Sex determination

In the usual human chromosomal system, females have two X sex chromosomes, written as XX, while males generally have one X and one Y, written as XY. An egg contributes an X chromosome, and an X-bearing sperm contributes the second X, producing an XX zygote. Therefore A is correct. YY is not a normal viable human combination, and XO represents a different chromosomal condition rather than the usual pattern.

In the usual human XX–XY system, the mother’s egg contributes an X chromosome in every fertilisation. The father produces two broad sperm types: X-bearing and Y-bearing. Fusion of an X sperm with the egg usually gives XX, while fusion of a Y sperm usually gives XY. Therefore the sperm contribution determines the chromosomal sex; this does not mean that either parent morally controls the outcome.

For the usual human XX–XY pattern, every egg carries X, whereas sperm may carry X or Y. Fertilisation with an X-bearing sperm generally produces XX, and fertilisation with a Y-bearing sperm generally produces XY. Thus blaming the mother for giving birth to a girl is scientifically unjustified. The result is a biological outcome of fertilisation, not a choice or fault of the mother.

In the standard human XX–XY system, every egg contributes X, while sperm are produced as X-bearing or Y-bearing types. If these two types are generated and participate in fertilisation with approximately equal probability, XX and XY zygotes also have approximately equal expected probabilities. This is a population-level expectation, not a guarantee for every family or a result controlled by the mother.

The governing concept is the human sex-chromosome system. A typical human female has XX sex chromosomes, while a typical human male has XY. The egg contributes X, and a Y-bearing sperm contributes Y, producing the XY combination. Therefore, option B is correct. XX is the usual female combination; YY is not a viable normal human combination, and a single X does not represent the usual diploid chromosome state.