Question 1
A pea plant has the genotype Tt. Which word describes this genotype?
- Homozygous dominant
- Homozygous recessive
- Heterozygous
- Codominant
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Answer: C (1)
Question 2
Explain the difference between a dominant allele and a recessive allele.
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- a dominant allele is expressed (in the phenotype) when one copy is present / in the heterozygous state (1)
- a recessive allele is only expressed when two copies are present / no dominant allele present (1)
- so a heterozygous individual shows the dominant characteristic (1)
- allow homozygous recessive for two copies
Question 3
Human height shows a wide range of values. Explain why this is not a characteristic that can be explained by a simple monohybrid cross.
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- it is controlled by many genes / polygenic inheritance (1)
- so there is a range of values rather than two clear categories / not one gene with a dominant and a recessive allele (1)
Question 4
In rabbits, black fur (B) is dominant to brown fur (b). Two heterozygous black rabbits are crossed. (a) Draw a genetic diagram to show the genotypes of the possible offspring. (3) (b) Give the probability that an offspring has brown fur. (1)
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- gametes B or b from each parent (1)
- offspring BB, Bb, Bb, bb (1)
- phenotypes: 3 black : 1 brown (1)
- probability = 1/4 / 0.25 / 25% (1)
Question 5
In a family, parents 1 and 2 do not have a genetic disorder. They have three children: child 3 (a daughter) has the disorder, child 4 (a son) does not and child 5 (a daughter) does not. (a) State whether the disorder is caused by a dominant or a recessive allele. Give a reason. (2) (b) Using the letters D and d, give the genotypes of parents 1 and 2 and explain your answer. (2)
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- recessive (1)
- two parents without the disorder have a child with it (1)
- both parents are Dd / heterozygous (1)
- each parent must have passed on one recessive allele (d) to child 3, but they do not have the disorder themselves (1)
Question 6
Describe how the sex of a baby is determined at fertilisation. Use a genetic diagram or describe the gametes in your answer.
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- females are XX and males are XY (1)
- all eggs carry an X chromosome (1)
- half the sperm carry X and half carry Y; whether an X or Y sperm fertilises the egg decides the sex (1)
- allow correct Punnett square for full credit