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Lesson Video: Dihybrid Inheritance Biology

In this video, we will learn how to construct and interpret genetic diagrams of dihybrid crosses.

12:37

Video Transcript

In this video, we will learn about dihybrid inheritance. We will learn how to construct a Punnett square to show the inheritance of two genes. And we will calculate phenotypic ratios of offspring.

Our characteristics are determined by our genetic material, DNA. This DNA is responsible for determining our natural hair color, eye color, our blood types, and even how tall we can grow. Inheritance refers to the process by which genetic material is passed from parents to offspring. Genes code for different characteristics. And when we’re looking at the inheritance of one particular gene, we refer to this as monohybrid inheritance. The word part mono- means one. And when we’re studying the inheritance of two genes, we call it dihybrid inheritance, with di- meaning two.

Let’s have a look at an example of dihybrid inheritance to better understand the process. In pea plants, seeds can have a few different characteristics. They could be yellow or green and can have a smooth or wrinkled surface. In this example, you can see a yellow and smooth seed as well as a green and wrinkled seed. One gene will contain the information that controls the seed color. And a second different gene contains the information that determines if the surface will be smooth or wrinkled. These genes have two alleles or versions each.

In genetics, we usually assign genes and their alleles letters. For the gene that controls seed color, let’s use the letter Y. The dominant allele, represented by a capital Y, codes for yellow seeds, while the recessive allele, represented by a lowercase y, codes for green seeds. For the gene that controls the seed’s surface, let’s use the letter R. The dominant allele for smooth seeds will be represented by an uppercase R, while the recessive allele for wrinkled seeds will be represented by a lowercase r. Remember, if an allele is dominant, only one copy needs to be present in the genotype for it to be expressed in the phenotype. If an allele is recessive, it can only be expressed when there are no dominant alleles in the genotype.

Now, let’s have a look at the genotypes of the plants that have produced these seeds. The plant that produced our yellow seed has the genotype uppercase Y uppercase Y uppercase R uppercase R. We can refer to this as homozygous dominant for both genes. Homozygous means the alleles are the same, and dominant means that they’re both dominant alleles. The plant that produced our green seed has the genotype lowercase y lowercase y lowercase r lowercase r. Again, this plant is homozygous for both genes, but this time it’s homozygous recessive.

Plants will produce sex cells or gametes for reproduction. These gametes contain half the genetic material of a plant cell found in the body of the organism, such as those in the stem or the leaves. Using the genotype, we can determine the possible combination of alleles in the gametes produced by each plant.

Let’s start with a plant that is heterozygous for its seed color and surface. We can use the FOIL method to determine the possible combination of alleles in the gametes. FOIL stands for first, outside, inside, last. To start, we take the first allele of each gene, in this case uppercase Y uppercase R. So the first possible combination is uppercase Y uppercase R. Next, we take the outside letters, so the alleles that appear first and last. So our next combination of alleles is therefore uppercase Y lowercase r. Now we take the inside letters. So our next combination of alleles is lowercase y uppercase R. And finally, we take the last alleles of each gene. So our final possible combination of alleles is lowercase y lowercase r.

Now, we have all the possible combinations of alleles that can be present in a gamete produced by this plant. But why do we need to know this? We can study dihybrid inheritance by using Punnett squares. Punnett squares are incredibly useful tools which allow us to make predictions about the genotypes and phenotypes of the offspring produced by certain organisms. Let’s try completing one to show the inheritance of seed color and shape.

We’ll start with our parent plants, which are both heterozygous for both genes. We just saw how to determine the combination of alleles in the gametes for these plants. So the gametes from the first plant will go here, and the gametes from the second plant will go here. This is the information we will use to complete this Punnett square. We can do this by combining the alleles shown in the column header with those in the row header. This gives us a total of four letters representing two alleles for each gene in each box of the Punnett square.

We will follow the general convention and write the letters for the same gene together as shown here. And we’ll write the dominant allele before the recessive allele for each gene. Let’s use this process to complete the rest of the Punnett square. There, now our Punnett square shows us all the possible genotypes of the offspring from this cross.

Next, let’s determine the phenotypes of these plants. Let’s remind ourselves what these letters actually mean. We’re looking at seed color and the type of surface of the seed. Yellow seeds are dominant to green seeds, and smooth seeds are dominant to wrinkled seeds. This means if the genotype contains an uppercase Y with an uppercase R, it will indicate that the plant will produce yellow smooth seeds. Let’s identify all of these genotypes. We can see that out of 16 possible genotypes, nine of them are dominant for both traits and will produce yellow smooth seeds.

Now, let’s identify the genotypes that will produce yellow seeds but they will have a wrinkled surface. For this to happen, the genotype needs to have at least one uppercase Y but two copies of lowercase r. Out of the 16 possible genotypes, three fit this description.

Next, we want to find out how many genotypes would give green seeds with a smooth surface. So now we need two lowercase y’s but at least one uppercase R. And there are three genotypes that fit this description. Finally, we can figure out how many genotypes will produce a plant that is recessive for both traits. This means the genotype will have exclusively lowercase letters. You may have spotted that there’s only one genotype out of a possible 16 that will produce this phenotype. The final tally of phenotypes can be written as a phenotypic ratio. This compares the number of genotypes, which will give the dominant phenotype for both traits, the dominant phenotype for one trait and the recessive phenotype for the other, and the recessive phenotype for both traits. Here, our phenotypic ratio is nine to three to three to one. We can use this ratio to calculate the probability of each offspring having a particular phenotype.

Now that we’ve learned more about dihybrid inheritance and how to model it, let’s try out a practice question.

Assume that, in plants, the allele for tall stems, uppercase D, is dominant to the allele for short stems, lowercase d, and the allele for purple flowers, uppercase P, is dominant to the allele for white flowers, lowercase p. A plant with genotype uppercase D lowercase d uppercase P lowercase p is crossed with a plant with genotype uppercase D lowercase d uppercase P lowercase p. What is the probability, blank over 16, that the offspring will have a tall stem and purple flowers? (A) Nine over 16, (B) three over 16, (C) one over 16, or (D) 16 over 16.

To answer this question, we’re going to have to use a Punnett square to demonstrate the dihybrid inheritance of alleles. First, however, we need to determine the alleles present in the gametes of these plants. Both plants have the genotype uppercase D lowercase d uppercase P lowercase p. Each gamete produced by these plants will contain one allele that determines the stem length and one allele that determines flower color. So we just need to work out all the different combinations of these two alleles.

To do this, we can use the FOIL method. FOIL stands for first, outside, inside, last. If we take the first two alleles for each gene, we end up with the combination uppercase D uppercase P. Now, let’s take the end or the outside alleles of each gene to give the combination uppercase D lowercase p. Now, we can take the inside allele of each gene to give the combination lowercase d uppercase P. And finally, we combine the last set of alleles to give the combination lowercase d lowercase p.

Now, let’s draw out a Punnett square. We take these allelic combinations and place them into the row and column headers of a four-by-four Punnett square. To complete the Punnett square, we take the alleles in the column header and the alleles in the row header and combine them to give a sequence of four letters that represent the possible genotype of the offspring. Repeating this gives us a complete Punnett square.

Note that we wrote DPDP in the top-left corner instead of DDPP. We did this to show you where the letters come from. But usually, we arrange the alleles of the same genes together, like you can see in the rest of the Punnett square. This makes it easier to interpret the phenotype.

The question is asking us about the probability of a certain phenotype: a tall stem and purple flowers. These traits are controlled by dominant alleles. This means we need to look in our Punnett square for genotypes that have at least one uppercase D and an uppercase P. In total, we have nine genotypes that fit this description out of a possible 16. So the probability that an offspring produced by this cross will have a tall stem and purple flowers is nine out of 16.

Now, let’s go over the key points that we covered in this video. Dihybrid inheritance refers to the inheritance of two genes that determine the expression of two different characteristics. 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 combinations 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.

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