Anthony P. Solli
In recent years, scientists have found the genetic basis for Down syndrome. This unfortunate condition results from an extra twenty-first chromosome in all of the body cells, resulting from nondisjunction during meiosis, in egg or sperm formation. Down syndrome is often referred to as trisomy-21 because of the presence of three twenty-first chromosomes (Figure 3). All other chromosomes are present in normal numbers.
Down syndrome is marked by moderate mental retardation and abnormal physical characteristics, including an enlarged tongue, slanted eyes, and muscle weakness. Palm and footprints are also abnormal.
About one in 800 babies are born with Down syndrome. The number is lower, about one in 1300 in mothers under 20 years of age, but increases to one in 32 in mothers over 45 years of age (Figure 4). Some scientists feel the result is from the decreased likelihood of the two chromosomes (pair 21) to separate during oogenesis in the older woman. Factors involved in faulty spermatogenesis are not yet discovered. It is known that one-quarter or more of Down syndrome persons have an extra chromosome 21 that originated with their fathers.
Figure 3. Female chromosome 21 trisomy (Down syndrome).
(figure available in print form)
Figure 4. Maternal age and the production of Down syndrome offspring.
(figure available in print form)
