A woman has a cell with 46 chromosomes in which chromosome 21 fails to separate…
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A woman has a cell with chromosomes in which chromosome 21 fails to separate. Her normal gametes carry chromosomes. Her partner's sperm are all normal and carry chromosomes.
- The failure takes place in meiosis I. How many of the four eggs have chromosomes?
- The failure takes place in meiosis II. How many of the four eggs are normal?
- How many chromosomes does the zygote have if an egg with chromosomes is fertilised by a normal sperm?
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The worked solution
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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.
Given. .
Plan. Follow the four gametes.
Step 1. (a) In meiosis I, one cell gets both chromosomes 21 and the other cell gets none. The two cells then divide normally. So eggs have chromosomes and have .
Step 2. (b) In meiosis II, only one of the two cells fails. It gives one egg with and one egg with . The other cell gives normal eggs with .
Step 3. (c) chromosomes, a trisomy of chromosome 21.
Answer: eggs; eggs; chromosomes.
Check. An egg with chromosomes and a normal sperm give , a monosomy.
Watch out. Meiosis I leaves no normal egg, but meiosis II leaves two.
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