The sex of a human child is determined by
Correct answer: D. the sperm, which carries either an X or a Y chromosome
- A. the ovum, which may carry either an X or a Y chromosome
- B. the hormones of the mother during pregnancy
- C. the total number of chromosomes in the zygote
- D. the sperm, which carries either an X or a Y chromosome
Explanation
The mother is XX and so every ovum carries an X, while the father is XY and produces X bearing and Y bearing sperm in equal numbers, so it is the sperm that decides. A Y bearing sperm gives an XY male and an X bearing sperm an XX female, which also explains the roughly equal sex ratio at birth. Blaming the mother for the sex of a child has no biological basis whatsoever.
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About X-linked Recessive Inheritance
X-linked recessive inheritance involves genes on the X chromosome, so males express a recessive allele with no corresponding allele on their single X chromosome, while females may be carriers. Pedigrees cover haemophilia and red-green colour blindness, including carrier mothers, affected sons and the absence of father-to-son transmission.
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