S4.2.4 For the adult

Boy or girl: X and Y

What decides whether a baby is a boy or a girl?

What this chapter teaches

In most people, XX develops as female and XY as male. Every egg carries an X, and a sperm carries X or Y, so the father's sperm decides. Each pregnancy is about a 1 in 2 chance of each, and chance has no memory.

Three questions to ask

There is no one right wording. Listen for the idea.

  1. Which parent's cell decides whether a baby has XX or XY, and why?
  2. A family has four girls. What is the chance the fifth baby is a boy?
  3. Why does a Punnett square give a chance and not a promise?

After the chapter, the learner can

  • Use a Punnett square to show how X and Y chromosomes decide sex
  • Explain why the chance of a boy or a girl is about even

What they copy into their notebook

XX usually develops as female and XY as male. Every egg carries X. A sperm carries X or Y, so the sperm decides. Each baby has about a 1 in 2 chance of each. Chance has no memory.

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