This is an example of trisomy 21 (47, XY, +21) also known as Down syndrome. Note the extra chromosome 21. Additions or deletions of genetic material are generally lethal in utero. Trisomy 21 is an example of one form of addition in which the fetus may occasionally survive to term and beyond. Of those born with Down syndrome, 1/6 die within the first year and the average life span is 16.2 years. The non-dysjunctional event in meiosis that produces this anomaly increases in incidence with increasing maternal age. Trisomy 21, one of the most common causes of mental retardation. The child can have an IQ between 25-74. An average person has an IQ between 90-110. This results in a number of characteristic features, such as short stature, broad hands, stubby fingers and toes, a wide rounded face, a large protruding tongue that makes speech difficult. Individuals with this syndrome have a high incidence of respiratory infections, heart defects, and leukemia. The average risk of having a child with trisomy 21 is 1/750 live births. Mothers in their early twenties have a risk of 1/1,500 and women over 35 have a risk factor of 1/70, which jumps top 1/25 for women 45 and over.
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