Three daughters in a row, a boy who can't tell red from green, and two healthy parents with a child who isn't. The same few rules of heredity explain all three.
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Students will explain that the father's sperm decides a child's sex with a one in two chance each time, show how a sex-linked trait passes on the X, read a simple pedigree, and explain why two carriers can have a child with a recessive disorder.
A sex-linked trait shows more often in males because a son has only one X chromosome, so a single recessive allele on it is enough. A daughter needs the recessive allele on both of her X chromosomes.
A carrier has one recessive allele and one dominant one, so usually has no symptoms. Two carriers have a one in four chance of an affected child with each pregnancy.
🚨 Thinking past girls make a boy more likely. Every child is a fresh coin flip, so after three girls the next child is still one in two.
Treating a disorder as someone's fault. Carrying a recessive allele is common, harms nobody, and isn't anyone's doing, which is why the lesson ends where it does.
Mixing up carrier and affected. A carrier has one recessive allele and no symptoms, and an affected person has two.
Go slowly through the disorders and let the student ask questions. If your family has a condition that runs in it, this is the lesson where it comes up, and the student should hear that the answer is nobody's fault.
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Before You Start
The words this lesson uses. Tap a card to see what it means, then come back to them whenever a question uses one.
Part One
Questions about the story you just read. Pick your answer straight off, or press Find it in the story if you would rather hunt for the sentence first.
Part Two
The word cards are at the top of this page. Look back at them as often as you need, then answer.