EasyBiologyHigh School (9–10)GeneralHeredity and Evolution › Heredity and InheritanceNumericalBoard Exam
A couple has three daughters. What is the chance, as a percentage, that their fourth child will be…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A couple has three daughters. What is the chance, as a percentage, that their fourth child will be a son? Explain why the earlier births do not change it.
Need a reminder? The key ideaShow
In some reptiles the temperature of the eggs decides the sex, and some snails change sex. Human women are XX and men are XY. Every egg carries X; half the sperm carry X and half Y. An X sperm gives a girl, a Y sperm a boy, with an equal chance each time.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The sperm. Half the father's sperm carry X and half carry Y, every time.
Step 2. The chance. So the chance of a son is one half, that is .
Step 3. Earlier births. Each fertilisation is a new event, so the three daughters do not change the chance.
Answer: ; each birth has an even chance.
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