The colour of onions is determined by two genes. The dominant R allele is for da
ID: 223252 • Letter: T
Question
The colour of onions is determined by two genes. The dominant R allele is for dark red colour and the recessive r allele is responsible for yellow coloured onions. The dominant I allele is epistatic to the R gene and prevents pigment from forming. When the I allele is present, the onion is white. In a dihybrid cross between red onions (RRii) and white onions (rrII), a 12 white: 3: red: 1 yellow ratio is observed in the F_2 progeny. a. What are the genotypes of the F_2 progeny in each of the three phenotypic classes? b. Of 200 F_2 onions from this cross, 145 were white, 40 were red and 15 were yellow. Use the chi^2 statistical test to check whether these numbers fit the expected 12:3:1 ratio.Explanation / Answer
1. In F1 generation
RRii * rrII
Red White
RrIi
White
F2 Self White genotypes - RRII, RRIi, RrII, RrIi, rrIi, rrIi
Red genotypes - RRii, Rrii
Yellow genotypes - rrii
2.
Using df = 2, 1.32 is less than the table value of 5.99, so the data are consistent.
O E O-E (O-E)2 (O-E)2 / E White 145 150 -10 100 0.66 Red 40 37.5 2.5 6.25 0.16 Yellow 15 12.5 2.5 6.25 0.5 Total 200 1.32