Topic 8: Origins of Genetic Variation — Edexcel A-Level Biology B
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Topic 8: Origins of Genetic Variation explained
Topic 8: Origins of Genetic Variation covers 4 specification points, including Origins of genetic variation i Understand that… and Transfer of genetic information i Understand….
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8.1 Origins of genetic variation i Understand that mutations are the source of new variations and that the processes of random assortment and crossing over during meiosis give rise to new combinations of alleles in gametes. ii Understand how random fertilisation during sexual reproduction brings about genetic variation.
Mutations change the base sequence of DNA and so create new alleles, which are the source of new variation. During meiosis, random assortment of homologous chromosomes in metaphase I and of sister chromatids in metaphase II shuffles whole chromosomes into gametes, while crossing over between non-sister chromatids in prophase I exchanges sections of homologous chromosomes. Together these processes produce new combinations of alleles in the gametes. Random fertilisation then combines gametes carrying different allele combinations, so the offspring receive a mixture of alleles from both parents and genetic variation in the population increases further.
8.2 Transfer of genetic information i Understand the terms ‘genotype and phenotype’,’ homozygote and heterozygote’, ‘dominance’, ‘recessive’, ‘codominance’ and ‘multiple alleles’. ii Be able to construct genetic crosses and pedigree diagrams. iii Understand the inheritance of two non-interacting unlinked genes. iv Understand that autosomal linkage results from the presence of alleles on the same chromosome and that the results of crosses can be explained by the events of meiosis, including black/grey body and long/vestigial wing in Drosophila. v Understand sex linkage on the X chromosome, including haemophilia in humans. vi Be able to use chi squared tests to test the significance of the difference between observed and expected results.
Genotype is genetic constitution; phenotype is observable characteristics. A homozygote has two identical alleles; a heterozygote has two different alleles. Dominance expresses in heterozygotes; recessiveness expresses only in homozygotes; codominance expresses both alleles; multiple alleles means >2 alleles exist in a population (e.g. ABO). Pedigree diagrams use squares for males, circles for females, and shading for affected individuals to infer genotypes. Dihybrid crosses of two non-interacting unlinked genes yield a 9:3:3:1 ratio. Autosomal linkage occurs when genes share a chromosome (e.g. Drosophila black body b and vestigial wings vg); crossing BbVgvg with bbvgvg produces mainly parental types, with recombinants formed only by meiotic crossing over. X-linked genes explain why recessive conditions like haemophilia affect males. Chi-squared tests assess ratio significance.
8.3 Gene pools i Understand that selection pressures acting on the gene pool change allele frequencies in the population, including: ● stabilising selection maintaining continuity in a population ● disruptive selection leading to changes or speciation. ii Understand that sometimes changes in allele frequencies can be the result of chance and not selection, including genetic drift.
A gene pool comprises all alleles within a population. Selection pressures alter allele frequencies over time. Stabilising selection maintains continuity by favouring average phenotypes and eliminating extremes, keeping allele frequencies constant in stable environments. Conversely, disruptive selection favours extreme phenotypes over intermediate ones, driving evolutionary changes that can lead to speciation. However, evolution is not always driven by selection. Genetic drift causes random fluctuations in allele frequencies purely by chance. This chance effect is highly significant in small populations, where random events can lead to the rapid loss or fixation of alleles regardless of their selective advantage.
8.3 Gene pools continued iii Understand that allele frequencies can be influenced by: ● population bottlenecks ● founder effect. iv Understand how the Hardy-Weinberg equation can be used to monitor changes in the allele frequencies in a population.
A gene pool is the total set of alleles in a population. Allele frequencies shift when the breeding population is reduced or newly founded. In a population bottleneck, an environmental event sharply cuts numbers, so the survivors carry only part of the original variation; drift then changes frequencies. In the founder effect, a few individuals colonise a new area, and their allele frequencies, not the source population's, seed the new gene pool. The Hardy-Weinberg equation models an ideal population: p + q = 1 for two alleles, and p² + 2pq + q² = 1 for genotypes. Comparing observed genotype proportions with those predicted shows whether allele frequencies are changing, so it monitors evolution in action.
Your focus
- State that mutations are the source of new variations.
- Explain how random assortment during meiosis produces new combinations of alleles in gametes.
- Explain how crossing over during meiosis produces new combinations of alleles in gametes.
Show all 13 objectives
- Explain how random fertilisation during sexual reproduction brings about genetic variation.
- Construct accurate genetic crosses and pedigree diagrams for various inheritance patterns.
- Explain how meiosis and crossing over affect the inheritance of autosomally linked genes in Drosophila.
- Perform a chi-squared test to evaluate the significance of deviations from expected phenotypic ratios.
- Explain how stabilising and disruptive selection pressures influence allele frequencies in a gene pool.
- Describe the process of speciation as a potential outcome of disruptive selection.
- Evaluate the impact of genetic drift on allele frequencies as a chance-driven evolutionary mechanism.
- Describe how a population bottleneck and the founder effect alter allele frequencies.
- Apply the Hardy-Weinberg equation to calculate allele and genotype frequencies.
- Interpret observed genotype frequencies against Hardy-Weinberg predictions to detect change in a population.
Topic 8: Origins of Genetic Variation exam tips
Marking Points
- Mutations are the source of new variations because they produce new alleles.
- Random assortment during meiosis gives rise to new combinations of alleles in gametes.
- Crossing over during meiosis gives rise to new combinations of alleles in gametes.
- Random fertilisation during sexual reproduction brings about genetic variation.
- Define genotype, phenotype, homozygote (two identical alleles), heterozygote (two different alleles), dominance, recessiveness, codominance, and multiple alleles.
- Construct and interpret pedigree diagrams using standard symbols (squares for males, circles for females, shaded for affected) to determine modes of inheritance and infer genotypes.
- Construct dihybrid crosses for two non-interacting unlinked genes to demonstrate independent assortment, yielding 9:3:3:1 phenotypic ratios.
- Explain autosomal linkage using the Drosophila example: genes for body colour (black/grey) and wing length (vestigial/long) reside on the same chromosome, producing parental phenotypes unless crossing over separates them during meiosis.
- Explain sex linkage on the X chromosome, accounting for higher incidence of recessive conditions like haemophilia in human males.
- Calculate chi-squared values and compare them with critical values at appropriate degrees of freedom to test statistical significance.
- Define a gene pool as the total collection of alleles in a population at a specific time.
- Explain that stabilising selection reduces phenotypic variation by selecting against extreme traits.
- Describe how disruptive selection can split a population into two distinct phenotypic groups, potentially causing sympatric speciation.
- Identify genetic drift as a random process that changes allele frequencies independently of environmental selection pressures.
- A gene pool is all the alleles present in a population.
- A population bottleneck is a sharp reduction in population size that leaves a small surviving group with reduced genetic variation.
- The founder effect occurs when a small group establishes a new population, so its allele frequencies differ from the source population.
- Genetic drift changes allele frequencies by chance, and its effect is stronger in small populations.
- For two alleles, p + q = 1, where p and q are the frequencies of the two alleles.
- Genotype frequencies are predicted by p² + 2pq + q² = 1, representing homozygous dominant, heterozygous and homozygous recessive proportions.
- Comparing observed genotype frequencies with Hardy-Weinberg predictions indicates whether allele frequencies are changing.
- Hardy-Weinberg assumptions include no selection, no mutation, no migration, random mating and a large population.
Examiner Tips
- 💡Name the meiotic stage for each process: crossing over in prophase I, random assortment in metaphase I and II.
- 💡Use the phrase new combinations of alleles when explaining assortment and crossing over.
- 💡Distinguish clearly between mutation, which creates new alleles, and fertilisation, which combines existing alleles.
- 💡For random fertilisation, state that any sperm can fuse with any egg, so many allele combinations are possible.
- 💡Always state the parental phenotypes, genotypes, gametes, and offspring genotypes clearly when drawing a genetic cross.
- 💡When evaluating Drosophila crosses, look for offspring ratios that deviate significantly from 9:3:3:1 as a strong indicator of autosomal linkage.
- 💡Use a null hypothesis stating there is no significant difference between observed and expected results before performing a chi-squared test.
- 💡Use clear examples, such as birth weights in humans, to illustrate stabilising selection in extended response questions.
- 💡When discussing speciation, explicitly link disruptive selection to the reproductive isolation of the extreme phenotypes.
- 💡Emphasise the role of chance when defining genetic drift to distinguish it clearly from directional or stabilising selection.
- 💡Write the equation and define p and q before substituting numbers, so the marker can follow your reasoning.
- 💡Check that p + q = 1 and that p² + 2pq + q² = 1 after calculating, to catch arithmetic slips.
- 💡When a question describes a disaster or a colonisation event, name the mechanism (bottleneck or founder effect) and explain the change in allele frequencies.
Common Mistakes
- Thinking mutations are always harmful: most mutations have no effect, and some can be beneficial or neutral.
- Confusing random assortment with crossing over: random assortment separates whole chromosomes, while crossing over exchanges sections between homologous chromosomes.
- Believing crossing over happens in mitosis: it occurs during prophase I of meiosis between non-sister chromatids of homologous chromosomes.
- Thinking random fertilisation creates new alleles: it combines existing alleles from two gametes, whereas mutation creates new alleles.
- Confusing codominance with incomplete dominance; correction: codominance means both alleles are fully expressed in the heterozygote phenotype, rather than blended.
- Assuming sex-linked traits can be passed from father to son; correction: fathers pass their Y chromosome to sons, so X-linked traits like haemophilia are inherited from the mother.
- Miscalculating degrees of freedom in chi-squared tests; correction: degrees of freedom for genetic crosses equal the number of phenotypic categories minus one (n-1).
- Confusing genetic drift with natural selection; correction: genetic drift is driven by random chance events, whereas natural selection is driven by specific environmental pressures.
- Assuming stabilising selection creates new species; correction: stabilising selection maintains the current population characteristics, whereas disruptive selection drives speciation.
- Believing genetic drift only occurs in small populations; correction: genetic drift occurs in all populations, but its effects on allele frequencies are much more pronounced and rapid in small populations.
- Thinking the founder effect and a bottleneck are the same: a bottleneck reduces an existing population, while the founder effect starts a new population from a few colonists.
- Assuming dominant alleles automatically become more common: dominance affects phenotype, not allele frequency, and selection acts on phenotype.
- Using p and q as genotype frequencies: p and q are allele frequencies, while p², 2pq and q² are genotype frequencies.
- Believing Hardy-Weinberg equilibrium proves no evolution is possible: it describes an ideal population, and real deviations show that allele frequencies are changing.