At what stage in the life history of a mammal is the sex of an individual set?

Step 1: Analyze the question The question asks when an individual mammal's sex is determined (set). This is about the genetic basis of sex.

Step 2: Explain the concept In mammals, sex is determined by the sex chromosomes:

• Females have two X chromosomes (XX).
• Males have one X and one Y chromosome (XY).

Every egg (ovum) from the mother carries an X chromosome. Sperm cells from the father carry either an X or a Y chromosome.

Step 3: Work through it

• If an X-bearing sperm fertilises the egg, the zygote is XX, so the baby is female.
• If a Y-bearing sperm fertilises the egg, the zygote is XY, so the baby is male.

The sex is therefore fixed at the moment of fertilisation, when the sperm and egg fuse. This moment is called conception.

Now consider the other options:

• A. Adolescence and B. Puberty: These are stages when secondary sexual characteristics develop (for example, breast development, a deeper voice, facial hair). The sex was already determined long before. These stages only bring out the characteristics.
• C. Birth: The baby's sex is only observed or revealed at birth. It was set much earlier.
• D. Conception: This is when the chromosomes combine and the sex is genetically determined.

Step 4: Answer The correct answer is D. at conception.

Tip: Remember "XX = girl, XY = boy, decided at fertilisation by the father's sperm." Words like adolescence, puberty and birth describe when sex is shown, not when it is set. If a question uses the word "determined" or "set", think "zygote" or "fertilisation".