In mammals, how is sex determined at conception?

Sex determination in mammals at conception is primarily governed by the combination of sex chromosomes that an individual receives from their parents. In mammals, there are two types of sex chromosomes: X and Y. Typically, females possess two X chromosomes (XX), while males have one X and one Y chromosome (XY). During fertilization, the egg contributes one X chromosome, and the sperm can contribute either an X or a Y chromosome.

If the sperm carries an X chromosome, the resulting combination will be XX, resulting in a female. Conversely, if the sperm carries a Y chromosome, the combination will be XY, leading to a male offspring. This chromosomal combination is the fundamental biological mechanism that determines the sex of the organism at the moment of conception.

The other options do not accurately describe the mechanism of sex determination. The number of chromosomes alone does not determine sex, as both males and females have the same total number of chromosomes; it is the type of sex chromosomes that matters. While dominant genes can influence traits and characteristics, they do not directly dictate the sex of the offspring. Lastly, external conditions can affect development after conception, but they do not influence the genetic determination of sex at the point of fertilization.