Final answer:
The absence of the Y chromosome in a fetus leads to the development of the Müllerian ducts into the female reproductive tract, as the default development pathway in humans is female.
Step-by-step explanation:
The absence of the Y chromosome in the fetus will further the development of the Müllerian ducts into the female reproductive tract. During fetal development, if there is no Y chromosome present, specifically no SRY gene expression, the bipotential gonads will naturally develop into ovaries. The Wolffian ducts, which would have developed into male reproductive structures, degenerate, whilst the Müllerian ducts form the uterine tubes, uterus, and are involved in the formation of the vagina alongside the cloaca.
The Y chromosome is crucial in determining the development of the male reproductive tract; its absence means that the fetus will follow the default pathway towards female sexual development, creating a uterus environment suitable for a developing embryo and fetus, characteristic of viviparity in mammals. This is why females, lacking the Y chromosome, do not develop male reproductive organs and complete their development of eggs before birth.