What is the primary genetic defect in androgen insensitivity syndrome?

The primary genetic defect in androgen insensitivity syndrome (AIS) is the lack of androgen receptors. This condition occurs due to mutations in the androgen receptor gene located on the X chromosome, which is responsible for coding for the receptors that bind and respond to androgens, such as testosterone. As a result, even though individuals with AIS produce testosterone, their cells are unable to respond to it due to the absence or malfunction of androgen receptors. This leads to the development of female characteristics despite having a male (XY) genotype, as the body cannot utilize the available androgens to promote male sexual development.

In the context of the other options, the absence of the Y chromosome is not relevant because individuals with AIS typically have one. Defective testosterone synthesis does not occur in AIS, as testosterone is normally produced; the issue lies in the body's inability to utilize it. Finally, the SRY gene is present, as it is located on the Y chromosome and crucial for initiating male sex differentiation, though in AIS, the downstream effects of this differentiation are compromised due to the lack of androgen receptor function.