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Old 10-02-2011
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Genetics Coarse facies, stiff joints, and mental retardation

A 4-year-old boy presents to the physician's office with coarse facies, short stature, stiffening of the joints, and mental retardation. Both parents, a 10-year-old sister, and an 8-year-old brother all appear unaffected. The patient's mother is pregnant. She had a brother who died at 15 years of age with similar findings that seemed to worsen with age.
She also has a nephew (her sister's son) who exhibits similar features. Based on the probable mode of inheritance, what is the risk that her fetus is affected?
A. 100%
B. 67%
C. 50%
D. 25%
E. Virtually 0%
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Old 10-02-2011
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d.25 % ....
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Old 10-02-2011
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The answer is: D
The fact that the mother of the affected child has an affected brother and an affected nephew through her sister suggests X-linked recessive inheritance. This is made more likely because the symptoms suggest a mucopolysaccharidosis (storage of glycosaminoglycans) and because one type exhibits X-linked recessive inheritance [Hunter's syndrome or MPS type II ]. When evaluating the possibility of an X-linked disorder, it is important to remember the pattern of inheritance of the Xchromosome . Females have two Xchromosomes, which are passed along in a random fashion. They pass any given X chromosome to 50% of their sons and 50% of their daughters. For an X-linked recessive condition, those daughters who inherit the affected allele are heterozygous carriers of the disorder but are not affected. Since males have only one X chromosome, those who inherit the affected allele are affected with the disorder. Given X-linked recessive inheritance, the mother must have the abnormal allele on one of her X chromosomes (she is an obligate carrier) in order for her son and brother to be affected. The fetus thus has a 1/2 chance of being a boy and a 1/2 chance of being affected given male sex, resulting in a 1/4 (25%) overall risk of being affected.
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