AP Biologymediummcq1 pt
Trisomy 21 (Down syndrome) results from
A.nondisjunction producing a gamete with two copies of chromosome 21
B.a point mutation in one gene
C.crossing over between nonhomologs
D.viral infection of the embryo
Explanation
Core Concept
Most cases of Down syndrome result when nondisjunction produces a gamete carrying an extra copy of chromosome 21, so fertilization yields trisomy 21. Less commonly, Down syndrome can arise through mechanisms such as Robertsonian translocation or mosaicism. The nondisjunction mechanism is therefore the major one, not the only possible origin.
Correct Answer
Anondisjunction producing a gamete with two copies of chromosome 21
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