).
A NYSSLS-level answer on inheritance for the New York Life Science: Biology Regents: alleles, genotype and phenotype, dominant and recessive traits, using Punnett squares to predict ratios and probabilities, and reading pedigrees.
- Describe the structure of DNA (the antiparallel double helix and base pairing) and explain how complementary base pairing allows DNA to be copied accurately during replication (NYSSLS LS3, structure and function; patterns).
A NYSSLS-level answer on DNA for the New York Life Science: Biology Regents: the double-helix structure, base pairing, why DNA is a stable store of information, and how complementary base pairing allows accurate replication.
- Compare sexual and asexual reproduction, explain fertilization and early development from zygote to embryo, and describe the role of reproductive structures in humans (NYSSLS LS1 and LS3, patterns; systems and system models).
A NYSSLS-level answer on reproduction for the New York Life Science: Biology Regents: sexual versus asexual reproduction, fertilization and the zygote, early development into an embryo, and the role of human reproductive structures.
- Explain how variation, overproduction, competition and differential survival lead to natural selection, and how this changes the proportion of traits in a population over time (NYSSLS LS4, cause and effect; patterns).
A NYSSLS-level answer on natural selection for the New York Life Science: Biology Regents: how variation, overproduction, competition and differential survival drive evolution, with the Beaks of Finches investigation and worked examples.
Sources & how we know this
- New York State P-12 Science Learning Standards (Life Science) — New York State Education Department (2016)
- Educator Guide to the Regents Examination in Life Science: Biology — New York State Education Department (2025)