6.7 Genetic terms and monohybrid crosses
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Define the terms genotype and phenotype.
Mark scheme
- genotype: the alleles present / combination of alleles (1)
- phenotype: the characteristic expressed / shown (1)
Model answer
The genotype is the alleles an organism has; the phenotype is the characteristic that is expressed.
What is meant by a heterozygous genotype? Give an example using the letters B and b.
Mark scheme
- two different alleles (1)
- Bb (1)
Model answer
A heterozygous genotype has two different alleles, for example Bb.
In pea plants, the allele for tall stems (T) is dominant to the allele for short stems (t).
Two heterozygous plants are crossed. Draw a Punnett square and give the ratio of tall to short offspring.
Mark scheme
- parents Tt × Tt (1)
- gametes T, t and T, t (1)
- offspring TT, Tt, Tt, tt (1)
- 3 tall : 1 short (1)
Model answer
Parents Tt × Tt, gametes T and t from each. Offspring: TT, Tt, Tt, tt. Ratio 3 tall : 1 short.
A heterozygous black mouse (Bb) is crossed with a white mouse (bb). Black is dominant.
What is the probability that an offspring is white? Give your answer as a percentage.
Mark scheme
- offspring Bb, Bb, bb, bb / 1:1 (1)
- 50% (1)
Model answer
Offspring: Bb, Bb, bb, bb, so the probability of white is 2 in 4 = 50%.
Two parents without a condition have a child with the condition. What does this tell you about the allele that causes it? Explain your answer.
Mark scheme
- the allele is recessive (1)
- both parents must carry it / be heterozygous (but not show it) (1)
Model answer
The allele must be recessive: both parents are heterozygous carriers, so they do not show it, but the child inherited a recessive allele from each.
Explain why most human characteristics cannot be predicted with a Punnett square.
Mark scheme
most are controlled by many genes (interacting) / also affected by the environment
Model answer
Most characteristics are controlled by several genes interacting, not a single gene.
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