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The Fun Mouse: Genetics: Inheritance, Punnett squares ~ 2 Loci

This section will show you how to determine all possible combinations of alleles offspring can inherit on 2 given loci. In order to understand this section, you must have a full understanding of the last section.

For the sake of example, I will do these punnett squares for a Blue father with B/b d/d genotype (blue carrying chocolate) and a Black mother with B/b D/d genotype (carrying both chocolate and blue).

As shown on the last page of this section, you need to make a punnett square for each locus. Like before, put the fathers alleles for the B locus on the top row and the mothers alleles for the same locus on the side column to complete your punnett square for that locus. Do the same for the D locus in the other punnett square. This will show you what possible combinations offspring can inherit on those 2 loci individually.

Completed punnett squares for each locus:

B locus
  B b
B B/B B/b
b B/b b/b
D locus
  d d
D D/d D/d
d d/d d/d

Next you will make a larger punnett square to include both loci which will help you determine the combinations of both loci together. This table will consist of 5 rows and 5 columns as shown below. Using the completed squares from above, take the 4 combinations of alleles the babies can inherit on the B locus (in purple) and add them to the left column in the following punnett square. Then add the D locus (in green) to the top row.

  D/d D/d d/d d/d
B/B        
B/b        
B/b        
b/b        

Next you will fill in the alleles from the B locus and the D locus to determine all possible genotypes for those loci that the offspring can inherit. You will fill them in similar to how you did the punnett square for a single locus, taking a set from the left column and a set from the top row to fill in each corresponding square.

Animated example of how to fill in the squares:

Completed punnett square:
  D/d D/d d/d d/d
B/B B/B D/d B/B D/d B/B d/d B/B d/d
B/b B/b D/d B/b D/d B/b d/d B/b d/d
B/b B/b D/d B/b D/d B/b d/d B/b d/d
b/b b/b D/d b/b D/d b/b d/d b/b d/d

You will notice from above that there are 16 squares of genotypes that the babies can inherit. This means that each square accounts for 6.25% of the litter (100 ÷ 16= 6.25).

Now it's time to start figuring out what percent of the litter will have a specific genotype as some squares will be repetitive.

  D/d D/d d/d d/d
B/B B/B D/d B/B D/d B/B d/d B/B d/d
B/b B/b D/d B/b D/d B/b d/d B/b d/d
B/b B/b D/d B/b D/d B/b d/d B/b d/d
b/b b/b D/d b/b D/d b/b d/d b/b d/d

The table above is color coded to easily show all genotypes that are the same.
Each square accounts for 6.25% as stated before. To figure out what percent of a litter should inherit each genotype you will multiply each corresponding genotype (shown in the same colored squares) by the number of squares that genotype occupies. Example: In the case of the green squares (shown above) with B/b D/d genotype, we multiply:
   6.25% the percent each square accounts for in the litter.
×4 the number of squares that same genotype occupies.
----------
25% that will have B/b D/d genotype.
Do that for all possible genotypes. The table below is a completed list of possible genotypes and percents for that genotype based on the punnett square above.

Genotypes %
B/B D/d 12.5%
B/b D/d 25.0%
B/B d/d 12.5%
B/b d/d 25.0%
b/b D/d 12.5%
b/b d/d 12.5%

Knowing all possible genotypes is just as important as knowing all possible phenotypes (explained below). This will help you with future breedings and knowing the chance that the babies have of carrying something recessive or hidden.

Now it's time to start figuring out what percent of the litter will have a specific phenotype based on genotype. I took the genotype table from above and added what phenotypes they are in the table below. As you can see, some genotypes can make the same phenotype.

Phenotype Genotype Percentage
Black B/B D/d 12.5%
Black B/b D/d 25.0%
Blue B/B d/d 12.5%
Blue B/b d/d 25.0%
Chocolate b/b D/d 12.5%
Dove (US) b/b d/d 12.5%

Using the numbers above, you can calculate what percentage of each litter that will have each of those phenotypes. To do this, add all corresponding phenotype percentages together, as shown below:

Phenotype Genotype Percentage   Total Phenotype %
Black B/B D/d 12.5% 37.5% Black
Black B/b D/d 25.0%
Blue B/B d/d 12.5% 37.5% Blue
Blue B/b d/d 25.0%
Chocolate b/b D/d 12.5% 12.5% Chocolate
Dove (US) b/b d/d 12.5% 12.5% Dove

The percentages for each phenotype for this litter will be; 37.5% Black, 37.5% Blue, 12.5% Chocolate, and 12.5% US Dove, as shown in the table above.

Always remember, what nature has planned isn't always what statistics will tell you. You may get more or less of one genotype than what the statistics say. In addition, smaller litters can not possibly pull out all possible genotypes (if you have 16 different genotypes and 5 babies, it's not possible to get all possible genotypes in a litter).


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