In a species of tree, seed color is determined by four independently assorting g
ID: 172883 • Letter: I
Question
In a species of tree, seed color is determined by four independently assorting genes: A, B, C, and D. The recessive alleles of each of these genes (a, b, c, and d) produce abnormal enzymes that cannot catalyze a reaction in the biosynthetic pathway for seed pigment. This pathway is diagrammed as follows: When both red and blue pigments are present, the seeds are purple. Trees with the genotypes Aa Bb Cc Dd and Aa Bb Cc dd were crossed. (a) What color are the seeds in these two parental genotypes? (b) What proportion of the offspring from the cross will have white seeds? (c) Determine the relative proportions of red, white, and blue offspring from the cross.
Please explain the following solution to this problem: (a) purple × red; (b) proportion white (aa) = 1/4; (c) proportion red (A- B- C- dd) = (3/4)(3/4)(3/4)(1/2) = 27/128, proportion white (aa) = 1/4 = 32/128, proportion blue (A- B- cc Dd) = (3/4)(3/4)(1/4)(1/2) = 9/128.
Explanation / Answer
a) The color of the seeds of the parental genotypes is purple and red because the first genotypes have the allele for the synthesis of red as well as the blue pigment which together gives purple pigment while the second genotype does not have the allele for the synthesis of blue pigment so the color of the seed is red.
b) the proportion of the getting the white seed color is 1/4 which means that the seed lacks the enzyme for the synthesis of the pigment for all the four genes. The color of the seed will be white only when all the four genes is present in the recessive condition which always occur once in a cross. therefore the probability of the occurrence is 1/4 among four genes.
c) the proportion of the red, white, and blue offspring is calculated by the calculation of the probability of the occurrence of the dominant and recessive alleles. the occurrence of red is 3/4*3/4*3/4*1/2= 27/128, this is calculated by taking the four genes separetely, that means the probability of occurrence of Allele A can be calculated by forming punnet square of Aa* Aa which will give three dominant genotypes out of four. similarly, for all the four genes are calculated that gives 3/4*3/4*3/4*1/2=27/128. The same procedure is taken for the calculation of proportion for white and blue is taken which gives the ratio 1/4 for white and 9/128 for blue.