What determines the sex of offspring in many animal species?

The determination of the sex of offspring in many animal species is primarily governed by the combination of sex chromosomes inherited from both parents. In many species, including humans, sex is determined by specific sex chromosomes: typically, females possess two X chromosomes, while males possess one X and one Y chromosome.

During reproduction, the mother contributes an X chromosome, while the father can contribute either an X or a Y chromosome. The resulting combination of these chromosomes dictates the sex of the offspring. For instance, if the offspring inherits an X chromosome from the mother and an X from the father, it will develop as female. Conversely, if it inherits an X from the mother and a Y from the father, it will develop as male. This illustrates how the genetic contributions from both parents are crucial in determining the sex of the offspring.

In contrast, the other answer choices do not accurately reflect the biological mechanisms of sex determination. The mother's genetic contribution alone or the father's does not account for the necessary combination that defines the sex of the child. Additionally, while environmental factors can influence certain traits and conditions in offspring, they are not the primary determinants of genetic sex in those species that follow chromosomal sex determination.