Video Transcript
What is the expected phenotypic
ratio of a dihybrid cross between two heterozygous individuals? (A) Nine to three to three to
one. (B) 12 to three to one. (C) 16 to zero to zero to zero. (D) Nine to six to one.
This question asked for the
expected phenotypic ratio of a dihybrid cross between two heterozygous
individuals. There’s a lot of terminology
involved in this question, so let’s review a few terms first. A dihybrid cross refers to a mating
experiment involving two different genes. This is easy to remember because
the root word di- means two. Dihybrid crosses can be done using
a Punnett square with 16 boxes.
Heterozygous refers to an
individual that has two different alleles for a gene in their genotype. So the heterozygous individuals in
this example would have the genotype uppercase Y lowercase y uppercase F lowercase
f. There are two different letters, Y
and F, to represent two different genes in our dihybrid cross. You may recall that a genotype is
the genetic makeup, or alleles, of an organism. So our four letters here uppercase
Y lowercase y uppercase F lowercase f is an example of a genotype that is
heterozygous.
This question is wanting the
phenotypic ratio. And the phenotype refers to the
observable traits of an organism determined by the genotype. In this example, uppercase Y will
represent the allele for yellow fur, while lowercase y will represent the allele for
green fur. And uppercase F will represent the
allele for a fuchsia tail, while lowercase f represents the allele for a red
tail.
In order to answer this question,
we can construct a theoretical dihybrid cross with our two heterozygous
individuals. We can use the FOIL method to
identify the possible gametes from the parents’ genotype. FOIL stands for first, outside,
inside, last. The gametes for the first
combination will be uppercase Y uppercase F. The gametes for the outside
combination will be uppercase Y lowercase f. The gametes for the inside
combination will be lowercase y uppercase F. And the gametes for the last
combination will be lowercase y lowercase f. These will be the same for both
parents as both parents have the same genotype.
When we construct a dihybrid
Punnett square, the four gamete possibilities are placed across the top of the
Punnett square for parent one and down the left side of the Punnett square for
parent two. Now, we fill in each square with
the possible gametes from parent one at the top and parent two on the left side so
that each square has four letters, two alleles for each trait. For example, the first square will
be all uppercase, YYFF, while this square will be heterozygous for both traits:
uppercase Y lowercase y uppercase F lowercase f.
Once all of the genotypes are
filled in, we can start determining phenotypes. Any genotypes with an uppercase Y
will have yellow fur. And any genotype with an uppercase
F will have a fuchsia tail. We can represent that by filling in
those squares like so, with the top half of the square representing the fur color
and the bottom half representing the tail color. Any individuals with two lowercase
y’s will have a green fur color. And any individuals with two
lowercase f’s will have a red tail color. Out of the 16 theoretical
individuals, nine exhibit the yellow and fuchsia phenotype. Three exhibit the green and fuchsia
phenotype. Three exhibit the yellow and red
phenotype. And one exhibits the green and red
phenotype. Therefore, the ratio of phenotypes
is nine to three to three to one.
So the correct answer to our
question about the expected phenotypic ratio of a dihybrid cross between two
heterozygous individuals is (A) nine to three to three to one.
