Chromosomal sex determination
Of the 46 chromosomes in each human cell except sperm and egg cells (which have only half that number), 44 are non-sex chromosomes or "autosomes." The other two are the sex chromosomes. Sex chromosomes come in only two varieties: an X and a Y. If one of the two chromosomes is an X and the other is a Y, the individual is male. If both chromosomes are X, the individual is female. In addition to its sex-determining role, the X chromosome includes a great deal of other information. This is because the X chromosome contains several times the number of genes contained in the Y chromosome.
X and Y Chromosomes - NASA
Of course, this was not always known. The first indication of a chromosomal mechanism for sex determination is traceable to experiments carried out by Thomas Hunt Morgan and his students in the early Twentieth Century. In researching a batch of fruit flies, Drosophila melanogaster, which typically have red eyes but in which there were some with white eyes, he noticed that all of the white-eyed flies present were male.
He already knew female flies have two X chromosomes, while males have only one. He correctly concluded from this that the white-eye color trait is on the X chromosome. Female flies rarely have white eyes because a white-eye trait on one X chromosome is likely to be cancelled out by a much more prevalent red version on the second X chromosome. Males, on the other hand, only have one X chromosome, and if contains the white trait, the eyes of the fly must be white.
This work demonstrates that the X chromosome is an important factor in sex determination. It also was the basis for further use of the fruit fly by later genetics researchers. For his work, Morgan was awarded the Nobel Prize in physiology in 1933.