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Review Question - QID 106554

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QID 106554 (Type "106554" in App Search)
A 1-year-old boy with blonde hair, blue eyes, and fair skin is found to have mental retardation, eczema, and vomiting. A mousy body odor is apparent. Which of the following pedigrees best resembles the inheritance pattern of this disease?
  • A
  • B
  • C
  • D
  • E

Figure A

21%

81/391

Figure B

51%

199/391

Figure C

10%

39/391

Figure D

9%

37/391

Figure E

4%

15/391

  • A
  • B
  • C
  • D
  • E

Select Answer to see Preferred Response

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Phenylketonuria (PKU) is an autosomal recessive disease due to a decrease in phenylalanine hydroxylase that occurs in approximately 1 in 10,000 people. Autosomal recessive diseases follow an inheritance pattern as shown in Figure B.

There are several modes of inheritance, with the most common being autosomal dominant, autosomal recessive, X-linked recessive, X-linked dominant, and mitochondrial inheritance. Autosomal recessive modes of inheritance affects males and females equally. The disease is silent in heterozygotes, but 1/4 of the children of carrier parents will be homozygous and affected by the disease. Examples of autosomal recessive diseases include cystic fibrosis, inborn errors of metabolism (e.g. PKU), sickle cell anemia, and hemochromatosis.

Figures explained in the Incorrect Answers section.

Incorrect Answers:
Answer 1: Figure A demonstrates the X-linked recessive mode of inheritance. Note that 50% of sons from the carrier mothers are affected. There is no male-to-male transmission.
Answer 3: Figure C demonstrates the X-linked dominant mode of inheritance. Note that all the daughters of the affected fathers have the disease.
Answer 4: Figure D demonstrates the autosomal dominant mode of inheritance. Note that every generation is affected.
Answer 5: Figure E demonstrates the mitochondrial inheritance pattern. Note that all the children of the affected mother have the disease.

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