Human Genetics Handout Spr08

Down syndrome children share a number of physical

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Unformatted text preview: Down Syndrome (190685): Trisomic 21. Down syndrome children share a number of physical characteristics, including a broad head; rounded face; protruding tongue; flattened bridge of the nose; epicanthic folds of the eye; small, irregular teeth; short, heavy stature; stubby fingers and toes. The modal IQ is 42. Many children with Down syndrome have congenital heart defects and are prone to respiratory infections and leukemia. Improvements in medical care in recent decades have increased survival rates dramatically, but few individuals with Down syndrome reach the age of 50. More than 90% of the extra copies of chromosome 21 are inherited from the mother and the frequency of Down syndrome is related to the age of the mother. For example, the frequency of Down syndrome offspring is 1/2000 for 20 year old mothers, increases to 1/250 by age 35, and to 1/100 by age 40. These increases may be due to a prolonged Meiosis I in aged females, but this possibility has not been proven, and there are other, equally tenable theories to explain these observations. Edwards Syndrome: Trisomic 18. Small at birth, overlapping fingers, malformed feet, heart and kidney deformities, severe mental retardation. Most die of heart failure or pneumonia early in their first year. Frequency = 1/11,000 live births; 80% are female (reason unknown). Patau Syndrome: Trisomic 13. Non-functional eyes, cleft lip and palate, small cranium, polydactyly, heart defects, severe mental retardation. Survival is rarely past the first few months after birth. Frequency = 1/15,000 live births. Like Down syndrome, both Edwards and Patau syndromes increase in frequency with the age of the mother. ***Cri du Chat: Loss of all or part of the short arm of chromosome 5. Affected individuals have a cry that sounds like a cat meowing in d...
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