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.