Video Transcript
Which of the following best
explains what is meant by dihybrid inheritance? (A) The inheritance of a single
characteristic that is controlled by one gene alone. (B) The formation of gametes using
alleles inherited from both parents. (C) The possibility that three or
more alleles may exist for each locus on a chromosome. Or (D) the inheritance of two
different genes, located on different chromosomes, together.
In order to answer this question
correctly, we need to review what dihybrid inheritance means. Dihybrid inheritance refers to a
mating experiment involving two different genes. You can remember this because the
root word di- means two.
Now the question becomes what are
genes? So genes are sequences of DNA that
contains the information needed to produce a certain characteristic. For example, a trait that you get
from a gene could be eye color. People with blue eyes have a
protein being made from a sequence of DNA, a gene, that makes their eyes blue.
One of the earliest experiments
studying genes was carried out by Gregor Mendel. He experimented with pea plants to
understand inheritance. One gene that Gregor Mendel studied
closely was pea plant color. Although, we commonly see green
peas, pea plants can also be yellow. This means there are alternate
versions for the gene for pea color. These are known as alleles. Mendel wanted to know what color
would be inherited from parents with different alleles. So Mendel mated a yellow pea plant
with a green pea plant to see what that offspring’s color would be. This would also tell him which
allele was dominant. The dominant allele is the allele
that is always observable if it’s inherited.
So, when Mendel crossed yellow and
green peas, he found that the offspring were yellow. This told him that the allele for
yellow peas is dominant to the allele for green peas. Mendel used the letter Y to
represent the trait for pea color. He represented the yellow allele
being dominant by using an uppercase Y and used lowercase y to represent the green
pea plant allele. Mendel eventually figured out that
you inherit two copies of each trait, one from each parent. So the letters below the pea plants
are how we would represent the alleles in our example.
Mendel could also predict the
outcomes of mating experiments, called crosses, using his Punnett squares. His use of Punnet squares split
each parental allele for a single gene. You fill in the Punnet square by
combining the alleles from the left and above for each square. In this example, all of our
offspring are the same, having one dominant allele from parent one and one recessive
allele from parent two.
When we make a cross looking at a
single trait, such as pea plant color, we call it a monohybrid cross. The root word mono- means one. Therefore, a monohybrid cross
examines the possible offspring outcomes for a cross of one gene. But remember, our question is about
dihybrid inheritance, which is when we cross two traits at the same time.
We know that pea plants can be
yellow or green, but did you know peas can also be round or wrinkled? In this trait, round is dominant,
so we will represent that will an uppercase R, and wrinkled seeds will be a
lowercase r. Let’s cross a yellow, round pea
plant with a recessive trait for green and wrinkled peas with a pea plant that is
fully recessive for both the green and wrinkled traits. Now we are crossing two traits, so
that parent has four alleles, two for each trait.
For a dihybrid cross, we will need
16 squares in the Punnet square because there are more possible combinations for
alleles. Parent one alleles go across the
top of the dihybrid Punnet square. With parent one, there are four
different combinations of alleles. Each is represented here by a
different color. You can look at the arrows to see
how those combinations were formed. Parent two only has one option of
allele combination, lowercase y and lowercase r. These are shown on the left side of
the dihybrid Punnet square.
You fill in a dihybrid Punnett
square in the same way as a monohybrid, by combining the alleles from the top and
left side for each square. All four letters go in the square,
keeping the letters for each trait together. This is what the filled-in dihybrid
square looks like. Because we have two genes, our pea
plant has four possible outcomes from this dihybrid cross. We could be yellow and round. Our offspring could be yellow and
wrinkled. Our offspring could be green and
round. Or our offspring could be green and
wrinkled.
So now that we have discussed
dihybrid inheritance, let’s go back to our question and take a look at our answer
choices. Because dihybrid inheritance looks
exclusively at the inheritance of two genes, the only answer choice that is
consistent with our definition is answer choice (D). The inheritance of two different
genes, located on different chromosomes, together.
