Biological Systems MCAT Practice Exam

Which chromosomes designate a male organism?

1. XX

2. XY

3. YY

4. XXY

The correct answer is: XY

In many species, including humans, the sex of an organism is determined by specific combinations of sex chromosomes. Males typically have one X chromosome and one Y chromosome, constituting an XY chromosomal configuration. This pair of chromosomes results in the development of male secondary sexual characteristics due to the presence of the Y chromosome, which carries the SRY gene responsible for initiating male sex determination and promoting the development of testes. The XX chromosome configuration is associated with females, as they possess two X chromosomes. The YY configuration is not viable for development in humans and would not result in a male organism. The XXY configuration is an atypical arrangement that can lead to conditions like Klinefelter syndrome, which affects male development but does not correspond to the standard male chromosomal pattern. Therefore, the XY combination is the definitive designation of a male organism in species where sex is determined chromosomally.