Video Transcript
Assume that in fruit flies, the
allele for a gray body, uppercase G, is dominant to the allele for a black body,
lowercase g, and the allele for normal wings, uppercase N, is dominant to the allele
for small wings, lowercase n. A fly with a genotype uppercase G
lowercase g uppercase N lowercase n is crossed with a fly with a genotype lowercase
g lowercase g lowercase n lowercase n. What is the probability, in
percent, that the offspring will have a black body and normal wings?
Looking at the question, we can see
that for each gene, G or N, there are two possible alleles. Uppercase and lowercase letters
represent the dominant and recessive alleles, respectively. As the question asks about the
inheritance of two genes and their alleles, we know that we are being asked about
dihybrid inheritance.
When these two flies are crossed,
their gametes will combine during fertilization. So we first need to determine the
combination of alleles present in their gametes. We can do this using the FOIL
method with each genotype. Each gamete needs one allele for
each gene. So, for the first possible
combination of alleles in the gametes, we take the first allele of each gene. Then, we take the outside alleles
of each gene, followed by the inside alleles of each gene. Finally, we combine the last
alleles for each gene. So we have four possible
combinations of alleles that could be present in this fly’s gametes. Why don’t you pause the video and
see if you can determine the possible combination of alleles in the gametes of the
second fly?
Good work! Now we have the information needed
to start constructing our Punnett square. Along the top of the Punnett square
we will put the possible combination of alleles for the gametes of the first fly and
down the side of the Punnett square the possible combination of alleles for the
gametes of the second fly. To complete each cell, we take the
alleles from the column head and the row head and combine them. Repeating this throughout the
Punnett square gives us all the possible genotypes for an offspring born to these
parents.
This question asks us to identify
the offspring with a black body and normal wings. We have been told that the allele
for a black body is recessive. So, for this to be expressed, we
would need two copies of this allele to be present in the genotype. The allele for normal wings is
dominant. So this will be expressed if one or
if two copies of this allele is present in the genotype. So how many cells in our Punnett
square fit this description?
There are four genotypes which
would produce a fly with a black body and normal wings. Our question asks us to express the
answer as a percentage. To calculate this, we divide four
by the total number of possible genotypes, which is 16. Then, we multiply this answer by
100 percent. So the probability that the
offspring born to these parents will have a black body and normal wings is 25
percent.
