Question

Blue flower color is produced in a species of morning glories when dominant alleles are present at two gene loci, A and B. Purple flowers result when a dominant allele is present at only one of the two gene loci, A or B. Flowers are red when the plant is homozygous recessive for each gene. What flower color ratio is expected from the cross of aaBb to AABb?

Answer 1

The flower colour ratio obtained is: Blue : Purple = 3 : 1.

Step-by-step explanation:

• According to the question,

1. the genotype, AABB or AaBb or AABb or AaBB gives blue phenotype.
2. the genotype, AAbb or aaBB or Aabb or aaBb gives purple phenotype.
3. Red phenotype is for the aabb genotype.

• As only the presence of dominant allele is mentioned but whether it is homozygous or heterozygous is not mentioned, in 1. we consider that presence of a single dominant allele A or B in either locus is capable of showing a blue phenotype, irrespective of the nature of the other allele.
• In 2. we consider that the presence of a single dominant allele, A or B in one locus with recessive pairs of alleles in the other locus is sufficient to develop the purple phenotype.
• The aaBb individual will produce the gametes: aB, ab.
• The AABb individual will produce the gametes: AB, Ab.
• Crossing them,

aB ab

AB AaBB AaBb

(Blue) (Blue)

Ab AaBb Aabb

(Blue) (Purple)

• Of the offspring obtained, 3 have blue phenotype and 1 has purple phenotype.
• Hence, they have the following phenotypic ratio:

Blue : Purple = 3 : 1.