What phenomenon explains the presence of both 45,X and 46,XX karyotypes in a patient with Turner syndrome?

• A. Genetic recombination
• B. Loss of heterozygosity
• C. Postfertilization nondisjunction
• D. Translocation

Answer: (C)

The phenomenon seen in Turner syndrome, where both 45,X and 46,XX karyotypes coexist in a patient, can be primarily attributed to postfertilization nondisjunction. This occurs when there is an error during cell division after fertilization, leading to loss of one sex chromosome in some of the daughter cells.

In the context of Turner syndrome, the presence of 45,X indicates that in certain cells, one X chromosome is missing due to this nondisjunction event. Meanwhile, some cells may retain the complete set, leading to the 46,XX configuration. This mosaicism is not due to a failure in the initial separation of chromosomes during meiosis in the parents (which would lead to a single karyotype) but rather happens during later cell division in the developing embryo.

This process is crucial in understanding the range of phenotypic expressions observed in Turner syndrome, as the proportion of 45,X versus 46,XX cells can influence the severity of clinical manifestations.