The recessive alleles k (kidney-shaped eyes), c (cardinal-colored eyes), and e (
ID: 57952 • Letter: T
Question
The recessive alleles k (kidney-shaped eyes), c (cardinal-colored eyes), and e (ebony body color) identify three genes on the same chromosome in Drosophila. 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:
+ c e 149
k + + 144
k c + 784
+ + e 790
+ + + 3
k c e 6
+ c + 68
k + e 70
a.Determine the order of genes and the map distances between them.
b.How much interference in double-recombination is there?
Explanation / Answer
a. Classes of progeny, k c + and + + e which are most common represent the parental type while k c e and + + + are least common and represent double crossovers. When least common and most common are compared we deduce that gene e is in the middle.
The recombinant classes must now be determined to calculate the map distance.
K + e, + c +, + c e and + c e are the remaining crossover events.
Therefore map distance between k and e is given by:
Recombinants= 70+68+6+3 = 147
Map distance = Recombinant individuals/ total progeny X 100%
= 147/2014 X 100%
= 7.29 m.u.
Map distance between e and c is given by:
Recombinants= 144+147+6+3 = 300
Map distance = Recombinant individuals/ total progeny X 100%
= 300/2014 X 100%
= 14.89 m.u.
The gene order is k---e----c
b. Expected frequency of double crossover = 7.89 X 14.89 = 1.08
Observed frequency = 6+3/ 2014X100 = 0.44
Interference is calculated as:
I = 1 – (Observed DCO/Expected DCO)
= 1- .407
= .593
This value is significant because it is positive and lies between 0 and 1. The interference is equal to .593
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