The primary reason that Down syndrome is more common in individuals with Turner syndrome (45,X) is

Core Concept

The primary reason that Down syndrome is more common in individuals with Turner syndrome (45,X) is the presence of an extra X chromosome. Individuals with Turner syndrome typically have only one X chromosome, which can lead to a greater risk of mosaic Down syndrome (47,XX,+21) due to the presence of an extra X chromosome in some cells. Option A is incorrect because the lack of a second X chromosome would not increase the risk of Down syndrome. Option C is incorrect because the presence of an extra Y chromosome is not related to Down syndrome. Option D is incorrect because the increased rate of mitotic errors is not the primary reason for the increased risk of Down syndrome in Turner syndrome.