3.2.2Unit 3: Practical Skills in Biology I
Meiosis and Variation — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 16 of 27 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Stages of Meiosis
Meiosis is the type of nuclear division that produces gametes with half the chromosome number.
1
Before meiosis the DNA replicates, so each chromosome consists of two sister chromatids in a diploid (2n) cell.
2
In meiosis I homologous chromosomes pair up as bivalents and then separate, forming two haploid (n) nuclei.
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The chromosome number halves in meiosis I, not meiosis II, and each chromosome still has two chromatids.
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In meiosis II the sister chromatids separate, so each chromosome now consists of a single chromatid.
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One diploid cell therefore produces four haploid nuclei that are genetically different from one another.
Part 2 of 5Sources of Variation
Independent assortment
- Independent assortment Homologous pairs line up at random at the equator in metaphase I,
- so maternal and paternal chromosomes are mixed in different combinations.
Formula
- Formula The number of possible chromosome combinations in gametes is 2ⁿ, where n is the haploid number.
Crossing over
- Crossing over In prophase I non-sister chromatids in a bivalent exchange sections of DNA at points called chiasmata.
Recombination
- Recombination Crossing over gives chromatids with new combinations of alleles, which are called recombinants.
Random fertilisation
- Random fertilisation Any sperm can fuse with any egg, so the number of possible zygotes is much greater than the number of gametes.
Mutation
- Mutation Random changes in DNA create new alleles, which are the ultimate source of all genetic variation.
Part 3 of 5Why Variation Matters
Why Variation Matters
Natural selection
- Natural selection Variation means some individuals are better suited to their environment, and they are more likely to survive and reproduce.
Adaptation
- Adaptation Populations with more genetic variation are more likely to include individuals able to cope with environmental change.
Chromosome number
- Chromosome number Halving the number in gametes keeps the chromosome number the same in each generation after fertilisation.
Offspring
- Offspring Offspring from sexual reproduction are genetically different from both parents and from each other.
Part 4 of 5Worked Example
Question
- Humans have a haploid number of 23.
- Calculate the number of different chromosome combinations in gametes by independent assortment alone, and the number of possible zygotes from random fertilisation.
- Combinations per gamete type = 2ⁿ = 2²³ = 8 388 608.
- Random fertilisation joins any one of these male gametes with any one of the female gametes.
- Zygote combinations = 8 388 608 × 8 388 608 = 7.04 × 10¹³ (3 s.f.).
Answer 8 388 608 combinations per parent, giving about 7.04 × 10¹³ possible zygotes.
Part 5 of 5Exam Checklist & Tips
3.2.2 – Key Points for the Exam
- Haploid A cell with one set of chromosomes (n).
- Bivalent A pair of homologous chromosomes joined together in meiosis I.
- Chiasma The point where non-sister chromatids cross over during prophase I.
- Independent assortment The random orientation of homologous pairs in metaphase I.
- Recombinant A chromatid or cell with a new combination of alleles.
Common Mistakes
- Saying that the chromosome number halves in meiosis II, when it halves in meiosis I.
- Describing crossing over between sister chromatids, when it occurs between non-sister chromatids.
- Confusing independent assortment (metaphase I) with crossing over (prophase I).
- Forgetting to include random fertilisation as a source of variation.
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