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Genetics Info & Tutorial: Double Mutants |
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Some animals have more than one recessive mutation. An example of this is a snow cornsnake. This snake is amelanistic, and anerythristic. Anerythrism, often shortened to “Anery,” is a mutation that keeps the snake from producing erythrin, the red pigment. (notice that putting the prefix “A” or “An” in front of a pigment means it is lacking) When mixing two different mutations, it is important to understand that they will not affect each other’s inheritance as long as they are not in the same locus. In other words, the gene that helps produce melanin and is mutated in amelanism, is in a different place than the gene that affects red pigment production in anery cornsnakes. As a result, when determining the sperm/egg possibilities for each parent, realize that which “A” gene they get is independent of which “B” gene they get. What this means is that the Punnett squares for two mutations can get a little more complicated... Let’s walk through a snow corn Punnett square. In this example, we will use “a” for amelanism and “e” for anerythrism. Note that it’s not important which letters or symbols you use for your Punnett squares as long as you know which gene each stands for, so pick what you are comfortable with. In this example we can first mate a snow corn to a (homozygous) normal, and then two of the offspring together. The Snow corn is going to be what genotype? Correct, it’s “aa ee” and the normal is “ AA EE.” Let’s build our Punnett square: |
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Notice that we can get the same offspring from mating an anery (AAee) to an amel (aaEE) |
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As you can see, all of the offspring will be the same. If you recall, Aa is “het for amel” and you’ve probably already figured out that Ee is “het for anery.” These animals can be called “het for amel and anery,” or “double het,” or “het for snow.” Now let’s cross two of these “het for snow” animals to each other. Since we have two mutations, and two different genes for each “locus” (the mutant and the wild-type) we need to figure out which combinations they can have. One way to do this is similar to a Punnett square. You take the first mutation as one parent, and the second as the other, like so: |
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One of the difficulties is knowing what to do with the results. One thing you can do is find out which offspring will show mutations. Remember, those with “aa” will be amels, those with “ee” will be anerys, and those with “aaee” will be snow. In this example, I’ve colored in the snow as white, the anery as black, and the amel as red. As you can see, out of 16 babies, you are likely to get 3 amel, 3 anery, and 1 snow. |
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Notice that of the remaining animals (the blue squares) that 2 out of 3 of each of them is het for anery, and 2 out of 3 are het for amel. In this clutch, each baby (including the amels and anerys) that is not showing a particular mutation is then 66% likely to be het for that mutation. |
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