Question

Two pairs of alleles interact to determine cyanide production in different strains of white clover: some strains have high cyanide content, while others have low cyanide content. When a true-breeding strain (“H”) that produces high amounts of cyanide was crossed to a true-breeding strain (“L”) that produces low amounts of cyanide, the F₁ plants all had high cyanide content. When the F₁ plants were crossed to each other, the following resulted:   

Low cyanide content: 350 plants High cyanide content: 450 plants

a). Establish a “key” which describes how the alleles interact. (4 pts)

A =                                         B =

a =                                          b =

b). List all of the genotypes that give rise to the following phenotypes: (3 pts)

i).  High cyanide plants:

ii).  Low cyanide plants:

c). What are the genotypes of the P₁ and the F₁ plants? What are the genotypes of the gametes that these plants produce? (6 pts)

d). What is the expected phenotypic ratio of the F₂ progeny? Does this ratio differ from the phenotypic ratio that Mendel described when he studied two independently assorting genes that determine two different characteristics? Explain. (5 pts)

e). When two F₁ plants mate, what is the probability that their first five offspring will be heterozygous at each loci or that they will be homozygous recessive at both loci? Use expected values, not observed values. (4 pts)

Answer 1

High cyanide content = A_B_

Low cyanide content = A_bb, aaB_, aabb

A.

A = high content

B = high content

a = low content

b = low content

B.

P1 : AABB (high) × aabb (low)

F1 : AaBb (high)

C.

Gametes by F1 : AB, Ab, aB, ab

D.

F2 : AaBb × AaBb

Expected phenotypic ratio = 9 : 7

9 = A_B_

7 = aaB_ + A_bb + aabb

This ratio different from that of the classical mendelian ratio, 9:3:3:1, obtained in dihybrid cross. It is because the alleles are not getting independently of each other during gamete formation. They are interacting with each other. They are showing a non mendelian type of inheritance which is epistasis.

Epistasis is non allelic interaction between genes Where alleles at one gene locus mask the effect of allele at other gene locus.

E.

# AaBb = 4

# aabb = 1

Probability that first five offsprings are AaBb = (4/16)^5

Probability that first five offsprings are aabb = (1/16)^5

Final answer = (4/16)^5 + (1/16)^5 = 1024 + 1 / 16^5 = 1025/1048576 = 0.000977

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