Origins of Genetic Variation | Edexcel A-Level Biology B (9BI0)
Origins of Genetic Variation
- 103 questions
- 8 subtopics
- Topics 8 to 10, examined on Paper 2 and Paper 3
- Paper 2 and Paper 3
Where genetic variation comes from, how it is passed on through monohybrid, dihybrid and sex-linked crosses, how a chi-squared test judges whether results fit a prediction, and how allele frequencies change in a population..
It covers mutation, meiosis and random fertilisation, genotype, phenotype and monohybrid inheritance, dihybrid crosses and pedigree diagrams, autosomal linkage and sex linkage, the chi-squared test in genetics, selection pressures and allele frequency, genetic drift and the founder effect and the Hardy-Weinberg equation.
Sample questions from Origins of Genetic Variation
Answer each one closed book first, then open the answer.
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Mutation, meiosis and random fertilisation
State the formula for the number of possible gamete combinations produced by independent assortment, and define the variable.
Show the answer
The formula is 2ⁿ, where n is the haploid chromosome number. -
Genotype, phenotype and monohybrid inheritance
What does it mean for an allele to be dominant?
Show the answer
A dominant allele is expressed in the phenotype when present in either homozygous or heterozygous form. -
Dihybrid crosses and pedigree diagrams
How are affected individuals indicated in a pedigree diagram?
Show the answer
By filled (shaded) symbols. -
Autosomal linkage and sex linkage
In Drosophila, what are the dominant and recessive phenotypes for body colour and wing length?
Show the answer
Grey body and long wings are dominant; black body and vestigial wings are recessive. -
The chi-squared test in genetics
After squaring the differences in a chi-squared test, what is done with each (O − E)² value?
Show the answer
Each is divided by the expected value (E) for that category. -
Selection pressures and allele frequency
Under what conditions does disruptive selection occur?
Show the answer
When two or more distinct extreme phenotypes are each favoured and the intermediate phenotype is at a disadvantage, for example where conditions differ sharply between parts of a habitat or between seasons. -
Genetic drift and the founder effect
Why might allele frequencies change following a population bottleneck?
Show the answer
The surviving individuals represent a random sample of the original population, so allele frequencies shift by chance rather than selection. -
The Hardy-Weinberg equation
What does the term 2pq represent in the Hardy-Weinberg equation?
Show the answer
The frequency of heterozygous individuals in the population.
The 8 subtopics
One subtopic is one session. Work down the list.
| Subtopic | What it covers | Questions |
|---|---|---|
| Mutation, meiosis and random fertilisation | Recall questions on mutation, crossing over, independent assortment and random fertilisation as sources of variation. | 14 |
| Genotype, phenotype and monohybrid inheritance | Recall questions on the key terms, dominance and codominance, and constructing a monohybrid cross. | 12 |
| Dihybrid crosses and pedigree diagrams | Recall questions on the symbols and lines of a pedigree diagram, what a dihybrid cross is, the 9:3:3:1 ratio, and why unlinked genes assort independently. | 8 |
| Autosomal linkage and sex linkage | Recall questions on autosomal linkage and why linked genes do not assort independently, crossing over and recombinant phenotypes, body colour and wing length in Drosophila, and sex linkage, hemizygous males and the X-linked inheritance of haemophilia. | 15 |
| The chi-squared test in genetics | Recall questions on the null hypothesis, calculating the statistic, degrees of freedom, and interpreting the result against the critical value. | 15 |
| Selection pressures and allele frequency | Recall questions on selection pressures, stabilising selection and birth mass, disruptive selection, bimodal distributions and speciation, and why selection changes allele frequencies without creating new alleles. | 10 |
| Genetic drift and the founder effect | Recall questions on chance change in small populations, genetic bottlenecks, and the founder effect. | 8 |
| The Hardy-Weinberg equation | Recall questions on the conditions the principle assumes, using the two equations, and what a departure from equilibrium indicates. | 21 |
How the guide is worked
Answering a question from memory stores it far better than reading the answer again. The guide runs that as a fixed procedure on one subtopic at a time, about twenty minutes a session.
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Step 1 · Closed book
Cover the answers. Work through one subtopic and write down what you can. Leave blanks where you have nothing.
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Step 2 · Open book
Go back to the top. Read each printed answer and write it out in full, including the ones you had right.
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Step 3 · Closed book again
Same questions, same order, from memory. The gap between pass one and pass three is the session result.
Read the full method, the return schedule and the research behind it.
Nearby topics
Edexcel A-Level Biology B Active Recall Guide
Every topic, not just this one. 2,073 questions with their answers.