Video Transcript
Flower color in sweet pea plants is
an example of a characteristic affected by complementary gene action. The flowers can be white or
purple. In this scenario, there must be a
dominant allele present for both genes for the purple flower color to be shown. Which of the following genotypes
would give a plant with white flowers? (A) Uppercase A, uppercase A,
uppercase B, uppercase B. (B) Uppercase A, lowercase a,
uppercase B, lowercase b. (C) Uppercase A, lowercase a,
uppercase B, uppercase B. (D) Lowercase a, lowercase a,
uppercase B, uppercase B. Or (E) uppercase A, uppercase A,
uppercase B, lowercase b.
Complementary gene action refers to
the interaction between multiple genes that control the expression of a particular
trait. Here, we’re told that there must be
a dominant allele present for both genes for the purple flower color to be present
in the phenotype. In genetics, we tend to represent
dominant alleles with uppercase letters and recessive alleles with lowercase
letters. We need to spot the genotype that
will give the phenotype of white flowers. This means that one or both genes
have two copies of a recessive allele.
Option (A) has two uppercase As and
two uppercase Bs. Because this genotype has only
dominant alleles, this will produce purple flowers. So, this can’t be the correct
option. Option (B) has one uppercase A and
one uppercase B. Because there is one dominant
allele present for each gene, this genotype will produce purple flowers. Again, this can’t be the correct
option. Option (C) has one dominant allele
for gene a and two dominant alleles for gene b. This will also produce purple
flowers. So, this is not the correct
choice.
Option (D) has two dominant alleles
for gene b. However, gene a has two recessive
alleles. We know that to produce purple
flowers, there must be at least one dominant allele present for each gene. This genotype will therefore
produce white flowers due to it having no dominant allele for gene a. This looks like our correct answer,
but let’s first double-check option (E) to make sure it’s incorrect. Option (E) has two dominant alleles
for gene a and one dominant allele for gene b. Because both genes have a dominant
allele present, this genotype will produce purple flowers. So, this choice is incorrect. Therefore, we can conclude that the
genotype that will produce a plant with white flowers is (D) lowercase a, lowercase
a, uppercase B, uppercase B.
