Dihybrid Crosses - Online Tutor, Practice Problems & Exam Prep

A dihybrid cross involves organisms that are heterozygous for two specific genes, resulting in a characteristic phenotypic ratio of 9:3:3:1. This ratio reflects the independent assortment of alleles during gamete formation. In a Punnett square for a dihybrid cross, each box represents a potential fertilization event, leading to diverse combinations of traits. Understanding dihybrid crosses is essential for grasping concepts like genetic variation and inheritance patterns in organisms, particularly in relation to dominant and recessive alleles.

Dihybrid Crosses

In this video, we're going to begin our lesson on dihybrid crosses. Now, a dihybrid is an organism that is heterozygous for 2 specific genes, and that's because the root di- is a root that means 2. Here, you can see an example of what a dihybrid genotype would look like. Notice that it is heterozygous with the letter 'R', meaning it has one capital 'R' and one lowercase 'r', and it is also heterozygous for the letter 'Y', meaning it has one capital 'Y' and one lowercase 'y'. So, it's heterozygous for 2 genes here, heterozygous for the shape gene and heterozygous for the color gene. What we have here is a heterozygous round yellow pea, and notice that it is a yellow pea, it is round, and it is heterozygous for the shape gene and heterozygous for the color gene.

Over here on the left-hand side, we're showing you the pea-shaped gene alleles. Notice that we have a capital 'R' here, which is the dominant allele over the lowercase 'r' allele, which is the recessive allele. The dominant capital 'R' here is the allele for the round shape of the pea, and the lowercase 'r' is the recessive allele for the wrinkled shape of the pea. This organism here is going to be heterozygous for this particular gene.

Then, over here on the right, we have the same alleles that we talked about in our previous lesson videos, the alleles for the color gene. It shows you the pea color gene alleles. Again, we have the capital letter 'Y', the dominant allele, for the yellow color of the pea. Then, we have the lowercase 'y' over here, which represents the recessive allele for the green color. Again, a dihybrid is simply an organism that is heterozygous for 2 specific genes, heterozygous for, say, the shape gene and heterozygous for the color gene.

In our next video, we'll be able to talk more about dihybrid crosses. So, I'll see you all in that video.

Dihybrid Crosses & Punnett Squares

In this video, we're going to talk directly about dihybrid crosses and their Punnett squares. A dihybrid cross is really just cross-fertilization between two dihybrid organisms. Recall from our previous lesson video that dihybrid organisms are organisms that are heterozygous for two specific genes. There is a very characteristic phenotypic ratio for dihybrid crosses that you should all be familiar with. That characteristic phenotypic ratio is 93 31. We'll be able to get some clarity on that once we get to our image down below.

Take a look at this image over here—a dihybrid cross crossing two dihybrid organisms. Notice that parent number 1 is a dihybrid organism because it's heterozygous for two genes: the shape and color genes. Parent number 2 is also dihybrid in the same way because it's heterozygous for these two genes. Dihybrid organisms have more possibilities when it comes to their gametes. They have more gametes than what we've seen before in our other Punnett squares. These increased possibilities of gametes create a larger, more complex Punnett Square, but it works in the same way as our previous Punnett squares, where each box represents a possible fertilization event of the gametes that you see aligned.

When we examine the results of this dihybrid cross Punnett square, you'll see a very characteristic phenotypic ratio where there are 9 possibilities that have the dominant trait for both genes. So 9 round and yellow phenotypes—dominant-dominant phenotypes. Then we have 3 dominant recessive phenotypes—3 round green phenotypes. There are another 3 that are recessive dominant, so 3 wrinkled yellow phenotypes, and then one that is recessive, recessive— one wrinkled green phenotype. This 93 31 phenotypic ratio is very characteristic of a dihybrid cross, and it's evidence to show that independent assortment occurs.

This concludes our introduction to dihybrid crosses and their Punnett squares and how they create this characteristic phenotypic ratio of 93 31. We'll be able to apply the concepts that we've learned here as we move forward in our course. So, I'll see you all in our next video.

Which of the following statements is correct in describing the terms monohybrid cross and dihybrid cross?

• A. A monohybrid cross involves a single parent, whereas a dihybrid cross involves two parents.
• B. A dihybrid cross involves organisms that are heterozygous for two characters that are being studied, and a monohybrid cross involves organisms that are heterozygous for only one character being studied.
• C. A monohybrid cross is performed for one generation, whereas a dihybrid cross is performed for two generations.
• D. A monohybrid cross results in a 9:3:3:1 phenotypic ratio, whereas a dihybrid cross gives a phenotypic 3:1 ratio.

Which of the following phenomena is a consequence of independent assortment?

• A. For any gene displaying complete dominance, heterozygous individuals exhibit the dominant phenotype.
• B. Pure breeding plants, when mated with each other, produce completely homozygous offspring.
• C. The phenotypic ratio produced from a F1 × F1 dihybrid cross is 9:3:3:1.
• D. Smooth seed trait is dominant to wrinkled seed trait in peas.

Black fur in mice (B) is dominant to brown fur (b). Short tails (T) are dominant to long tails (t). What fraction of the progeny created by crossing BbTt × BBtt will be expected to have black fur and long tails?

• A. 1/16.
• B. 3/8.
• C. 1/2.
• D. 9/1.

In the dihybrid cross of AaBb x AaBb, what fraction of the offspring will be homozygous recessive for BOTH traits?

• A. 1/16.
• B. 1/8.
• C. 3/16.
• D. 1/4.
• E. 3/4.

Here’s what students ask on this topic:

What is a dihybrid cross and how does it differ from a monohybrid cross?

A dihybrid cross involves organisms that are heterozygous for two specific genes, whereas a monohybrid cross involves organisms that are heterozygous for only one gene. In a dihybrid cross, the Punnett square is larger and more complex because it accounts for the independent assortment of two different genes, leading to a characteristic phenotypic ratio of 9:3:3:1. In contrast, a monohybrid cross typically results in a 3:1 phenotypic ratio. Understanding dihybrid crosses is crucial for grasping genetic variation and inheritance patterns.

What is the phenotypic ratio of a dihybrid cross?

The phenotypic ratio of a dihybrid cross is 9:3:3:1. This ratio represents the likelihood of different combinations of traits appearing in the offspring. Specifically, 9 offspring will exhibit the dominant trait for both genes, 3 will exhibit the dominant trait for the first gene and the recessive trait for the second gene, another 3 will exhibit the recessive trait for the first gene and the dominant trait for the second gene, and 1 will exhibit the recessive trait for both genes. This ratio is a result of the independent assortment of alleles during gamete formation.

How do you set up a Punnett square for a dihybrid cross?

To set up a Punnett square for a dihybrid cross, follow these steps: 1) Determine the genotypes of the parent organisms, which should be heterozygous for two genes (e.g., RrYy). 2) List all possible gametes each parent can produce. For RrYy, the possible gametes are RY, Ry, rY, and ry. 3) Create a 4x4 grid and label the rows and columns with the possible gametes from each parent. 4) Fill in each box by combining the alleles from the corresponding row and column. This will give you the genotypes of the potential offspring. 5) Analyze the results to determine the phenotypic ratio.

What is the significance of the 9:3:3:1 ratio in a dihybrid cross?

The 9:3:3:1 ratio in a dihybrid cross is significant because it demonstrates the principle of independent assortment, one of Mendel's key genetic laws. This ratio shows that alleles for different traits segregate independently during gamete formation, leading to a variety of genetic combinations in the offspring. The 9:3:3:1 ratio specifically indicates that 9 offspring will have both dominant traits, 3 will have the first dominant and second recessive trait, another 3 will have the first recessive and second dominant trait, and 1 will have both recessive traits. This ratio is foundational for understanding genetic variation and inheritance patterns.

Can you explain the concept of independent assortment in the context of a dihybrid cross?

Independent assortment is a principle stating that alleles for different genes segregate independently of one another during gamete formation. In the context of a dihybrid cross, this means that the inheritance of one trait (e.g., seed shape) does not affect the inheritance of another trait (e.g., seed color). This principle is illustrated by the 9:3:3:1 phenotypic ratio observed in the offspring of a dihybrid cross. Each combination of alleles for the two traits is equally likely, leading to a variety of genetic outcomes and demonstrating the genetic diversity that results from sexual reproduction.