A trait such as skin colour is controlled by three genes, A, B and C…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A trait such as skin colour is controlled by three genes, A, B and C. Each dominant (capital) allele adds one unit of pigment, and each small allele adds none. Two people with the genotype AaBbCc have children. The offspring are sorted by the number of capital alleles they carry, from to . The numbers of alleles follow the binomial coefficients of .
- How many children have exactly capital alleles, like their parents?
- How many children have or capital alleles and so are darker than both parents?
- How many children have no capital allele at all?
Need a reminder? The key ideaShow
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. AaBbCc × AaBbCc, children. The classes come in the ratio , out of .
Plan. One part is children. Multiply by the number of parts in each class.
Step 1. (a) Exactly capital alleles has parts: children.
Step 2. (b) Five capital alleles has parts and six capital alleles has part, together parts: children.
Step 3. (c) No capital allele has part: children.
Answer: children; children; children.
Check. The seven classes hold children.
Watch out. The class with the most children is the middle class, the same as the parents. Very dark and very light children are rare.
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