Question

In a unique species of plants, flowers may be either yellow, blue, red, or mauve. A pure-breeding line with blue flowers is crossed to a pure-breeding red-flowered line, and the resulting F₁ plants have yellow flowers. In the F₂ the following ratio is observed:

9/16 yellow: 3/16 blue: 3/16 red: 1/16 mauve

If pure-breeding yellow and mauve plants are crossed, the same F₂ ratio is observed.

A. Give a concise genetic hypothesis for the inheritance of this trait (include number of genes, number of alleles and dominance relations between alleles).

B. Designate your own gene symbols and list the genotypes and ratios for each F₂ class.

C. Determine the expected phenotypic ratios if the F₁ from above is crossed to a pure-breeding mauve plant.

Answer 1

A).

Non epistatic Inter allelic genetic interaction:

In some cases two pairs of genes determine same phenotype but assorted independently, produce new phenotypes by mutual epistatic interaction. The F₂ ratio remains the same as 9:3:3:1.

Gene A consists of two alleles A(dominant) & a (recessive)

Gene B consists of two alleles B(dominant) & b (recessive)

A_B_ = Yellow

A_bb = Blue

aaB_ = Red

aabb = Mauve

B).

Purple (AABB) x Mauve (aabb)

             AaBb (Mauve) ---F1

F1 x F1

	AB	Ab	aB	ab
AB	AABB (Yellow)	AABb(Yellow)	AaBB(Yellow)	AaBb(Yellow)
Ab	AABb(Yellow)	AAbb (blue)	AaBb(Yellow)	Aabb (blue)
aB	AaBB(Yellow)	AaBb(Yellow)	aaBB (red)	aaBb (red)
ab	AaBb(Yellow)	Aabb (blue)	aaBb (red)	aabb (Mauve)

Yellow : blue : Red : Mauve = 9 : 3 : 3 : 1

C).

AaBb (F1) x aabb—Test cross

	Ab
AB	AaBb (yellow)
Ab	Aabb (blue)
aB	aaBb(red)
ab	aabb(mauve)

Purple : Blue : Red : Mauve = 1 : 1 : 1 : 1