alleryStart learning →Alkaptonuria is recessive. Neither parent in a family is affected, yet their child is. What must the parents be?
ABoth heterozygous carriers
BBoth homozygous recessive
CBoth homozygous dominant
DOne affected and one carrier
Answer & Solution
Correct answer: A. Both heterozygous carriers
1. Affected individuals have the genotype aa, and unaffected individuals are AA or Aa.
2. The affected child is aa, so it received one a allele from each parent.
3. Each parent therefore carries at least one a allele.
4. Neither parent is affected, so neither can be aa.
5. The only genotype left that carries an a allele without showing the disorder is Aa.
6. Both parents must therefore be heterozygous carriers.
7. Two homozygous dominant parents could not supply an a allele at all, so no affected child could arise.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 12 "Mendel's Experiments and Heredity", section 12.2 Characteristics and Traits_
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