Solving Single-Trait Genetics Problems

Single-Trait Cross Example #1: Let's try an example of a single-trait 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 = blue scales  

G is completely dominant over g, and thus:
GG = green scales, Gg = green scales, gg = blue scales

genotype of male fish = Gg (sperm are G or g)
genotype of female fish = Gg (eggs are G or g)
genotypes crossed in this mating = Gg X Gg               

We then set up a Punnett square like this:

 
sperm
G
g
eggs
G
.
.
g
.
.

Then we combine the gametes to fill in the rest of the chart with the possible genotypes for the offspring:

 
sperm
G
g
eggs
G
GG
Gg
g
Gg
gg

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%.

probability of genotypes

  probability of phenotypes
1/4 (25%) GG ---------------> 1/4 (25%) green scales
2/4 (50%) Gg ---------------> 2/4 (50%) green scales
1/4 (25%) gg ---------------> 1/4 (25%) blue scales

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.


Single-Trait 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 blue-scaled fish. What genotypic and phenotypic ratios do we get if we cross these two fish?

Our "legend":

G = green scales
g = blue scales  

G is completely dominant over g, and thus:
GG = green scales, Gg = green scales, gg = blue scales

genotype of male fish = Gg (sperm are G or g)
genotype of female fish = gg (eggs are g)
genotypes crossed in this mating = Gg X gg               

We then set up a Punnett square like this:

 
sperm
G
g
eggs
g
.
.
g
.
.

Then we combine the gametes to fill in the rest of the chart with the possible genotypes for the offspring:

 
sperm
G
g
eggs
g
Gg
gg
g
Gg
gg

Our final answer to the question:

probability of genotypes

  probability of phenotypes
2/4 (50%) Gg ---------------> 2/4 (50%) green scales
2/4 (50%) gg ---------------> 2/4 (50%) blue scales

We have thus shown the probabilities for both the genotypes and phenotypes for a cross between these two fishes.

 

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