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
Biology only (what this means)
Biology only: only in International GCSE Biology. Double Award students can skip it. What the labels meanCattle with one allele for red hair (R) and one allele for white hair (W) have a mixture of red and white hairs, called roan. (a) Name this type of inheritance and explain what it means. (2) (b) Two roan cattle (RW) are crossed. Give the expected ratio of phenotypes in the offspring. (1)
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- codominance (1)
- both alleles are expressed in the phenotype (of the heterozygous animal) (1)
- reject blending / mixing of the alleles for the second marking point
- 1 red : 2 roan : 1 white (1)
- allow 1 RR : 2 RW : 1 WW if phenotypes are clearly linked
Question 4
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 5
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 6
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 7
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