Video Transcript
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.
