Sex-Linked Inheritance Follows Chromosome Size

Lab Brief #6

🎯 Your mission

Learn the rule. Then catch nature using it.

⚡ The twist

Nature loves to surprise the careful watcher.

What you'll learn

The X chromosome carries many genes that the much shorter Y chromosome does not, so a male carrying one recessive allele on his X has no second copy to mask it, while a female would need two. Red-green colour blindness and haemophilia therefore appear far more often in males, and are typically inherited from a carrier mother rather than from an affected father. The pattern is a direct consequence of chromosome size, not of anything about the trait itself.

Key words

sex-linked
Carried on the X or Y chromosome, so the inheritance pattern differs between males and females.
carrier
A female heterozygous for a recessive X-linked allele, who does not show the trait but can pass it on.

Make a hypothesis first

Before you start: write down what you think will happen, and why.

Say your guess out loud — then find out if you were right.

Step 1 of 4 · Read

Think about what you just learned: say what "sex-linked" and "carrier" mean in your own words before you start.

Step 2 of 4 · Do

Complete Punnett squares for a carrier mother with an unaffected father, and for an affected father with an unaffected non-carrier mother. Compare the proportion of affected sons and daughters in each, and explain the difference from chromosome size alone.

Step 3 of 4 · Observe

Record what actually happened, including anything that did not match what you expected.

Step 4 of 4 · Record

Write a conclusion that answers the question this lesson opened with, and state your uncertainty.

Question 1 of 3

Red-green colour blindness is commoner in males because they have:

How sure are you?😐 maybe

Question 2 of 3

A carrier is a female who:

How sure are you?😐 maybe

Question 3 of 3

Sex-linked patterns arise because the Y chromosome:

How sure are you?😐 maybe

Mind = blown

🔬 The more you look, the more nature shows you.

Nice work!

You got 0 of 3 right.

All lessons

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👨‍👩‍👧 For the dinner table: “What's the most surprising thing you learned today?”

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Sex-Linked Inheritance Follows Chromosome Size

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