How does meiosis make gametes and create genetic variation?
Use a model to explain how meiosis halves the chromosome number to make gametes and creates genetic variation through crossing over and independent assortment (Ohio's Learning Standards for Science, Biology, B.H.2).
A standard-level answer on meiosis for Ohio's Biology EOC: how meiosis halves the chromosome number to make gametes, how it differs from mitosis, and how crossing over, independent assortment, and random fertilization create variation.
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What this topic is asking
Ohio standard B.H.2 states that "in sexual reproduction, offspring receive some genetic information from each parent." Ohio's Biology EOC turns this into items on meiosis: the special cell division that halves the chromosome number to make gametes (sex cells), and that creates genetic variation. The crosscutting idea is cause and effect: meiosis is the cause of the variation that makes every offspring genetically unique. The EOC loves to contrast meiosis with mitosis, so know both side by side.
Why halving is necessary
Body cells are diploid (2n): they carry chromosomes in matching pairs, one from each parent (the homologous chromosomes from chromosomes, genes, and alleles). If gametes were also diploid, then joining two of them at fertilization would double the chromosome number every generation, which would quickly become unworkable.
Meiosis solves this by halving the number. Gametes are haploid (n): they carry only one chromosome from each pair. When a haploid sperm () fertilizes a haploid egg (), the offspring is diploid () again. So meiosis followed by fertilization keeps the chromosome number constant across generations.
Meiosis versus mitosis
This comparison is one of the most reliable EOC items. Learn it as a table.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Cells produced | 2 | 4 |
| Chromosome number | Same as parent (diploid) | Half of parent (haploid) |
| Genetically | Identical to parent | Different from parent and each other |
| Purpose | Growth, maintenance, repair | Making gametes for sexual reproduction |
| Variation created | None | Yes (crossing over, independent assortment) |
The shorthand the EOC rewards: "identical, same number, growth and repair" means mitosis; "different, half the number, reproduction" means meiosis.
How meiosis creates variation
Sexual reproduction produces offspring that differ from their parents and from each other, and meiosis is the source of most of this genetic variation. There are three mechanisms.
- Crossing over. Early in meiosis, homologous chromosomes pair up and swap matching segments. This shuffles alleles between the chromosome that came from the mother and the one that came from the father, creating new combinations.
- Independent assortment. When the homologous pairs line up to be separated, each pair orients randomly and independently of the others, so the mix of maternal and paternal chromosomes in each gamete is random.
- Random fertilization. Any one of millions of genetically different sperm can fertilize any one of the genetically different eggs, multiplying the variation again.
Why variation matters
The genetic variation that meiosis creates is the raw material for evolution. Because offspring differ, some will by chance be better suited to the environment, and natural selection can favor them. This connects meiosis directly to natural selection and adaptation: without the variation from meiosis (and mutation), there would be nothing for selection to act on.
Try this
Q1. State two ways meiosis differs from mitosis. [2]
- Cue. Meiosis makes four cells (mitosis makes two); meiosis halves the chromosome number and makes genetically different haploid cells (mitosis keeps the number the same and makes identical diploid cells).
Q2. Name two processes in meiosis that create genetic variation. [2]
- Cue. Crossing over (homologous chromosomes swap segments) and independent assortment (homologous pairs separate randomly).
Exam-style practice questions
Practice questions written in the style of ODEW exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Ohio Biology EOC (style)1 marksA body cell of an organism has 24 chromosomes. How many chromosomes are in a gamete produced by meiosis? (A) 48. (B) 24. (C) 12. (D) 6.Show worked answer →
A 1-point item on the outcome of meiosis.
The correct answer is C. Meiosis halves the chromosome number, so a diploid body cell with 24 chromosomes produces haploid gametes with 12. Fertilization then combines two gametes to restore 24 in the offspring. 48 would mean doubling, and 6 would mean halving twice.
The rule the EOC tests: meiosis takes one diploid (2n) cell and makes four haploid (n) cells.
Ohio Biology EOC (style)3 marksCompare mitosis and meiosis. (a) State the number and type of cells each produces. (b) State one source of genetic variation that occurs in meiosis but not mitosis.Show worked answer →
A 3-point compare item.
(a) 2 points: mitosis produces two genetically identical diploid cells with the full chromosome number (1 point); meiosis produces four genetically different haploid cells with half the chromosome number (1 point).
(b) 1 point: any one of crossing over (homologous chromosomes swap segments) or independent assortment (homologous pairs separate randomly). Either earns the mark.
Related dot points
- Use Punnett squares and the laws of segregation and dominance to predict the genotypes and phenotypes of offspring from a monohybrid cross (Ohio's Learning Standards for Science, Biology, B.H.2).
A standard-level answer on Mendelian genetics for Ohio's Biology EOC: dominant and recessive alleles, Mendel's law of segregation, how to set up and read a Punnett square, and how to work out genotype and phenotype ratios.
- Distinguish patterns of inheritance beyond simple dominance, including incomplete dominance, codominance, multiple alleles, and polygenic traits (Ohio's Learning Standards for Science, Biology, B.H.2 and B.H.3).
A standard-level answer on inheritance patterns for Ohio's Biology EOC: incomplete dominance, codominance, multiple alleles (ABO blood type), and polygenic traits, with how each differs from simple Mendelian dominance.
- Use a model of the cell cycle to explain how cell division and differentiation support growth, maintenance, and repair, and how a loss of control leads to cancer (Ohio's Learning Standards for Science, Biology, B.C.1).
A standard-level answer on the cell cycle and mitosis for Ohio's Biology EOC: interphase and the phases of mitosis (PMAT), how mitosis supports growth and repair, cell differentiation, and how a mutation in cell-cycle genes leads to cancer.
- Explain that genes are segments of DNA located on chromosomes, and distinguish between genes, alleles, genotype, and phenotype (Ohio's Learning Standards for Science, Biology, B.H.1).
A standard-level answer on chromosomes, genes, and alleles for Ohio's Biology EOC: how DNA is packaged into chromosomes, the difference between a gene and an allele, homologous chromosomes, and the meaning of genotype and phenotype.
- Explain how mutations change the DNA sequence and therefore proteins and traits, and how they can be harmful, neutral, or beneficial (Ohio's Learning Standards for Science, Biology, B.H.4 and B.H.5).
A standard-level answer on mutations for Ohio's Biology EOC: what a mutation is, the main types (substitution, insertion, deletion), how a changed base can change a protein, mutagens, and why mutations can be harmful, neutral, or beneficial.
Sources & how we know this
- Ohio's Learning Standards and Model Curriculum for Science — Ohio Department of Education and Workforce (2022)
- Biology State-Tested Course Resources — Ohio Department of Education and Workforce (2024)