The probability of a baby being a boy or a girl depends on the contribution of the

Analyze the question: It asks which parent's contribution decides whether a baby is a boy or a girl.

Concept: Humans have 46 chromosomes: 44 autosomes (somatic chromosomes) and 2 sex chromosomes.

• Female: XX
• Male: XY

Step-by-step solution:

1. The mother is XX, so every egg she produces carries an X chromosome. She always contributes X, so she cannot cause a difference in the baby's sex.
2. The father is XY, so his sperm cells are of two kinds: about half carry X and half carry Y.
3. If a sperm carrying X fertilises the egg, the baby is XX, a girl. If a sperm carrying Y fertilises the egg, the baby is XY, a boy.
4. So the sex of the baby depends on which type of sex cell (sperm) the father contributes.

Checking the options:

• A. Father's sex cells: correct. The sperm carries either X or Y.
• B. Father's somatic chromosome: wrong. Somatic chromosomes (autosomes) do not determine sex.
• C. Mother's sex cell: wrong. All eggs carry X, so there is no variation.
• D. Mother's X-chromosome: wrong. The mother always gives X.

Exam tip: Remember "the Father decides". The father is the only parent who can give either X or Y, so he determines the sex of the child. Whenever you see a question about sex determination, look for the option with the father's sperm or sex cell.