Final answer:
Sex determination in many organisms is based on the presence and combination of sex chromosomes; in mammals, XX denotes female and XY denotes male, while avian species use a ZW system.
Step-by-step explanation:
The type of chromosome that provides the basis for sex determination in many sexually reproducing organisms is referred to as the sex chromosomes. In mammals, including humans, sex determination is typically based on the XY system, where the presence of an X and Y chromosome combination results in a male (XY), and the presence of two X chromosomes results in a female (XX). Pioneering scientist Nettie Stevens was instrumental in discovering this, noting differences in chromosomes between sexes in mealworms through microscopy, which then extended to other species. Remarkably, the mechanism of sex determination can vary among different organisms. For example, avian species utilize a ZW system, where ZZ results in a male and ZW in a female. Additionally, some species, like certain fish, may have even more complex systems involving multiple sex chromosomes. In humans, the presence of the Y chromosome initiates the development of male characteristics, while its absence results in female characteristics. Furthermore, in human females, one of the X chromosomes becomes inactivated and is known as a Barr body, balancing the dosage of X chromosome genes between the sexes.