Chapter 3, Problem 24P

a.	How would you describe inheritance of flower color? Describe how specific alleles influence each other and therefore affect phenotype.
b.	A white F2 plant is allowed to self-fertilize. Of the progeny, 3/4 are white-flowered, and 1/4 are purple-flowered. What is the genotype of the white F2 plant?
c.	A purple F2 plant is allowed to self-fertilize. Of the progeny, 3/4 are purple-flowered, and 1/4 are white flowered. What is the genotype of the purple F2 plant?
d.	Two white F2 plants are crossed with each other. Of the progeny, 1/2 are purple-flowered, and 1/2 are white-flowered. What are the genotypes of the two white F2 plants?

Two true-breeding white strains of the plant Illegitimati noncarborundum were mated, and the F progeny were all white. When the F plants were allowed to self-fertilize, 126 white-flowered and 33 purple-flowered F plants grew.

Summary Introduction

a.

To determine:

The inheritance of white colored flowers in the plant Illegitimati noncarborundum along with the description of the influence of alleles on each other and the effect on the phenotype of the plant.

Introduction:

The cross between white strains of plant Illegitimati noncarborundum results in the production of white colored flowers in the F₁ generation. The self cross between offsprings of F₁ generation produces 126 offsprings with white flowers and 33 offsprings with purple flowers.

Explanation of Solution

Let the genotype of white flowers of F₁ generation be AABB and aabb respectively. The cross between AABB and aabb is as follows:

	AB	Ab	aB	ab
AB	AABB (White)	AABb (White)	AaBB (White)	AaBb (White)
Ab	AABb (White)	AAbb (White)	AaBb (White)	Aabb (White)
aB	AaBB (White)	AaBb (White)	aaBB (Purple)	aaBb (Purple)
ab	AaBb (White)	Aabb (White)	aaBb (Purple)	Aabb (White)

The phenotypic ratio of flower color is as follows:

• White flowers with alleles A and B = $\frac{9}{16}$

• White flowers with alleles a and b = $\frac{1}{16}$

• White colored flower with alleles A and b= $\frac{3}{16}$

• Purple colored flower with alleles B and a= $\frac{3}{16}$

It can be observed that the phenotypic ratio of the F₂ generation is 13:3. This implies that A is epistatic over B resulting in 3 flowers with alleles A and b to be white whereas flowers with alleles B and a to be purple. It indicates that the presence of dominant allele A masks the effect of other alleles resulting in the production of white colored flowers. Therefore, the inheritance pattern is found to be dominant epistasis.

Summary Introduction

b.

To determine:

The genotype of white F₂ plants which produced offsprings with 3 out of 4 flowers as white and 1 out of 4 as purple.

Introduction:

Epistasis is a phenomenon in which one gene suppresses the expression of other genes. The gene suppressing the effect of other genes can be a dominant gene as well as recessive gene.

Explanation of Solution

According to the given information, the cross between a white F₂ plant produces 3/4 white flowers and 1/4 purple flowers. Let us assume the genotype of white parent plants to be AaBB.

The cross between AaBB and AaBB is as follows:

	AB	AB	aB	aB
AB	AABB (White)	AABB (White)	AaBB (White)	AaBB (White)
AB	AABB (White)	AABB (Unknown)	AaBB (White)	AaBB (White)
aB	AaBB (White)	AaBB (White)	aaBB (Purple)	aaBB (Purple)
aB	AaBB (White)	AaBB (White)	aaBB (Purple)	aaBB (Purple)

The phenotypic ratio of flower color is as follows:

• White flowers = $\frac{12}{16}=\frac{3}{4}$

• Purple flowers = $\frac{4}{16}=\frac{1}{4}$

The cross depicts that the assumption about parent plants having heterozygous alleles is correct. The ratio of 3:1 would be produced if the parents have genotype consisting of an amalgamation of the homozygous and heterozygous gene. Since A is epistatic over B, then all the offsprings having dominant A would produce white flowers. Therefore, the genotype of parent plant with white flowers is AaBB.

Summary Introduction

c.

To determine:

The genotype of purple F₂ plant which produced offsprings with 3 out of 4 flowers as purple and 1 out of 4 as white.

Introduction:

Epistasis is a phenomenon which affects the expression of genes. If dominant allele of one gene suppresses other genes, then it is known as dominant epistasis. On the contrary, if recessive allele of one gene suppresses other genes, then it is known as recessive epistasis.

Explanation of Solution

According to the given information, the cross between white F₂ plant produces 3/4 purple flowers and 1/4 white flowers. Let us assume the genotype of white parent plants to be aaBb.

The cross between aaBb and aaBb is as follows:

	aB	ab	aB	ab
aB	aaBB (Purple)	aaBb (Purple)	aaBB (Purple)	aaBb (Purple)
ab	aaBb (Purple)	aabb (White)	aaBb (Purple)	aabb (White)
aB	aaBB (Purple)	aaBb (Purple)	aaBB (Purple)	aaBb (Purple)
ab	aaBb (Purple)	aabb (white)	aaBb (Purple)	aabb (White)

The phenotypic ratio of flower color is as follows:

• Purple flowers = $\frac{12}{16}=\frac{3}{4}$

• White flowers = $\frac{4}{16}=\frac{1}{4}$

The cross depicts that the assumption about parent plants having heterozygous alleles is correct. The ratio of 3:1 would be produced if the parents have genotype consisting of an amalgamation of homozygous and heterozygous gene. Since A is epistatic over B, then all the offsprings having dominant B and recessive a would produce purple flowers. Therefore, the genotype of parent plant with purple flowers is aaBB.

Summary Introduction

d.

To determine:

The genotype of two white F₂ plants which produced offsprings with the phenotypic ratio of white and purple flowers as 1/2.

Introduction:

Genes are present on chromosomes. Each gene is responsible for the expression of a specific phenotype. The expression of genes in the progeny is known as phenotype, whereas the gene set which is responsible for a particular phenotype is known as genotype.

Explanation of Solution

According to the given information, the cross between two white F₂ plant produces 1/2 purple flowers and 1/2 white flowers. Let us assume the genotype of one parent as aabb and another parent plant as AaBB.

The cross between aabb and AaBB is as follows:

	ab	ab	ab	ab
AB	aAbB (White)	aAbB (White)	aAbB (White)	aAbB (White)
aB	aabB (Purple)	aabB (Purple)	aabB (Purple)	aabB (Purple)
AB	aAbB (White)	aAbB (White)	aAbB (White)	aAbB (White)
aB	aabB (Purple)	aabB (Purple)	aabB (Purple)	aabB (Purple)

The phenotypic ratio of flower color is as follows:

• Purple flowers = $\frac{8}{16}=\frac{1}{2}$

• White flowers = $\frac{8}{16}=\frac{1}{2}$

The cross depicts that the assumption about one parent having homozygous alleles and other parents having combination of homozygous and heterozygous gene is correct. The ratio of 1:1 would be produced. Therefore, the genotype of one parent plant with white flowers is aabb, and another parent plant with white flowers is AaBb.