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Pedigree evidence for heterozygosity
Polydactyly is an inherited disorder caused by a dominant allele. Figure 10 shows the inheritance of polydactyly in one family.
Key
- D
- allele for having polydactyly
- d
- allele for not having polydactyly
Person 1 is heterozygous. Explain how Figure 10 shows that person 1 is heterozygous.
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Marking points
- 1 Uses person 1's polydactyly phenotype to infer a D allele.
- 2 Uses unaffected child 5 (dd) to infer that person 1 also supplied a d allele.
Full-mark answer
Person 1 has polydactyly, so he has a D allele. Person 5 does not have polydactyly and must be dd, so person 5 inherited a d allele from person 1. Person 1 is therefore Dd.
Why this answer loses marks
Person 1 is affected, so they must be heterozygous.
Being affected shows a dominant allele but not the recessive one; the unaffected child proves the recessive allele was passed on.