How does meiosis make gametes with half the chromosomes, and where does the variation between offspring come from?
Describe the process of meiosis and explain how it results in genetic variation in gametes (NGSSS SC.912.L.16.4; Reporting Category 2, Classification, Heredity, and Evolution).
A benchmark-level answer on meiosis for the Florida Biology 1 EOC: halving the chromosome number, the difference from mitosis, and how crossing over and independent assortment create variation in gametes.
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What this topic is asking
The NGSSS benchmark SC.912.L.16.4 asks you to describe meiosis and explain how it produces genetic variation in gametes. For the Florida Biology 1 EOC you need to know that meiosis halves the chromosome number to make gametes, how it differs from mitosis, and the two events (crossing over and independent assortment) that make every gamete genetically unique. Items commonly ask for the chromosome number in a gamete or for the source of variation.
What meiosis does
In humans, a body cell is diploid with 46 chromosomes (23 pairs). Meiosis halves this to make gametes that are haploid with 23 chromosomes. The halving matters because:
Meiosis versus mitosis
The EOC frequently contrasts the two divisions, so keep them straight:
- Mitosis makes two cells that are genetically identical to the parent and have the same chromosome number. It is used for growth, repair, and replacement of body cells.
- Meiosis makes four cells that are genetically varied and have half the chromosome number. It is used to make gametes for reproduction.
So mitosis is "same number, identical, body cells," and meiosis is "half the number, varied, gametes." (Mitosis is covered in the cell cycle topic.)
How meiosis creates variation
The whole point of meiosis for evolution is that it makes gametes different from one another. Two events do this:
- Crossing over. Early in meiosis, homologous chromosomes (the matching pair, one from each parent) line up and exchange matching segments. This swaps alleles between the chromosomes, producing new combinations that neither parent's chromosome had.
- Independent assortment. When the chromosome pairs line up to be separated, each pair orients randomly and independently of the others. With 23 pairs, this alone gives millions of possible gamete combinations.
Together with the random combination of gametes at fertilization, these processes ensure that offspring are genetically unique. This variation is the raw material that natural selection acts on.
Try this
Q1. A cell with 12 chromosomes undergoes meiosis. State the number of cells produced and the chromosome number in each. [2]
- Cue. Four cells, each with 6 chromosomes (half of 12).
Q2. State two ways meiosis increases genetic variation. [2]
- Cue. Crossing over (exchange of segments between homologous chromosomes) and independent assortment (random orientation of chromosome pairs).
Exam-style practice questions
Practice questions written in the style of FLDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
FL Biology 1 EOC (2023 released style)1 marksA human body cell has 46 chromosomes. How many chromosomes are in a human gamete (egg or sperm) produced by meiosis? (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 gamete has half the body-cell number: . When two gametes join at fertilization, the offspring returns to 46. B is the body-cell number (the result of mitosis), and 92 would be a doubling.
Meiosis halves; fertilization restores. Divide the body-cell number by two for a gamete.
FL Biology 1 EOC (2024 released style)1 marksWhich process during meiosis increases genetic variation by exchanging segments between homologous chromosomes? (A) DNA replication. (B) Crossing over. (C) Cytokinesis. (D) Transcription.Show worked answer →
A 1-point item on the source of variation in meiosis.
The correct answer is B. Crossing over, in which homologous chromosomes exchange matching segments, produces new combinations of alleles and is a major source of genetic variation. Independent assortment (the random lineup of chromosome pairs) adds more. DNA replication copies DNA without shuffling it, and the other options are unrelated.
Related dot points
- Use Mendel's laws of segregation and independent assortment, with Punnett squares, to analyze patterns of inheritance and predict the genotype and phenotype ratios of monohybrid crosses (NGSSS SC.912.L.16.1; Reporting Category 2, Classification, Heredity, and Evolution).
A benchmark-level answer on inheritance for the Florida Biology 1 EOC: alleles, genotype and phenotype, dominant and recessive traits, Mendel's laws, and using Punnett squares to predict ratios and probabilities.
- Explain the cell cycle and mitosis, and the relationship between mutation, the cell cycle, and uncontrolled cell growth that can result in cancer (NGSSS SC.912.L.16.5; Reporting Category 1, Molecular and Cellular Biology).
A benchmark-level answer on the cell cycle for the Florida Biology 1 EOC: interphase and the phases of mitosis, the purpose of mitosis, checkpoints that regulate division, and how mutations cause uncontrolled growth and cancer.
- Describe how mutation and genetic recombination increase genetic variation, and the possible effects of mutations (NGSSS SC.912.L.15.15; Reporting Category 2, Classification, Heredity, and Evolution).
A benchmark-level answer on mutation and variation for the Florida Biology 1 EOC: types of mutations, harmful, neutral, and beneficial effects, genetic recombination through meiosis and fertilization, and why variation matters for evolution.
- Discuss observed inheritance patterns caused by various modes of inheritance, including dominant, recessive, codominant, incomplete dominance, sex-linked, polygenic, and multiple alleles (NGSSS SC.912.L.16.2; Reporting Category 2, Classification, Heredity, and Evolution).
A benchmark-level answer on inheritance patterns for the Florida Biology 1 EOC: incomplete dominance, codominance, multiple alleles (ABO blood type), sex-linked traits, and polygenic inheritance, with how to recognize each.
- Describe the structure of DNA and the basic process of DNA replication, and how it relates to the transmission and conservation of genetic information (NGSSS SC.912.L.16.3; Reporting Category 1, Molecular and Cellular Biology).
A benchmark-level answer on DNA for the Florida Biology 1 EOC: the double helix and nucleotide structure, complementary base pairing, semiconservative replication, and why copying conserves genetic information.
Sources & how we know this
- Next Generation Sunshine State Standards: Science (Biology 1) — Florida Department of Education (2024)
- Biology 1 End-of-Course Assessment Test Item Specifications — Florida Department of Education (2024)