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AQA · GCSE · Biology · Higher

Punnett square inheritance probability

Q06.3 4 marks

Polydactyly is an inherited disorder caused by a dominant allele. Figure 10 shows the inheritance of polydactyly in one family.

Figure 10
Figure 10

Figure 10

Enlarged question image

Figure 10
Key
D
allele for having polydactyly
d
allele for not having polydactyly

Persons 6 and 7 are expecting a fourth child. A doctor states that the probability of having a child with polydactyly is 0.5. Explain how the doctor determined this probability.

  • draw a Punnett square diagram
  • give the genotype of person 6 and the genotype of person 7
  • identify all the offspring that will have polydactyly

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Marking points

  1. 1 Gives person 6 as Dd and/or her gametes as D and d.
  2. 2 Gives person 7 as dd and/or his gametes as d and d.
  3. 3 Derives Dd, Dd, dd and dd in a correct Punnett square.
  4. 4 Identifies the two Dd outcomes as affected and links two of four outcomes to probability 0.5.

Full-mark answer

Person 6 is Dd and produces D or d gametes. Person 7 is dd and produces d gametes only. A Punnett square gives Dd, Dd, dd and dd. The two Dd offspring have polydactyly, so 2 out of 4 outcomes are affected: 2/4 = 0.5.

Why this answer loses marks

The previous unaffected children mean the next child is likely to be unaffected.

Previous births do not change the cross; derive both parents’ genotypes, their gametes and all Punnett outcomes.