Edexcel A Level Biology B Topic 8: Origins of Genetic Variation Exam Questions

About Edexcel A Level Biology B Topic 8: Origins of Genetic Variation

Topic 8 of Edexcel A Level Biology B explains where genetic variation comes from, how it is inherited and how it changes in populations. You learn about gene and chromosome mutations and how meiosis and random fertilisation generate variation. Inheritance covers monohybrid and dihybrid crosses, codominance, multiple alleles, linkage and sex linkage, and you use the chi-squared test to compare observed and expected ratios. The gene pool section covers allele frequencies, the Hardy-Weinberg principle, genetic drift, founder effects and bottlenecks, and how reproductive isolation leads to speciation. Expect genetic diagram questions and statistical analysis of cross data.

What you need to know

• Origins of genetic variation Gene mutations (substitution, insertion, deletion) change the base sequence; chromosome mutations change structure or number, such as non-disjunction. Meiosis creates variation by crossing over and independent assortment.
• Inheritance Use genetic diagrams for monohybrid and dihybrid crosses. A dihybrid cross of two heterozygotes with unlinked genes gives a 9 : 3 : 3 : 1 ratio.
• Linkage and sex linkage Linked genes on the same chromosome tend to be inherited together. Sex-linked genes are carried on the X chromosome, so recessive conditions are more common in males.
• Chi-squared Chi-squared = sum of (O - E)^2 / E, with degrees of freedom = number of categories minus 1, compared with the critical value at p = 0.05.
• Gene pools A gene pool is all the alleles in a population. Hardy-Weinberg: p + q = 1 and p^2 + 2pq + q^2 = 1. Genetic drift, founder effects and bottlenecks change allele frequencies in small populations.
• Speciation Geographical, ecological or behavioural isolation stops gene flow; different selection pressures change allele frequencies until populations can no longer interbreed to produce fertile offspring.

Common mistakes in Topic 8: Origins of Genetic Variation

• Writing gametes with both alleles of a gene; each gamete has one allele of each gene.
• Assuming every mutation is harmful; many are neutral and some are beneficial.
• Treating p^2 as the frequency of the dominant allele; p is the allele frequency, p^2 the homozygous dominant genotype frequency.
• Concluding that chi-squared proves a hypothesis; it only shows whether the difference is significant.
• Confusing genetic drift with natural selection; drift is random and stronger in small populations.

Try three Topic 8: Origins of Genetic Variation exam-style questions