K327 / K328 / 2027

Inheritance

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Which description correctly defines an allele?

  1. A complete set of chromosomes in a gamete
  2. Any characteristic caused only by the environment
  3. An alternative form of the same gene
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An alternative form of the same gene

Two homologous chromosomes can carry different alleles at corresponding positions.

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Which comparison is correct?

  1. AB is an intermediate point on a continuous scale from A to B.
  2. Height is continuous; ABO blood group is discontinuous.
  3. Dividing heights into two groups removes all intermediate heights.
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Height is continuous; ABO blood group is discontinuous.

Height has intermediate measurements, while ABO has distinct categories.

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R is dominant to r. Which statement about Rr is correct?

  1. It is homozygous and has no recessive allele.
  2. Its phenotype must be halfway between the two homozygotes.
  3. It is heterozygous and expresses the dominant phenotype.
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It is heterozygous and expresses the dominant phenotype.

Both alleles remain present, but the dominant phenotype is expressed in this model.

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What is the expected phenotype ratio for Rr x Rr when R is completely dominant?

  1. 1 dominant : 2 intermediate : 1 recessive
  2. 3 dominant : 1 recessive
  3. All dominant, because recessive alleles cannot be inherited
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3 dominant : 1 recessive

RR, Rr and rR show the dominant phenotype; rr shows the recessive phenotype.

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What offspring genotypes are expected from Rr x rr?

  1. Rr and rr in equal proportions
  2. Homozygous RR and rr in equal proportions
  3. RR, Rr and rr in a 1:2:1 ratio
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Rr and rr in equal proportions

The Rr parent supplies R or r; the rr parent always supplies r.

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For Rr x Rr, the first three offspring are round. What is the probability that the next is wrinkled?

  1. 0, because the parents can make only round offspring
  2. 1/4 under the same inheritance model
  3. 1, because the ratio must now be completed
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1/4 under the same inheritance model

A fresh random combination of gametes has the same probability regardless of the earlier results.

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A cross of two heterozygous tall pea plants gives 316 tall and 104 short offspring. Which conclusion is best?

  1. The result is close to the expected 3 : 1 ratio
  2. The result proves that tall is recessive to short
  3. The result is far from 3 : 1, so the model must be wrong
  4. The result is a 1 : 1 ratio, so one parent was homozygous
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The result is close to the expected 3 : 1 ratio

316 : 104 is about 3.04 : 1, which is close to 3 : 1; random fertilisation explains the small difference.

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Two parents who are both carriers of a recessive allele have a child. What is the chance that the child is an affected boy?

  1. 1/8, because 1/4 affected x 1/2 boy.
  2. 1/2, because half the children are boys.
  3. 3/4, because three in four children have the dominant allele.
  4. 1/4, because one in four children is affected.
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1/8, because 1/4 affected x 1/2 boy.

Being affected and being a boy are independent, so multiply the two chances.

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Which combination produces XY in the usual human model?

  1. An X-bearing egg and a Y-bearing sperm
  2. A Y-bearing egg and an X-bearing sperm
  3. An egg with both X chromosomes and a sperm with no sex chromosome
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An X-bearing egg and a Y-bearing sperm

Their chromosome contributions combine to give XY.

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Two parents are both carriers (Ff) of a recessive allele for cystic fibrosis. What is the probability that their next child is a boy with cystic fibrosis?

  1. 1/2
  2. 3/4
  3. 1/4
  4. 1/8
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1/8

P(ff) = 1/4 and P(boy) = 1/2, so P(boy with ff) = 1/4 x 1/2 = 1/8.

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Which is an example of a chromosome-number change?

  1. A plant grows taller because it receives more light.
  2. A cell has 47 chromosomes because it has an extra chromosome 21.
  3. One DNA base changes within a haemoglobin gene, giving sickle-cell anaemia.
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A cell has 47 chromosomes because it has an extra chromosome 21.

The chromosome number differs from the usual 46, as in the common trisomy-21 form of Down syndrome.

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Question 1 of 9

Construct F1 and F2 crosses

5 marks

In the stated pea model, R gives round seeds and is completely dominant to r. Cross RR with rr to obtain F1, then cross two F1 plants to obtain F2. Give parental gametes, offspring genotypes and phenotype ratios. Calculate the expected number of wrinkled offspring among 80 F2 offspring. [5]

Define the symbols, combine one allele from each parent, then distinguish genotype from phenotype.

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