How does meiosis halve the chromosome number and create genetic variation?
Describe meiosis as the division that produces gametes with half the chromosome number, and explain how crossing over, independent assortment, and fertilization create genetic variation (Virginia 2018 Biology SOL BIO.3.d, supporting BIO.5).
A SOL-level answer on meiosis for the Virginia Biology EOC: producing haploid gametes, the contrast with mitosis, and how crossing over, independent assortment, and fertilization generate genetic variation.
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What this topic is asking
Virginia Biology SOL standard BIO.3.d (and supporting BIO.5) asks how meiosis produces gametes and creates genetic variation. The Biology EOC expects you to know that meiosis halves the chromosome number to make eggs and sperm, to contrast it with mitosis, and to explain the sources of variation: crossing over, independent assortment, and the random combination of gametes at fertilization. Variation is the link to genetics and to evolution, so this topic connects across the whole exam.
What meiosis does
The purpose of halving the chromosome number is to keep it constant across generations. If gametes had the full number, fertilization would double the chromosome number every generation, which would be disastrous. By halving it in the gametes, fertilization restores the correct number when two gametes join.
Haploid and diploid: the chromosome numbers
This arithmetic is a common EOC item. If you are told the body-cell number, halve it for the gamete; if you are told the gamete number, double it for the body cell. The logic, halve in meiosis, restore at fertilization, is the key.
Meiosis compared with mitosis
The contrast is one of the most tested ideas in the course:
- Mitosis produces two cells, genetically identical, with the same chromosome number, for growth and repair.
- Meiosis produces four cells, genetically varied, with half the chromosome number, for sexual reproduction.
A quick way to keep them straight: mitosis is for the body (identical copies), meiosis is for reproduction (varied, halved gametes).
How meiosis creates variation
Sexual reproduction produces offspring that differ from their parents and from each other, and meiosis is the source. Three processes generate this genetic variation:
- Crossing over. Early in meiosis, homologous chromosomes pair up and exchange segments, producing new combinations of alleles on each chromosome.
- Independent assortment. The homologous pairs line up and separate randomly, so each gamete gets a random mix of maternal and paternal chromosomes.
- Random fertilization. Any one of millions of genetically different sperm can fertilize any egg, combining the genetic material of two parents in a new way.
Together these produce an enormous number of possible offspring, which is why siblings differ. This variation is essential for natural selection and the survival of a species.
Try this
Q1. State two differences between mitosis and meiosis. [2]
- Cue. Mitosis makes two identical cells with the same chromosome number; meiosis makes four varied cells with half the chromosome number (any two valid contrasts).
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, rather than doubling every generation.
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
VA Biology SOL (2023 released style)1 marksA human body cell has 46 chromosomes. How many chromosomes does a human gamete (egg or sperm) contain? (A) 92. (B) 46. (C) 23. (D) 12.Show worked answer →
A 1-point multiple-choice item on the outcome of meiosis.
The correct answer is C. Meiosis halves the chromosome number, so a human gamete contains 23 chromosomes. Fertilization then combines two gametes to restore the full 46. A would be a doubling, B is the body-cell number, and D is incorrect.
The test rewards knowing that gametes are haploid, with half the body-cell chromosome number.
VA Biology SOL (2024 released style)2 marksSexual reproduction produces offspring that are genetically varied. (a) Identify two processes during meiosis that create genetic variation. (b) State one further source of variation that occurs at fertilization.Show worked answer →
A 2-point item on the sources of genetic variation.
(a) 1 point: crossing over (exchange of segments between homologous chromosomes) and independent assortment (the random arrangement and separation of homologous pairs into gametes).
(b) 1 point: the random combination of two gametes at fertilization, so that any egg can be fertilized by any sperm, mixing the genetic material of two parents.
Markers reward two correct meiosis processes and the random fertilization point.
Related dot points
- Describe the cell cycle and mitosis as the process that produces two genetically identical cells for growth and repair, and relate uncontrolled cell division to cancer (Virginia 2018 Biology SOL BIO.3.c, BIO.3.d).
A SOL-level answer on the cell cycle for the Virginia Biology EOC: interphase and the stages of mitosis, why the two daughter cells are identical, the role of growth and repair, and how loss of control leads to cancer.
- Use alleles, genotype and phenotype, dominant and recessive, and Punnett squares to predict the genotype and phenotype ratios and probabilities of monohybrid crosses (Virginia 2018 Biology SOL BIO.5.b).
A SOL-level answer on inheritance for the Virginia Biology EOC: alleles, genotype and phenotype, dominant and recessive traits, and using Punnett squares to predict ratios and probabilities of monohybrid crosses.
- Explain that a mutation is a change in the DNA base sequence with harmful, beneficial, or neutral effects, and that genetic variation (from mutation and sexual reproduction) is important to the survival of a species (Virginia 2018 Biology SOL BIO.5.c).
A SOL-level answer on mutations for the Virginia Biology EOC: what a mutation is, its harmful, beneficial, or neutral effects, the difference between body-cell and gamete mutations, and why genetic variation matters for survival.
- Describe the structure of DNA (the antiparallel double helix and base pairing) and explain how complementary base pairing allows DNA to be replicated accurately (Virginia 2018 Biology SOL BIO.5.a).
A SOL-level answer on DNA for the Virginia Biology EOC: the double helix, base pairing, why DNA is a stable information store, and how complementary base pairing allows accurate replication.
- Explain how the role of variation and mutations drives natural selection, producing adaptation and changing the heritable traits of a population over generations (Virginia 2018 Biology SOL BIO.7.b).
A SOL-level answer on natural selection for the Virginia Biology EOC: variation and mutations as the raw material, overproduction and competition, differential survival and reproduction (fitness), and how selection produces adaptation and shifts allele frequencies, with antibiotic resistance as the worked example.
Sources & how we know this
- 2018 Science Standards of Learning (Biology) — Virginia Department of Education (2018)
- SOL Practice Items (All Subjects) — Virginia Department of Education (2024)