Example 1: Testing for a dihybrid ratio
Recall that in a dihybrid cross, you expect a 9:3:3:1 ratio of phenotypes -- if you don't recall this, you can review it in the module called "Counting Mice with Fangs". In that module, we considered two (hypothetical!) genes in mice. T and t coded for normal teeth or vampire fangs, respectively, and F and f coded for smooth or fuzzy fur, respectively.
A dihybrid mouse would have one copy of each of the 4 alleles and would look completely normal:
However, if
two such mice mated, they would have offspring that showed the whole range of possible phenotypes:
normal ( ),
fanged (
),
fuzzy (
), and
fuzzy and fanged (
).
Furthermore, these phenotypes should appear in approximately the ratio 9:3:3:1, resulting in a nursery
that looks something like this (if 16 babies were born):
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But, as we discussed in that module, the process of generating new mice is random, so the ratio will not be exact. Now we have the tools to test whether an actual litter approximates the 9:3:3:1 ratio.
If Mr. and Mrs. Mouse are both heterozygous for both traits (TtFf), then their offspring should follow the 9:3:3:1 ratio. So if they had 30 babies, how of each type would you expect?
all normal |
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fanged |
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fuzzy |
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fuzzy and fanged |
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Mr. and Mrs. Mouse have 15 normal, 7 fanged, 6 fuzzy, and 2 fuzzy fanged babies. Does Mr. Mouse have any cause for jealousy?
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