In a typical female with two X chromosomes, how many Barr bodies are present?

In a typical female with two X chromosomes, how many Barr bodies are present?

• A. 0
• B. 1
• C. 2
• D. 3

Answer: (B)

In female mammals, one of the X chromosomes is inactivated to balance gene expression with males. This inactivation turns the chromosome into a densely packed Barr body and renders it transcriptionally silent, while the other X remains active to provide necessary gene products. With two X chromosomes, one X becomes a Barr body and the other stays active, so there is one Barr body. If more X chromosomes were present, there would be additional Barr bodies—specifically, the number of X chromosomes minus one. Males, having a single X, typically have no Barr bodies.

In female mammals, one of the X chromosomes is inactivated to balance gene expression with males. This inactivation turns the chromosome into a densely packed Barr body and renders it transcriptionally silent, while the other X remains active to provide necessary gene products. With two X chromosomes, one X becomes a Barr body and the other stays active, so there is one Barr body. If more X chromosomes were present, there would be additional Barr bodies—specifically, the number of X chromosomes minus one. Males, having a single X, typically have no Barr bodies.