Dihybrid Cross Problem: AP® Biology Crash Course (2024)

The dihybrid cross problem can be a very challenging topic for AP® Biology students to master. Luckily, with practice and an organized method for completing the problem, the dihybrid cross problem can be solved! In this AP® Biology Crash Course Review, we will work together to solve a dihybrid cross problem. We will begin by reviewing the monohybrid cross. We will then discuss why a dihybrid cross would be done by scientists. Finally, we will walk through a dihybrid cross with a problem that you could see on your AP® Biology exam.

Monohybrid Cross

In order to understand a dihybrid cross, we must first review a monohybrid cross. A monohybrid cross is when mating occurs between two individuals with different alleles at a single locus of interest. When we consider these problems, plants will often be the focus because their mating can be most easily controlled by scientists.

We will review a case of tall-stemmed flowers and short-stemmed flowers. A tall stem is a dominant allele, and a short stem is a recessive allele (for the purpose of this example). Let’s review several different mating situations and discuss their outcomes.

In this case, we are crossing a tall-stemmed flower and a short-stemmed flower. As you can see, this cross will result in 100% heterozygous offspring. Heterozygous means that the individual has both a dominant and a recessive allele. Because the tall stem is a dominant allele, all of these plants will have tall stems.

In this case, we are crossing a tall-stemmed flower with a tall-stemmed flower. We will produce 100% hom*ozygous dominant plants. hom*ozygous means that the individual has two of the same alleles. Each of the offspring will be tall stemmed.

Lastly, this case shows the cross of two heterozygous plants. In this example, there are a variety of outcomes that may occur.

Phenotypic and Genotypic Ratios

If the AP® Biology exam were to ask you about monohybrid crosses, the question will likely focus on the ratios that the cross produces. It will be necessary for you to first be able to fill out the Punnett square. After you have filled the Punnett square with the offspring genotypes, you can easily figure out the offspring ratios.

Let’s look at the Punnett squares that we completed above. The phenotypic ratio will be the ratio of phenotypes that you will see in the offspring organisms. For organisms with Tt or TT, the plant will have a tall stem. Organisms with tt will have a short stem. The genotypic ratio will be the ratio of genotypes in the offspring. In the genotypic ratio, even though Tt and TT both produce tall stem offspring, they are different.

In the final example, the genotypic ratio is 1:2:1. The order that we write the genotypic ratio is the dominant hom*ozygous, the heterozygous, and the recessive hom*ozygous. The phenotypic ratio is 3:1 because the heterozygous offspring will still be tall-stemmed.

Theory of Dihybrid Cross

Monohybrid crosses do occur in science; however, organisms are much more complicated than one single allele. Many times during cross-breeding, scientists will have to deal with organisms that have two different alleles at two different loci. For example, a dihybrid cross would be between a tall (TT) and blue plant (BB) with a short (tt) and red plant (bb).

It is important also to understand Mendel’s second law of independent assortment. Mendel’s second law states that alleles of one gene sort independently of alleles of another gene. Basically, when performing a dihybrid cross, you can think of it as two separate monohybrid crosses.

Now that we have reviewed the monohybrid cross and the theory of a dihybrid cross, we will work through an example of a dihybrid cross.

Dihybrid Cross Problem

We are going to work through a dihybrid cross problem step-by-step. These problems can be complicated to learn, but when we have walked through one problem, you should be able to use the same method for all of the dihybrid problems.

In guinea pigs, the allele for black fur (B) is dominant over that for brown fur (b). Similarly, the allele for short fur (S) is dominant over that for long fur (s).

We will do an example of a cross between BbSs and BBss.

Step 1:

The very first step that you should complete when doing a dihybrid cross is to figure out the possible gametes of the parents. We must figure out all of the ways possible for the alleles to sort themselves based on Mendel’s second law of independent assortment. We will fill this part of the table out first.

If you’re having trouble with this step, start with just one allele first and fill in the second allele after.

Step 2:

In this step, we will fill in the offspring by combining the gametes. This section can be complicated, but we will use the same method for filling in a dihybrid cross, as we did for a monohybrid cross.

Step 3:

In this final step, we will figure out the phenotypic ratios. The genotypic ratios can get very complicated in dihybrid cross problems. For that reason, the AP® Biology exam will not require you to understand those ratios.

In order to create the phenotypic ratios, we will have to go back to the problem to see which traits are dominant; black fur (B) is dominant over brown fur (b), and short fur (S) is dominant over long fur (s). Now, we can look at our cells and determine what the offspring will display as a phenotype based on their genotype.

Now we can create a ratio. The phenotypic ratio for this dihybrid cross will be 8 (black fur, short fur): 8(black fur, long fur): 0 (brown fur, short fur): 0 (brown fur, long fur). The ratios that you will see will always be in this convention: dominant trait 1, dominant trait 2 : dominant trait 1, recessive trait 2 : recessive trait 1, dominant trait 2 : recessive trait 1, recessive trait 2.

AP® Biology Exam Problem

Because these problems are a major sticking point for many students, and practice only makes these problems easier, we are going to do one more problem in this Dihybrid Cross: AP® Biology Crash Course Review. I won’t walk through all of the steps, but I will give you the problem and solution to see if you understand the dihybrid cross problems.

Problem:

Consider pea plants, in which green color is dominant to yellow color, and two leaves are dominant to three. A cross between two plants that are heterozygous green and heterozygous tall is completed. What is the phenotypic ratio?

The phenotypic ratio for the problem above is 9:3:3:1. Note that when crossing two heterozygous (for both alleles) parents this will always be the ratio.

Summary

In this article Dihybrid Cross: AP® Biology Crash Course Review, we began by reviewing the monohybrid cross and the possibilities that could result from crossing parents with different alleles. We worked through several monohybrid crosses and learned that we can find the genotypic and phenotypic ratios by using Punnett squares and creating fractions of every genotype and phenotype. We then talked about the importance of dihybrid cross problems when dealing with very complex organisms. Finally, we walked through the steps to completing a dihybrid cross problem with guinea pigs, and then you completed a dihybrid cross independently to practice for the AP® Biology exam.

Thank you for reading this article! If you are interested in reading more, please check out our article Chi Square Test: AP® Biology Crash Course.

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