Genetics, variation & evolution — questions and answers
5 high-yield, syllabus-aligned questions on Genetics, variation & evolution, covering 13 individually creditable mark points. Each one shows the answer that scores full marks under our guide, which phrase earns which mark, and a common incomplete answer — so you can see the difference rather than guess at it.
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Every question and solution is reviewed by examiners versed in WAEC and JAMB, each with more than thirty years of experience.
Mark allocations are TopMarks’ own, written against the published syllabus and Chief Examiners’ reports. They are a guide to how these answers are usually credited, not an official marking scheme.
Define2 marks
Define the term gene.
TopMarks model answer
A gene is a unit of heredity, consisting of a segment of DNA on a chromosome, which carries the coded information for a particular characteristic and is transmitted from parent to offspring.
Suggested marking guide
1A unit of heredity / segment of DNA on a chromosome
2Determines a characteristic passed from parent to offspring
Why it scores
A two-mark definition needs two ideas: WHAT the gene physically is, and WHAT it does. Give only the physical description and the examiner has no evidence you know what a gene is for; give only the function and there is nothing to show you know where it sits.
A common incomplete answer
“A gene is something in the body that makes you look like your parents.”
What it costs: Nothing. There is no mention of DNA, chromosomes or heredity, and “look like your parents” is not the same as determining a characteristic that is transmitted.
Key ideas to include: unit of heredity, DNA, chromosome, characteristic, offspring.
Explain3 marks
A pure-breeding tall pea plant was crossed with a pure-breeding short pea plant. All the F1 offspring were tall. Explain this result.
TopMarks model answer
The tall parent is homozygous dominant, TT, and the short parent is homozygous recessive, tt. Each parent contributes one allele through its gametes, so every F1 plant receives T from one parent and t from the other and is heterozygous, Tt. Tallness is dominant over shortness, so the dominant allele is expressed in the phenotype and all the F1 plants are tall while still carrying the recessive allele.
Suggested marking guide
1Parental genotypes given as homozygous TT and tt
2F1 genotype is heterozygous Tt
3Tallness is dominant, so it is expressed
Why it scores
Genetics marks are for the SYMBOLS as much as the words. Write the genotypes, show the gametes, then state the conclusion. A candidate who explains the reasoning perfectly in prose but never writes TT, tt and Tt has left the examiner nothing to tick on the mark scheme.
A common incomplete answer
“All the F1 plants are tall because tall is dominant over short.”
What it costs: One mark of three. The dominance is credited, but no genotypes are given for the parents or the offspring, so two of the three marks have nothing to attach to.
State three differences between mitosis and meiosis.
TopMarks model answer
Mitosis produces two daughter cells while meiosis produces four. The daughter cells of mitosis are diploid and genetically identical to the parent cell, whereas those of meiosis are haploid and genetically different. Mitosis occurs in body cells for growth and repair, while meiosis occurs in the reproductive organs to form gametes.
Suggested marking guide
1Two daughter cells versus four
2Diploid and identical versus haploid and different
3Body cells for growth versus reproductive organs for gametes
Why it scores
Each mark is one PAIRED difference — both halves in the same sentence. Three marks, three features compared. Listing facts about mitosis in one paragraph and facts about meiosis in another leaves the examiner to do the pairing, and the mark scheme does not allow it.
A common incomplete answer
“Mitosis produces two cells and meiosis produces four cells.”
What it costs: One mark of three. A correctly paired difference, but only one of the three the question asked for.
Explain the term continuous variation and give one example.
TopMarks model answer
Continuous variation is variation in which individuals show a complete range of intermediate forms between two extremes, with no distinct groups, because the character is controlled by many genes and is influenced by the environment. Height in humans is an example.
Suggested marking guide
1A complete range with no distinct groups
2A valid example
Why it scores
When a question says “and give one example”, the example is a separate mark. It costs one line and it is the easiest mark on the paper — yet it is left out constantly because candidates stop as soon as they have finished defining.
A common incomplete answer
“Continuous variation is when characteristics vary. An example is height.”
What it costs: One mark of two. The example is credited, but the definition never mentions a continuous range with no distinct groups, which is the whole point of the word “continuous”.
Key ideas to include: range, intermediate forms, no distinct groups, polygenic, environment.
State3 marks
State three ways in which variation is important to organisms.
TopMarks model answer
Variation provides the raw material on which natural selection acts, so that individuals with favourable characteristics survive and reproduce. It enables a species to adapt to changes in the environment and so avoid extinction. It is also the basis of selective breeding for improved crop and animal varieties, and it makes individuals identifiable for forensic and blood-transfusion purposes.
Suggested marking guide
1Basis of natural selection and survival
2Allows adaptation to a changing environment
3Basis of selective breeding or of identification
Why it scores
Three marks means three DIFFERENT kinds of importance. Survival, adaptation and human application are three distinct categories; three restatements of “it helps them survive” are one.
A common incomplete answer
“It helps organisms to survive in their environment.”
What it costs: One mark of three. Survival is credited once, but adaptation to change and the practical uses of variation are both absent.
Practise Genetics, variation & evolution on real questions
Reading a full-mark answer is the first half. Writing one under time is the other. A free account opens exam-standard practice in Biology with the full solution on every question.