Explain Me This: Two-Trait and Dihybrid Crosses of Unlinked Genes

Before we dive into the cross, let's quickly recap what we have covered so far:

 

1. A two-trait cross is a cross between two organisms of two genes.
2. Unlinked genes are genes that are inherited independently as they are either located far apart on the same chromosome, or on different chromosomes all together. (For a revision of linked vs. unlinked genes, see here.)

 

This blog post is going to tackle the two-trait crossing and dihybrid crossing of unlinked genes.

Two-Trait Crossing of Unlinked Genes

This is similar to doing regular Mendelian crosses, but with two genes this time. The dominant and recessive rules for genes apply here. However, due to the existence of different gamete combinations, it is more complicated than just crossing one gene.

 

Let's say that we are looking at two characteristics: texture and color. The information we have includes that:

 

1. Round ( R ) peas are dominant over wrinkled ( r ) peas.
2. Yellow ( Y ) peas are dominant over green ( y ) peas.

 

The cross is between a RrYy (round, yellow) pea plant and a rrYy (wrinkled, yellow) pea plant. This can be represented as:

 

RrYy x rrYy

 

To find the genotypic and phenotypic ratio of the two plants' offspring:

 

Step 1

 

Determine the possible gamete combinations for both parent organisms. This means that you should determine all the possible combinations of gametes that the offspring from each parent can have.

 

TIP: To prevent silly mistakes, you should always write the genes in the same order, and write down the dominant allele first if there is one.

 

The best way to do this is the FOIL method, which you may be familiar with from mathematics. FOIL stands for first (the first letters), outside (the two outermost letters), inside (the two innermost letters), last (the last letters). This will give you all the possible combinations from one parent.

 

For the first parent pea plant, it is as follows:

 

If you want to visualize it in terms of chromosomes, then it would look like this:

And for the second parent pea plant:

As the second parent has two "rY" combinations, we can note it down as one. This is the same for "ry" combinations.

 

 

Step 2

 

Set up a Punnett Square. To set up a Punnett Square, one parent's gamete combinations go across the top, and the other parent's gamete combinations go down the left side of the table.

 

Keep in mind that the largest possible Punnett Square for a two-trait cross is a 4 x 4 square. This is because the maximum number of possible gamete combinations is four. Thus, if you find that your square is larger than that, then you should check again. Moreover, it is definitely possible that your Punnett Square is less than a 4 x 4. In our example, the square is a 4 x 2.

	RY	Ry	rY	ry
rY
ry

Step 3

 

Fill in the Punnett Square to determine the gamete combinations for the offspring. This is the easy part, but also the part where students are the most likely to make mistakes. Try to follow the rules of writing the genes in the same order and writing dominant alleles first to avoid errors!

 

	RY	Ry	rY	ry
rY	RrYY	RrYy	rrYY	rrYy
ry	RrYy	Rryy	rrYy	rryy

 

 

Step 4

 

Determine the genotypic and phenotypic ratios. Be careful not to confuse the two! The genotypic ratio is the ratio of the various gamete combinations, while the phenotypic ratio is the ratio of the characteristics of the offspring.

 

1. Genotypic Ratio (arrange numbers from large to small)
   
   
2. Phenotypic Ratio
   

Genotype	Ratio Number	Phenotype
RrYy	2	Round yellow
rrYy	2	Wrinkled yellow
RrYY	1	Round yellow
Rryy	1	Round green
rrYY	1	Wrinkled yellow
rryy	1	Wrinkled green

 

Phenotype	Ratio Number Total
Round yellow	3
Wrinkled yellow	3
Round green	1
Wrinkled green	1

Dihybrid Cross of Unlinked Genes

 

Now that you know how to do a cross with two traits, let's move on to a specific case: the dihybrid cross. In this situation, the genotypic and phenotypic ratios will always end up the same way (you will see this later). A dihybrid cross is a cross between two organisms of two genes differing in two traits.

 

Let's continue using the example of pea plants, and cross two RrYy (round, yellow heterozygote) pea plants together.

 

This can be represented as:

 

RrYy x RrYy

 

 

To find the genotypic and phenotypic ratios for the offspring: (the process is very similar to the two-trait crossing, and thus, will be simplified this time)

 

Step 1

 

Determine the possible gamete combinations for both parent organisms.

 

For both parents, the following applies:

Step 2 + 3

 

Set up a Punnett Square and fill in the Punnett Square to determine the gamete combinations for the offspring.

 

	RY	Ry	rY	ry
RY	RRYY	RRYy	RrYY	RrYy
Ry	RRYy	RRyy	RrYy	Rryy
rY	RrYY	RrYy	rrYY	rrYy
ry	RrYy	Rryy	rrYy	rryy

 

 

Step 4

 

Determine the genotypic and phenotypic ratios.

1. Genotypic Ratio:
   
2. Phenotypic Ratio:

Genotype	Ratio Number	Phenotype
RrYy	4	Round yellow
RRYy	2	Round yellow
RrYy	2	Round yellow
rrYy	2	Wrinkled yellow
Rryy	2	Round green
RRYY	1	Round yellow
rrYY	1	Wrinkled yellow
RRyy	1	Round green
rryy	1	Wrinkled green

 

Phenotype	Ratio Number Total
Round yellow	9
Wrinkled yellow	3
Round green	3
Wrinkled green	1

From the dihybrid cross, we can conclude that if a Punnett Square is filled out with a cross between two heterozygotes for both genes, then the phenotypic ratio would be 9:3:3:1. Thus, this information could be useful for quickly determining whether or not two organisms are dihybrids for two traits.

 

In the next post, we will have a look at how to do two-trait crosses with linked genes.