In guppies, body color is controlled by one gene and gray is dominant to gold. T
ID: 91489 • Letter: I
Question
In guppies, body color is controlled by one gene and gray is dominant to gold. The size of spots on the tail is controlled by one gene with large spots being dominant to small spots. Suppose that a true breeding strain with gray body and large spots is crossed with a strain having gold body and small spots. The F_1 generation guppies are intercrossed to produce an F_2 generation. Which of the following diagrams shows correct use of the forked line method to predict the results of this cross? Select one:Explanation / Answer
Ans. Two genes regulating the phenotype are-
I. Gene for body color: Alternate phenotype = Gold (dominant), Gray (Recessive)
II. I. Gene for spot size: Alternate phenotype = Large (dominant), small (Recessive)
While using fork method of making cross (or gametes), only one gene (you can take any gene, say gene 1) is taken in first step. In next step, each alternate forms of gene 1 (here gene for body color) is combined with the alternate forms of gene 2 (here, gene for spot size). It successively continues till the last gene (nth gene) alternate forms are included in the fork.
Note that the sequence of gene (which gene to take first or in which order) does NOT matter. However, the fork method is based on the fact that the alternate forms of one gene must be taken at same time.
Figure a shows phenotype of two different genes (gray, large spot) taken at same time in the fork. It’s incorrect method.
Figure b shows a gene’s alternate forms taken at same time- gray and gold at one time, large and small spot at second time. It’s the correct method.