Diagnosis: Prenatal Testing for Trisomy 21
• Robertsonian translocations are whole-arm translocations between acrocentric chromosomes. In humans, the acrocentric chromosomes are 13, 14, 15, 21, and 22. As a result of this translocation, the derivative chromosome contains the complete long arms of the two acrocentric chromosomes fused together and this derivative chromosome lacks at least some short-arm chromatin.
• Robertsonian translocation carriers are increased risk for trisomy or monosomy because of unbalanced segregation at meiosis.
• Advanced maternal age is the most important etiologic factor in risk for aneuploidy and human reproductive failure. Association of advanced maternal age and Down syndrome was known even before the chromosomal basis of Down syndrome was discovered.
• Women with the karyotype 45,XX,t(21;21)(q10;q10) can have only trisomy 21 or monosomy 21 conceptions. Conceptions with monosomy 21 result in spontaneous abortions. A trisomy 21 conception may survive and result in a live birth with the infant having Down syndrome.
• Female carriers of the Robertsonian translocation (14;21) are at increased risk, a 10% to 15% risk of trisomy 21. The risk for male carriers is very different; it is less than 1%.
