Solving SingleTrait Genetics Problems SingleTrait Cross Example #1: Let's try an example of a singletrait cross. In a particular type of fish, a "G" allele produces green scales, which is completely dominant over the "g" allele which produces blue scales. What are the genotypic and phenotypic ratios from a cross beween two heterozygous fish? By definition, a heterozygous fish would have a genotype of Gg. It could produce gametes (sperm or eggs) that are either G or g. We recommend that you always first write out a "legend" based on what you know from the question. Do it like this: G = green scales G is completely dominant over g, and thus: genotype of male fish = Gg (sperm are G or g) We then set up a Punnett square like this:
Then we combine the gametes to fill in the rest of the chart with the possible genotypes for the offspring:
Our final answer to the question: Note that in the Punnett square 2 of the 4 rectangles contain Gg, and thus we give Gg a probability of 2/4, or 50%.
Note that we can combine the two probabilities for green scales to make our phenotypic ratios simply 3/4 (75%) green scales and 1/4 (25%) blue scales. We have thus shown the probabilities for both the genotypes and phenotypes for a cross between these two fishes. SingleTrait Cross Example #2: Let's try another example using the same species of fish from example 1. Let's cross a male heterozygous fish with a female bluescaled fish. What genotypic and phenotypic ratios do we get if we cross these two fish? Our "legend": G = green scales G is completely dominant over g, and thus: genotype of male fish = Gg (sperm are G or g) We then set up a Punnett square like this:
Then we combine the gametes to fill in the rest of the chart with the possible genotypes for the offspring:
Our final answer to the question:
We have thus shown the probabilities for both the genotypes and phenotypes for a cross between these two fishes.

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