Final answer:
The combination of sex chromosomes (XX vs XY or ZW vs ZZ) determines gender and development of certain traits.
Step-by-step explanation:
The sex of an individual is determined by their sex chromosomes. In humans, females have two X chromosomes (XX), while males have one X chromosome and one Y chromosome (XY). The presence of the Y chromosome triggers the development of male characteristics, while its absence results in female characteristics.
Other organisms, such as birds, fish, crustaceans, insects, and reptiles, have different systems of sex determination. For example, in birds, sex determination is dependent on the combination of Z and W chromosomes. Homozygous for Z (ZZ) results in a male, while heterozygous (ZW) results in a female.
Overall, the combination of sex chromosomes, whether it is XX vs XY or ZW vs ZZ, determines the gender and development of certain traits in an individual.