In this explainer, we will learn how to construct and interpret genetic diagrams of dihybrid crosses.
During the process of fertilization, gametes (sex cells) combine to produce a new organism. In this process, thousands of genes are passed from parent to offspring. These genes determine many of the characteristics that the offspring will have. The variations in the characteristics we see in organisms are called traits.
A gene is a section of DNA that contains the information needed to produce a functional unit, for example, a protein.
A characteristic is an observable, heritable feature of an organism.
A trait is a variation of a characteristic.
Dihybrid inheritance refers to the inheritance of two genes that determine two different characteristics or influence the same characteristic. To represent the inheritance of two genes, a dihybrid cross can be used. Dihybrid crosses are visual representations of the inheritance of the different versions of these genes, termed “alleles.”
Definition: Dihybrid Inheritance
Dihybrid inheritance is the inheritance of two genes that control different characteristics or influence the same characteristic.
An allele is an alternative version of a gene.
The cells of plants, animals, and fungi contain their genetic material in long molecules of DNA called chromosomes. An organism inherits half of their chromosomes from one parent and half from the other parent. These chromosomes “pair up” to form two sets of chromosomes in the nucleus. Each gamete produced by an organism has half the genetic material of a normal body cell from that same organism. Gametes are haploid; this means they contain only one set of chromosomes, unlike a normal body cell, which contain two sets of paired chromosomes. Consequently, for each gene, a gamete will contain only one allele. This is so when two gametes combine during fertilization, the resulting zygote, the embryo, has the correct amount of genetic information (two alleles for each gene). The inheritance of chromosomes and the basic process of fertilization are outlined in Figure 1.
A haploid cell is a cell that only has a single set of chromosomes (n).
A diploid cell is a cell that has two complete sets of chromosomes (2n), arranged into homologous pairs.
Let’s look at an example of dihybrid inheritance and a dihybrid cross.
The seeds of pea plants can be green or yellow in color, and smooth or wrinkled in shape. The characteristics of seed color and seed shape are controlled by two different genes. Both of these genes have two alleles that determine the different traits. We will use the following letters for the alleles and the traits they code for:
We will use “R” and “r” to represent the alleles for smooth seeds rather than “S” and “s” because there is a clearer difference between the uppercase and lowercase letters.
From these alleles, we can determine that yellow seeds and smooth seeds are the dominant color and shape, whereas green seeds and wrinkled seeds are the recessive traits. The dominant traits are those that will always be expressed in an organism if the alleles are present. Recessive traits, however, are only expressed if no dominant alleles for that gene are present in the organism. It is scientific convention to write dominant alleles using uppercase letters (Y) and recessive alleles using lowercase letters (y).
Definition: Dominant Trait
A dominant trait is a trait that is always expressed in the phenotype if the allele that codes for it is present in the genotype.
Definition: Recessive Trait
A recessive trait is a trait controlled by recessive alleles that is only expressed in the phenotype if there are no dominant alleles present in the genotype.
Let’s say we have two parent pea plants. One is homozygous dominant for both traits, which means both alleles are dominant for each gene. The word part “homo” means “the same.” The other plant is homozygous recessive for both traits, which means both alleles are recessive for each gene. This would give the genotypes and phenotypes outlined in Figure 2 below.
An individual is heterozygous for a characteristic if they have two different alleles for a gene.
An individual is homozygous for a characteristic if they have a pair of identical alleles for a gene.
To construct a dihybrid cross, we need to determine the combination of alleles that will be present in each gamete that is produced by these parent plants. For this, we need to combine each allele for seed color with each allele for seed shape present in a single genotype. The FOIL method is a useful tool to find the possible combinations.
How To: Determining the Combination of Alleles in a Gamete from a Given Genotype Using the FOIL Method
When constructing genetic diagrams, we need to know what the possible combinations of alleles are in the gametes of organisms in order to predict how these alleles will combine when producing a new organism.
Let’s say we want to construct a dihybrid cross between two plants with the following genotypes: YyRr and Yyrr. The dominant Y allele produces yellow seeds, and the recessive y allele produces green seeds. The dominant R allele produces smooth seeds, and the recessive r allele produces wrinkled seeds.
We can determine all the possible ways the alleles can combine in the gametes of these organisms by using the FOIL method. FOIL stands for first, outside, inside, last.
Let’s start with the first genotype, YyRr.
We start by taking the “first” letters, which represent an allele, for each gene. Our first combination of alleles is therefore YR.
We then take the “outside” letters for each gene. Our second combination is Yr.
Next, we take the two “inside” letters. Our third possible combination is yR.
Finally, we take the two “last” letters of each gene. Or you can simply remember these as the “last” letters left to match! Our final combination is yr.
To conclude, the possible combinations of alleles we may find in the gametes produced by this organism are as follows:
Example 1: Determining the Combination of Alleles Present in Gametes from a Given Genotype
Assume that in flies, body color and the size of wings are determined by two different genes, as outlined in the table provided.
|Body Color Alleles||G (gray)||g (black)|
|Wing Size Alleles||N (normal)||n (small/vestigial)|
In a fly that has the genotype ggNN, what combination of alleles can be produced in the gametes?
- gg, NN
- GN only
- gN only
- GN, gN
When organisms reproduce sexually, the gametes (sex cells) of the parents combine in a process called fertilization. To ensure the offspring produced by fertilization have the correct amount of genetic material, sex cells only contain half the amount of genetic material of a normal body cell of that organism. This means that for every gene, a gamete will only contain one allele.
We can determine the alleles present in the gametes produced by an individual organism by looking at their genotype. In this example, the genotype is ggNN. We can see from the table given that this organism has two recessive alleles for body color, so it is likely to have a black body. It has two dominant alleles for wing size, so it is likely to have normal-sized wings. Each gamete produced by this organism is going to have one allele for body color and one allele for wing size. We need to determine all the possible combinations for these alleles.
Using the genotype, let’s arrange these alleles into gametes. We can do this using the FOIL method. FOIL stands for first, outside, inside, last.
Taking the first g and the first N, we end up with the combination gN.
We will then repeat this, taking the first g and the second N, or the “outside” letters.
Now, we repeat using the second g and pairing that with the first N—these are the “inside” letters.
Now, we combine the last letters of each allele: the second g and the second n.
We now have our four possible combinations of these alleles. As you might have noticed, they are all the same!
Therefore, the combination of alleles that can be produced in the gametes is gN only.
If we recall the genotypes of the parent plants we were given in Figure 2, they were YYRR and yyrr. The next step in constructing our dihybrid cross is to determine the possible combinations of alleles in their gametes.
For YYRR, we should find that the allele combinations are YR, YR, YR, and YR.
For yyrr, we should find that the allele combinations are yr, yr, yr, and yr.
You may have noticed by now that for each individual there is only one possible combination of alleles that will be present in their gametes! This makes this example nice and easy. But be careful! Not all dihybrid crosses will be this straightforward.
We now need to start our dihybrid cross, by using a Punnett square. The “How To” box below explains how to construct and complete a dihybrid cross.
How To: Constructing a Punnett Square to Show Dihybrid Inheritance
Dihybrid inheritance refers to the inheritance of two different genes. To construct our Punnett square, we must first determine the combinations of alleles in gametes produced by both parents. We do this using the information provided about the genotypes of the parents.
For our example, let’s use two parents that are both heterozygous for two genes. This will give the following genotypes:
AaBb (mother) AaBb (father).
We know that each gamete produced by the mother and the father will only have one allele for each gene. We now need to determine the combinations that can occur in the gametes. These are
for both the mother and the father.
Let’s put these into the top column and the first row of a Punnett square. We have four possible combinations of alleles, so our Punnett square will need to be four rows by four columns.
We then complete the Punnett square by filling in each of the cells. We take the letter that is in the column header and the letter that is in the row header and combine them to give a combination of four letters. This is outlined in the diagram below.
Generally, it is good practice to write the same letters together (e.g., AA, Bb) and write the dominant allele first.
Let’s use this process to complete the rest of the squares.
Our Punnett square is complete! From here, we can use this to predict the genetic makeup and the physical appearance of the offspring born to these parents.
If we follow this method to complete our Punnett square for our example of seed color and shape in pea plants, we should produce the following dihybrid cross.
As you might have noticed from Figure 3, all the genotypes are the same! This makes predicting our phenotypes fairly straightforward. We can see that each genotype—the collection of letters in each square—has a dominant allele for both seed color (Y) and seed shape (R). This means that of the offspring will produce smooth, yellow seeds.
Example 2: Determining the Probability of Phenotypes from Dihybrid Crosses
Assume that in guinea pigs, the allele for black fur (B) is dominant to the allele for white fur (b), and the allele for smooth fur (F) is dominant to the allele for rough fur (f). A guinea pig with genotype Bbff is crossed with a guinea pig with genotype bbFF. What is the probability, in percent, that the offspring will have black, smooth fur?
This question is asking for us to use the information provided to carry out a genetic cross and determine the likelihood of a particular phenotype being shown by the offspring. As we have been provided with the alleles for two different genes, we know that we should construct a dihybrid cross.
Let’s start by determining the combination of alleles that will be present in the gametes of each parent. We can do this by combining the alleles for each of the two genes in all the possible ways, as shown.
For the genotype Bbff, the possible combinations are Bf and bf. For the genotype bbFF, bF is the only possible combination.
Now, we need to take these possible allele combinations and place them into the row and column heads of a Punnett square.
By filling in the cells, we are demonstrating all the possible ways that these alleles can be combined during the process of fertilization. For each cell, we take the alleles in the column head and the alleles in the row head and combine them. A completed cross should look like the following.
Each cell represents a possible genotype for each offspring and has a combination of two alleles for each gene. From this, and the information provided in the question about which alleles are dominant and recessive, we can determine the probability that an offspring born to these parents will have black, smooth fur.
The question tells us that the allele that produces black fur is dominant (B) and the allele that produces smooth fur is dominant (F). This means any genotypes with both a B and an F, even if it is only one copy, will give offspring with black, smooth fur. In this example, the genotype that has these alleles is only BbFf. If we count how many times we see this genotype, it is 8 out of 16.
To calculate this as a percentage, we can do the following: or we can simply spot that 8 is half of 16!
Therefore, the probability, in percent, that the offspring will have black, smooth fur is .
From dihybrid crosses, we can calculate the probability of offspring inheriting certain genotypes and phenotypes. We can also determine the phenotypic ratio if we know which alleles are dominant and which are recessive. Phenotypic ratios are generally given following the convention outlined below:
For the cross shown above in Figure 3, the probability of offspring inheriting the genotype YyRr is , so the phenotypic ratio is .
Let’s use an example that introduces more variation into the phenotypes.
In fruit flies, body color and eye color are coded for by different genes. Having a brown body (B) is dominant to having a black body (b), and having red eyes (E) is dominant to having brown eyes (e).
We have two parent flies, and both are heterozygous for both body color and eye color. This gives them the genotypes of BbEe. If we work out the combination of alleles in each gamete, we should end up with the following for both mother and father: BE, Be, bE, be. Let’s take these combinations, and use them to complete a dihybrid cross, as before.
As we can see from Figure 4, this produces various genotypes. The possible genotypes that can be produced from this cross are BBEE, BBEe, BbEE, BbEe, BBee, Bbee, bbEe, bbEE, and bbee. We can use what we have been told about dominant and recessive alleles to determine what phenotypes each of the genotypes will give.
- BBEE, BbEe, BBEe, and BbEE genotypes have dominant alleles for both body and eye color. In our Punnett square, there are 9 of these genotypes. These genotypes will give a brown body with red eyes.
- BBee and Bbee have the dominant allele for body color, but no dominant allele for eye color and instead two recessive alleles. In our Punnett square, there are 3 of these genotypes. These genotypes will result in a brown body with brown eyes.
- bbEE and bbEe have no dominant allele for body color, and instead two recessive alleles, but do have a dominant allele for eye color. In our Punnett square, there are 3 of these genotypes. These genotypes will result in a black body with red eyes.
- bbee is the only genotype to have no dominant allele for either of these characteristics but instead have two recessive alleles for both body color and eye color. In our Punnett square, there is only 1 of these genotypes. This will result in a black body with brown eyes.
Using the information above, and the cross shown in Figure 4, we can determine that the phenotypic ratio is: .
Example 3: Identifying Correct Dihybrid Crosses and Calculating Phenotypic Ratios
Assume that in plants, the allele for tall stems (D) is dominant to the allele for short stems (d), and the allele for purple flowers (P) is dominant to the allele for white flowers (p).
- Which of the following Punnett squares correctly crosses two plants that are both heterozygous for these traits?
- What is the phenotypic ratio of this cross?
A dihybrid cross, in the form of a Punnett square, is used to show the inheritance of two genes that control different characteristics. In this example, the two characteristics are stem length and flower color. Stem length is controlled by two different alleles: a dominant allele for tall stems (D) and a recessive allele for short stems (d). Flower color is also controlled by two different alleles: a dominant allele for purple flowers (P) and a recessive allele for white flowers (p).
If an organism is heterozygous for a characteristic, the two alleles it possesses for this characteristic in its genotype will be different. In this scenario, the organism would be heterozygous for both stem length and flower color. This means that they would have both the dominant and the recessive alleles for stem length, and both the dominant and the recessive alleles for flower color. This would give the organism the genotype DdPp.
We then need to establish the combination of alleles that would be present in the gametes produced by each heterozygous parent plant. We do this by combining the alleles for stem length and flower color in all the possible ways within a genotype. For the genotype DdPp, the possible combinations are
If we then take these possible allele combinations for each gamete and place them into the row and column heads for a dihybrid cross, we can see that it should look something like the following.
So, for part 1, we can conclude that the correct option is B.
Using this dihybrid cross, part 2 asks us to state the phenotypic ratio. Phenotypic ratios are given in the following format:
These phenotypes can be determined from the genotypes in the completed Punnett square given for part 1.
The genotypes that include a dominant allele for both stem length and flower color, so a D and a P, will produce phenotypes that show both dominant traits. There are 9 genotypes in the square that have at least one D and one P.
Genotypes that are dominant for stem length but recessive for flower color will have at least one D, but no P and instead pp. They will produce phenotypes that have tall stems with white flowers. There are 3 genotypes that fit this trend.
Genotypes that are dominant for flower color but recessive for stem length will have at least one P but no D and instead dd. They will produce phenotypes with short stems and purple flowers. There are 3 genotypes that fit this trend.
Genotypes that have no dominant alleles for stem length and flower color will produce phenotypes that are recessive, and there is only 1 genotype that fits this trend, ddpp.
Therefore, for part 2, we can conclude that the phenotypic ratio for this cross is .
Let’s review what we have learned from this explainer.
- Dihybrid inheritance refers to the inheritance of two genes that determine the expression of two different characteristics or influence the same characteristic.
- We can display the inheritance of these genes using a Punnett square to show a dihybrid cross.
- When constructing a dihybrid cross, we must determine the possible combination of alleles present in the gametes produced by each parent.
- We can use completed dihybrid crosses to determine the possible genotypes of the offspring and the phenotypic ratios.