Inheritance
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Which description correctly defines an allele?
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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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R is dominant to r. Which statement about Rr is correct?
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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?
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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?
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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?
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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?
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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?
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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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Why is IA IB codominant?
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Both A and B antigens are expressed in the AB phenotype.
Both alleles are expressed together rather than one masking the other.
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A mother is blood group AB and the father is blood group O. Which blood groups can their children have?
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A or B
The mother gives IA or IB and the father gives IO, so children are IA IO (A) or IB IO (B).
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Which combination produces XY in the usual human model?
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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?
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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?
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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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Which comparison is correct?
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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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Which chain best explains evolution by natural selection?
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Heritable variants differ in reproductive success, so their frequencies change over generations.
This connects inherited variation, environmental selection and population change.
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