Cyanogenesis is an anti-predation mechanism in the white clover, Trifolium repen

Question

Cyanogenesis is an anti-predation mechanism in the white clover, Trifolium repens. It is regulated by two loci. The first (A/a) ensures production of glucoside and the second (L/i) encodes the enzyme linamarase(B-d-glucosidase) which catalyzes the production of hydrogen cyanide from the glucoside. thus, if the two loci are in the dominant state (A-L-), leaves readily produce hydrogen cyanide when crushed. If only the A locus is producing a functional product, cyanogenesis still occurs, but at a slow, spontaneous rate. the plant is acyanogenic if the first gene product is absent. from a cross invovlving plants heterozygous at both loci, whis is the probability that any one of the resulting clover plants will be:

A. cyanogenic

B. acyanogenic

C. show slow cyanogenesis

Explanation / Answer

The relationship berween different genotypes is shown below.

A. The genotypes A-L- produces the cyanogenic clover plants. From the above table, there are 9 cases pf AL genotypes out of 16 combinations (AALL, AaLL, AALi, AaLi, AaLL, AaLi, AALi, AaLi, AaLi).

So, the probability of cyanogenic clover plants = 9/16

B. The plant is acyanogenic if the first gene product is absent, i.e. the gene A is inactive. There are 4 cases, where the gene A is inactive (aaLL, aaLi, aaLi, aaii). So the probaibility that the plant is acyanogenic = 4/16 = 1/4

C. When the genotype A is in dominance, the plant shows slow cyanogenesis. From the above table, there are 3 cases, where the genotype A is in dominance (AAii, Aaii, Aaii) . So the probability that the plant shows slow cyanogenesis = 3/16

Gametes AL aL Ai ai AL AALL AaLL AALi AaLi aL AaLL aaLL AaLi aaLi Ai AALi AaLi AAii Aaii ai AaLi aaLi Aaii aaii

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