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Examination of a karyotype taken from a metaphase preparation reveals the presence of an extra
chromosome. Which of the following is the most common mechanism of producing this phenomenon?
A. Balanced translocation
B. Chromosomal breakage
C. Fertilization by two sperm
D. Nondisjunction
E. Unbalanced translocation
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The correct answer is D. Nondisjunction can occur in both meiosis and mitosis, and refers to a
failure of paired chromosomes to separate and go to different daughter cells. When this happens,
one daughter cell gets an extra chromosome and the other daughter cell is "short" one
chromosome.
A balanced translocation (choice A) is an exchange of genetic material between non-homologous
chromosomes that preserves all critical genetic material.
Chromosomal breakage (choice B) produces fragmented chromosomes and can contribute to
tumorigenesis.
Fertilization by two sperm (choice C) produces triploidy and is seen in many spontaneously
aborted fetuses.
An unbalanced translocation (choice E) occurs when non-homologous chromosomes exchange genetic
material with a net loss or gain of critical genetic material.
