Meiosis and genetic variationEdexcel International A Level Biology: Revision notes
Section 1
Chromosomes, loci and alleles
A gene is a length of DNA that codes for a polypeptide. The position of a gene on a chromosome is its locus (plural loci). Alleles are different versions of a gene and occupy the same locus on homologous chromosomes.
Homologous chromosomes form a pair, one from each parent. They carry the same genes in the same order but may carry different alleles. A human body cell is diploid (2n = 46, or 23 pairs); a gamete is haploid (n = 23).
Section 2
Gene linkage
Genes at loci on the same chromosome are linked. Linked genes are inherited together because the whole chromosome passes into one gamete, so they do not assort independently.
Example: a heterozygous fly GL/gl (G and L on one chromosome, g and l on the other) test-crossed with a black vestigial fly (gl/gl) gives mostly parental types, grey long and black vestigial. A few recombinants (grey vestigial and black long) appear, because crossing over between the loci has separated alleles that were on the same chromosome.
Recombination frequency = recombinants ÷ total offspring × 100. For 80 recombinants in 1000 offspring this is 8%.
If two genes are unlinked and the cross is a test cross, you expect four phenotypes in a 1:1:1:1 ratio. Mostly parental types means the genes are linked.
Section 3
The stages of meiosis
Meiosis is two nuclear divisions after one round of DNA replication, producing four haploid cells. Names of the stages of prophase are not needed.
Meiosis I (homologous chromosomes separate)
- Prophase I: chromosomes condense and homologous chromosomes pair to form bivalents; crossing over occurs.
- Metaphase I: bivalents line up at the equator.
- Anaphase I: homologous chromosomes are pulled to opposite poles.
- Telophase I: two haploid cells form.
Meiosis II (sister chromatids separate)
- Chromosomes line up and in anaphase II the sister chromatids are pulled apart, giving four haploid cells.
Section 4
Crossing over in prophase I
During prophase I, homologous chromosomes pair up. Non-sister chromatids break at points called chiasmata (singular chiasma) and exchange equivalent sections of DNA. Alleles are swapped between the chromatids, giving recombinant chromatids with new combinations of alleles.
For a cell AB/ab with a chiasma between the two loci, the four gametes are AB, Ab, aB and ab. Ab and aB are the recombinants.
Crossing over is between non-sister chromatids of homologous chromosomes, not between the two sister chromatids of one chromosome.
Section 5
Independent assortment in metaphase I
In metaphase I each bivalent lines up at the equator, and the way each pair faces the poles is random. The maternal or paternal chromosome of each pair can go to either pole, independently of the other pairs. This is independent assortment.
The number of different combinations of chromosomes in the gametes (ignoring crossing over) is 2ⁿ, where n is the number of homologous pairs. For humans n = 23, giving 2²³ = 8 388 608. For a species with 2n = 14, n = 7 and 2⁷ = 128.
Section 6
Why gametes are non-identical
Meiosis produces gametes that are genetically different from each other and from the parent cell because of:
- Crossing over in prophase I, which gives chromatids new combinations of alleles.
- Independent assortment in metaphase I, which gives new combinations of maternal and paternal chromosomes.
Together these produce genetic variation between gametes, so offspring differ from their parents and from each other. Linked genes on the same chromosome are separated only by crossing over.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Meiosis and genetic variation
- A diploid human cell in the testis contains 46 chromosomes and begins meiosis to produce sperm cells.Ignoring crossing over, calculate the number of genetically different combinations of maternal and paternal chromosomes that could be present in the sperm cells produced by this person.2 marks
- In the fruit fly Drosophila the gene for body colour (grey G, dominant to black g) and the gene for wing length (long L, dominant to vestigial l) are close together on the same chromosome. A fly heterozygous for both genes, with G and L on one chromosome and g and l on its homologue, was crossed with a black, vestigial-winged fly. Out of 1000 offspring, 462 were grey with long wings, 458 were black with vestigial wings, 42 were grey with vestigial wings and 38 were black with long wings.Calculate the percentage of the offspring that were recombinants.2 marks
- A diploid cell has the genotype AaBb. Alleles A and B are on one chromosome of a homologous pair, and alleles a and b are on the other chromosome of the pair. In this cell, one chiasma forms between the loci of the two genes during prophase I of meiosis.Describe what happens during crossing over in this cell.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).