Question

If AB/ab is test crossed, we get AB/ab 180; ab/ab 170, Ab/ab 50, aB/ab 60. Then what is the distance between A and b?
A. 48 Morgan
B. 24 Morgan
C. 12 Morgan
D. 76 Morgan
E. 38 Morgan

Answer 1

Hint: Test cross can be defined as a cross in which the recessive phenotype parent is mated with a given phenotype. The formula for calculating the distance between two genes = (number of phenotypes arrived after crossing over / total number of phenotypes) * 100. The unit of distance between two genes is the map unit (mu) or centimorgan (cM). 1 Morgan = 100 cM.

Complete answer: According to the question,
AB/ab is test crossed with ab/ab
Given,
AB / ab = 180
ab / ab = 170
Ab / ab = 50
aB / ab = 60
When we perform a test cross between AB/ab and ab/ab, we get 4 offsprings with phenotype AB / ab, ab / ab, Ab / ab, aB / ab.
From this AB/ab and ab/ab are parental phenotype and Ab/ab and aB/ab are crossed over phenotype.
According to the formula, the distance between A and b will be = [(50+60) / (180+170+50+60)] * 100 Morgan
Distance between A and b = 23.9 Morgan or 24 Morgan.

Thus, the correct answer is option B. 24 Morgan.

Note: The highest two numbers are parental type; the lowest numbers are crossed over type. In the case of three gene mapping, the lowest two numbers are double crossover type.