A mutant screen of jimsonweed turns up a single plant that haswhite flowers, rat

Question

A mutant screen of jimsonweed turns up a single plant that haswhite flowers, rather than the normal purple flowers. This mutantplant was crossed with a true-breeding purple-flowered plant andall of the F1 offspring had purple flowers. When theseplants were self-pollinated, their offspring were a mixture ofphenotypes: 49 white and 147 purple.

i) Write the genotypes and phenotypes of the parental plants,their F1 offspring, and the F2 offspring. Usecorrect notation of your own choosing.

ii) If a white-flowered F2 plant is self-pollinated,what phenotypes are expected in the offspring, and in whatproportions?

iii) What proportion of the purple-flowered F2 plantsdo you expect to breed true when self-pollinated?

iv) If a white-flowered F2 plant is backcrossed to anF1 plant, what phenotypes are expected in the offspring,and in what proportions?

Explanation / Answer

Let the purple allele be 'P' and the white allele be 'p'. Youshould notice that the phenotypes of the F1 offspring are producedat a ratio of about 1:3. This is the classic Mendelian ratioproduced from the cross of two heterozygous individuals (i.e. Pp).This also fits with the data. Let's say the white mutant is 'pp'and the purple plant, since it is true breeding, is 'PP' (ratherthan 'Pp'). All of the F1 generation is then 'Pp', and thephenotypes are all purple. The F2 generation genotypes are thenproduced in a ratio of 1 PP : 2 Pp : 1 pp (draw a Punnett square ifyou are unsure), and the phenotypes are produced in a ratio of 3purple : 1 white, which corresponds to the data. We know that the white-flowered plant is 'pp'. Thus the onlypossibly phenotype that can be produced from self-pollination is'pp' = white. (looks like this my have been a trick question...) The probability of us selecting the 'PP' plant from thepurple F2 generation is 1/3. If a 'PP' purple plant isself-pollinated, 100% will be true-breeding purple. The probabilityof selecting a 'Pp' plant from the purple plants is 2/3. If a 'Pp'plant is self-pollinated we will again get 1 PP : 2 Pp : 1 ppplants. So the probability of getting a true-breeding purple plantfrom a 'Pp' self pollination is 2/3 * 1/4 = 2/12 = 1/6. Thus theoverall probability of selecting a purple plant is (Ithink!) 1/3 + 1/6 = 3/6 = 0.5. Not quite sure of my answer to this one - you may want to checkwith someone else. The white flowered plant is 'pp'. The F1 plant is 'Pp'. Thus thegenotypes will be 1 Pp : 1 pp, or equal proportions of purple towhite plants.

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