Sex determination

Correct answer: A. Because a father’s sperm can carry either an X or a Y chromosome

In the usual human XX–XY system, every egg produced by the mother carries an X chromosome. A father produces two types of sperm: some carry X and some carry Y. Fertilisation by an X-bearing sperm gives XX, while fertilisation by a Y-bearing sperm gives XY. Thus the sperm determines whether the zygote is XX or XY, making option A correct. This does not mean that the father controls the outcome consciously.

Correct answer: A. The mother always contributes X, while the father can contribute either X or Y

In the usual human chromosomal system, the mother is XX, so every ovum carries an X chromosome. The father is XY, so sperm may carry either X or Y. Fertilisation by an X-bearing sperm produces XX, while fertilisation by a Y-bearing sperm produces XY. Therefore option A is correct. The mother contributes a chromosome, and the other options incorrectly deny chromosomal determination or the father’s two sperm types.

Correct answer: A. The mother contributes X, while the father’s X- or Y-bearing sperm determines the sex-chromosome combination

The usual human XX-XY mechanism shows why blaming the mother is scientifically unjustified. Every normal ovum contributes an X chromosome. A sperm contributes either X or Y; XX generally develops as female and XY generally as male. Thus the father’s sperm determines which sex-chromosome combination forms, although fertilisation is a contribution from both parents. Option A is correct; B, C and D contradict this mechanism.

Correct answer: A. Sperm carrying an X chromosome

In the standard human system, the ovum always contributes one X chromosome because the mother is XX. A female chromosomal combination is XX, so the fertilising sperm must also contribute X: X from the ovum plus X from the sperm gives XX. A Y-bearing sperm would produce XY, while a sperm without a chromosome or both sperm together does not describe normal fertilisation. Therefore option A is correct.

Correct answer: A. XY, usually male

The human ovum normally contributes X, while a Y-bearing sperm contributes Y. Their fusion gives X + Y = XY, the usual male sex-chromosome combination. An X-bearing sperm would instead give X + X = XX, generally female. Only one sperm normally fuses with one ovum, so both combinations do not form together, and fertilisation does produce a definite chromosomal combination. Therefore option A is correct.

Correct answer: A. Because sperm can contribute either X or Y chromosome

The governing concept is the XX–XY mechanism of human sex determination. A mother’s egg normally contributes an X chromosome, whereas a father produces sperm carrying either X or Y. Fertilisation with an X-bearing sperm gives XX, while fertilisation with a Y-bearing sperm gives XY. Thus the sperm determines which combination forms. Option A is correct; B, C, and D incorrectly describe the chromosomes contributed by the mother or father.

Correct answer: A. A sperm can contribute one of two types of sex chromosomes

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.

Correct answer: A. The mother always contributes an X chromosome, while sperm type creates the sex-chromosome difference

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.

Correct answer: A. Because the two types of sperm are produced with nearly equal probability

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.