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Meiosis: two divisions, and why no two gametes come out the same questions
The two divisions of meiosis, homologous pairing and bivalents, crossing over at chiasmata, independent assortment and the 2 to the power n arithmetic, chromosome and chromatid counts through both divisions, how meiosis differs from mitosis, and the three sources of genetic variation it creates.
6 original questions · 21 marks · the meiosis: two divisions, and why no two gametes come out the same notes · Inheritance and population genetics
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Describe how crossing over happens during prophase I of meiosis, and describe its effect on the chromatids involved.
Mark scheme
- B1 each chromosome finds its homologue and the two lie alongside each other along their whole length, forming a bivalent of four chromatids
- B1 non-sister chromatids, one maternal and one paternal, break at the same point and each joins to the other's cut end
- B1 the visible point at which they cross is a chiasma, and equivalent sections of DNA have been exchanged
- A1 the two chromatids that took part leave carrying a combination of alleles that neither parental chromosome had, so sister chromatids are no longer genetically identical; the other two leave unchanged
Explain how meiosis produces gametes that are genetically different from one another, naming the stage at which each process happens.
Mark scheme
- B1 crossing over in prophase I exchanges sections of DNA between non-sister chromatids of a homologous pair
- B1 this makes new combinations of alleles along a single chromosome, combinations that were on neither chromosome the cell started with
- B1 independent assortment at metaphase I decides which member of each bivalent faces which pole, and the decision is made separately for every bivalent
- A1 this makes new combinations of whole chromosomes, giving 2 to the power n kinds of gamete where n is the number of homologous pairs
A species of plant has a diploid number of 14. Calculate the number of different chromosome combinations its gametes could carry from independent assortment alone, and calculate the number of different chromosome combinations possible in a zygote formed by two such plants.
Mark scheme
- M1 halve the diploid number to get the number of homologous pairs, so n = 7, because one decision is made per pair
- A1 2 to the power 7 is 128 chromosome combinations in a gamete
- M1 a zygote takes one gamete from each parent and the two are independent, so multiply 128 by 128
- A1 16 384 combinations, and crossing over would make the true figure larger still
Compare mitosis with meiosis, referring to the number of divisions, the chromosome number of the daughter cells, the behaviour of homologous chromosomes and the genetic result.
A chemical prevents chiasmata from forming in prophase I but allows the rest of meiosis to run normally. Suggest what this does to the genetic variation of the gametes produced, and suggest why they would still not all be the same.
State the stage of meiosis at which homologous chromosomes are separated, and state the stage at which sister chromatids are separated.
Practise meiosis: two divisions, and why no two gametes come out the same one question at a time
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