GENETICS Drosophilia females that are heterozygous for three recessive mutations
ID: 53499 • Letter: G
Question
GENETICS
Drosophilia females that are heterozygous for three recessive mutations [ e (ebony body), st (scarlet eyes), and ss (spineless bristles)] were testcrossed, and the following progeny were obtained. NOTE: the ‘+’ symbol in the table below represents the wild type phenotype; E represents the mutant phenotype of ebony body; ST represents the mutant phenotype of scarlet eyes, and SS represents the mutant phenotype of spineless bristles.
F1 Phenotype
Number of Offspring
+ (or +, +, +)
67
E, +, +
8
E, ST, +
68
E, +, SS
347
E, ST, SS
78
+, ST, +
368
+, ST, SS
10
+, +, SS
54
What is the order of these genes on the chromosome? (i.e. identify the gene that is in the middle of this group).
A) e/e+
B) ss/ss+
C) st/st+
F1 Phenotype
Number of Offspring
+ (or +, +, +)
67
E, +, +
8
E, ST, +
68
E, +, SS
347
E, ST, SS
78
+, ST, +
368
+, ST, SS
10
+, +, SS
54
Explanation / Answer
1. What is the order of these genes on the chromosome?
It is a double cross over event , there are two double crossover between the genes Ebony, spinless bristles ,scarlet eyes . the genotype of the F1 will be st+ e+ ss+/st e ss crossed st e ss / st e ss .these two double crossovers have two parental phenotypes each.there for the gene which is in the middle is spineless
Option B ; spineless is in the middle SS/SS+
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