Practice question

Down syndrome most common viable autosomal aneuploidy, frequency about 1 in 700 births rising sharply with maternal age after 35 years due prolonged oocyte arrest. Cytogenetically majority cases free trisomy 21 with 47,XX+21 or 47,XY+21 karyotype showing extra small acrocentric chromosome in metaphase. Phenotype includes characteristic facial appearance upslanting palpebral fissures, single palmar crease, hypotonia, intellectual disability variable, increased risk congenital heart defects, duodenal atresia, early Alzheimer-like pathology by fourth decade. Chromosome 21 smallest autosome containing about 233 protein coding genes explaining relative viability.