Why must gametes have half the usual number of chromosomes, and how does meiosis create variation?
Explain the role of meiosis in producing gametes with half the chromosome number and in generating genetic variation, and contrast meiosis with mitosis (TEKS Biology, Reporting Category 2; patterns; cause and effect).
A TEKS-level answer on meiosis for the Texas STAAR Biology EOC: chromosomes and the role of meiosis in halving the chromosome number, how crossing over and independent assortment create variation, and how meiosis differs from mitosis.
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What this topic is asking
The Biology TEKS ask you to explain the role of meiosis in forming gametes and in creating variation, and to contrast it with mitosis. For STAAR Reporting Category 2 you need to know that meiosis halves the chromosome number, why that matters for fertilization, and how meiosis generates variation through crossing over and independent assortment. This is a patterns and cause-and-effect topic, and keeping meiosis and mitosis straight is essential.
Chromosomes and chromosome number
Why meiosis halves the number
So in humans, a sperm (23) and an egg (23) join to form a fertilized cell with 46, the correct full number. The cause-and-effect logic here, halving so that combining restores the correct number, is a frequent short-constructed-response prompt.
How meiosis creates variation
Meiosis does more than halve the number; it shuffles the genes so each gamete is genetically unique:
- Crossing over. During meiosis, paired chromosomes exchange matching segments, creating new combinations of alleles on a chromosome.
- Independent assortment. The chromosome pairs line up and separate randomly, so gametes get different mixes of the parents' chromosomes.
Together with the random combination of gametes at fertilization, these produce the genetic variation seen among offspring of the same parents. This variation is the raw material that natural selection acts on.
Meiosis versus mitosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Cells produced | Two | Four |
| Chromosome number | Same as parent (diploid) | Half of parent (haploid) |
| Genetic result | Identical to parent | Genetically varied |
| Purpose | Growth, repair, asexual reproduction | Forming gametes for sexual reproduction |
Mitosis is covered in the context of the cell cycle and growth; meiosis is the division that makes sexual reproduction and variation possible. Mixing them up is one of the most common errors on this topic.
Try this
Q1. State the number of cells produced by meiosis and their chromosome number compared with the parent cell. [2]
- Cue. Four cells, each with half the chromosome number of the parent (haploid).
Q2. Name two processes during meiosis that create genetic variation. [2]
- Cue. Crossing over and independent assortment.
Exam-style practice questions
Practice questions written in the style of TEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
STAAR Biology (2023 released style)1 marksA human body cell contains 46 chromosomes. How many chromosomes are in a human gamete, and which process produces it? (A) 46, by mitosis. (B) 23, by meiosis. (C) 92, by meiosis. (D) 23, by mitosis.Show worked answer →
A 1-point multiple-choice item on chromosome number.
The correct answer is B. Meiosis halves the chromosome number, so a human gamete has 23 chromosomes. Fertilization then combines two gametes to restore the full 46. Mitosis (A and D) keeps the number the same, and 92 (C) would be double, not half.
Gametes are made by meiosis and have half the chromosome number.
STAAR Biology (2024 SCR style)2 marksExplain why it is important that meiosis produces gametes with half the chromosome number of a body cell. Support your answer with reasoning about fertilization.Show worked answer →
A 2-point short constructed response on the purpose of halving.
Full credit (2 points): meiosis halves the chromosome number so that when two gametes join at fertilization, the offspring has the correct full chromosome number rather than double. If gametes kept the full number, the number would double every generation, which would be harmful.
Partial credit (1 point): states that halving keeps the number correct without linking it to fertilization combining two gametes. The science is scored.
Related dot points
- Apply Mendel's laws and use Punnett squares to predict the genotype and phenotype ratios of monohybrid crosses, and identify patterns of inheritance including dominant, recessive, codominant, and incomplete dominance (TEKS Biology, Reporting Category 2; patterns; using mathematics).
A TEKS-level answer on inheritance for the Texas STAAR Biology EOC: alleles, genotype and phenotype, dominant and recessive traits, using Punnett squares to predict ratios and probabilities, and codominance and incomplete dominance.
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A TEKS-level answer on DNA for the Texas STAAR Biology EOC: the components of a nucleotide, the double helix and complementary base pairing, and how DNA replication produces two identical copies before a cell divides.
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A TEKS-level answer on protein synthesis for the Texas STAAR Biology EOC: transcription of DNA into mRNA, translation of codons into amino acids at the ribosome, and how the base sequence determines the protein and the trait.
- Recognize the types of gene mutations and explain how a change in the DNA base sequence may be harmful, beneficial, or neutral and how it can be inherited (TEKS Biology, Reporting Category 2; cause and effect; stability and change).
A TEKS-level answer on mutations for the Texas STAAR Biology EOC: what a mutation is, substitution, insertion, and deletion, why an effect can be harmful, beneficial, or neutral, and how mutations in gametes are inherited and supply variation.
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A TEKS-level answer on the mechanisms of genetic change for the Texas STAAR Biology EOC: mutation, gene flow, and genetic drift as sources of change in a population, and how reproductive isolation leads to speciation.
Sources & how we know this
- Texas Essential Knowledge and Skills for Science (Biology) — Texas Education Agency (2024)
- STAAR Biology Assessed Curriculum — Texas Education Agency (2024)