Question

XX males and XY females are too rare to occur in human populations. If it happens so it would be due to

• A. Genes of Y chromosomes are on X
• B. Crossing over occurs between X and X
• C. Non-disjunction of chromosomes
• D. All of the above

Answer 1

The correct option is A Genes of Y chromosomes are on X

The Y-chromosome carries SRY gene which encodes TDF (testis determining factor). The TDF serves as a transcription factor that binds to specific DNA sequence to start the process to develop the embryo into male child. Loss of this gene hinders the development of testes and the embryo develop female gonads despite male genotype (XY female). If the SRY gene is copied on the X-chromosome, the female genotype will show the male phenotype (XX male).

So, the correct answer is option A.