In recessive alleles k(kidney-shaped eyes), c(cardinal-colored eyes), and e(ebon
ID: 57480 • Letter: I
Question
In recessive alleles k(kidney-shaped eyes), c(cardinal-colored eyes), and e(ebony body color) identify three genes on the same chromosome in Drosphila. Females with kidney-shaped, cardinal-colored eyes were mated with ebony males. The F1 were all wild-type. When F1 females were testcrossed with ebony males with kidney-shaped, cardinal-colored eyes, the following progeny phenotypes were obtained:
k
A) Determine the order of genes and the map distance between them
B) How much interference in double-recombination is there ?
+ c e 149k
+ + 144 k c + 784 + + e 790 + + + 3 k c e 6 + c + 68 k + e 70Explanation / Answer
Identify the parental classes - these are the largest, so kc+ and ++e
Identify double recombinants - smallest classes, so kce and +++
k and c are together in both parentals and doubles, so they are on the outside, the gene order is kec.
ke recombinants are ++c and ke+
% recombination = (no. recombinants/total progeny)*100% = ((58 + 67 + 3 + 4)/2000)*100% = 6.6%
ec recombinants are k++ and +ec so % recombination = ((49 + 44 + 3+ 4)/2000)*100% = 5%
% recombination between k and c = ((58 + 67 + 49 + 44)/2000)*100% = 10.9% Theoretically this should = smaller map distances combined, but in the real world usually does not, but this is a good enough match, so our calculations seem to be correct.
Distance in map units = % recombination, so k and e are 6.6 map units apart, e and c 5 map units apart ( k and c 10.9 map units apart).
As the parental chromosomes were (in the correct gene order) k+c and +e+, then the genotype of the F1 was k+c/+e+. Parental mother was kidney cardinal wild type body so her genotype was k+c/k+c and the parental male was +e+/+e+
coefficient of coincidence = no. observed double crossovers/no. expected double crossovers
Expected no. doubles = f(ke recombinants) x f(ec recombinants) x no. progeny
= 0.066 x0.05 x2000 = 6.6
Observed no. doubles = 7, so coefficient of coincidence = 7/6.6 = 1.06
Interference = 1- coefficient of coincidence = -0.06. Negative interference is very unusual and difficult to explain. Suggests that the presence of 1 crossover encourages the second to occur, but more likely rates of crossing over are variable. Bad example for a beginner as it is very rare
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