A 22-year-old man complains about his inability to conceive a child. On physical examination, the patient is noted to be tall (6 ft, 5 in) and exhibits gynecomastia and testicular atrophy. Laboratory studies demonstrate increased serum levels of follicle-stimulating hormone. Cytogenetic studies reveal a chromosomal abnormality. What is the most common cause of this patient's chromosomal abnormality?
Klinefelter syndrome, or testicular dysgenesis, is related to the presence of one or more X chromosomes in excess of the normal male XY complement. Most persons with Klinefelter syndrome (80%) have one extra X chromosome (47, XXY karyotype). The additional X chromosome(s) arises as a result of nondisjunction during gametogenesis. In half of cases, nondisjunction occurs during paternal meiosis I, leading to a sperm containing both an X and a Y chromosome. Fertilization of a normal oocyte by such a sperm gives a zygote with a 47, XXY complement of chromosomes. Klinefelter syndrome occurs in 1 per 1,000 male newborns, which is roughly comparable to the incidence of Down syndrome. None of the other choices are associated with trisomy.Diagnosis: Klinefelter syndrome
Expansion of a trinucleotide repeat
Isochromosome formation
Meiotic nondisjunction
Nonreciprocal translocation
Genetics