- A cross was made between pure wild type males and brown eyed curled winged females of D. melanogaster. The F1 females were test crossed. The F2 progeny obtained was as follows:
Wild type 200
Brown eyes, curled wings 150
Brown eyes, normal wings 30
Normal eyes, curled wings 20
Total400
The genetic distance (cM) between brown eye and curled wing loci is:
(1) 12.5 (2) 50
(3) 150 (4) 25
Introduction:
Genetic distance is a crucial concept in genetics that measures how far apart two genes are on a chromosome, often calculated from recombination frequencies observed in progeny. This article explains how to calculate the genetic distance in centimorgans (cM) between the brown eye and curled wing loci in Drosophila melanogaster using the data from a test cross.
Understanding Genetic Distance and Recombination Frequency
Genetic distance between two gene loci is estimated by the recombination frequency—the proportion of recombinant offspring produced from a test cross. This frequency is converted into map units or centimorgans (cM), where 1% recombination equals 1 cM. Higher recombination frequencies indicate genes are farther apart on the chromosome, while lower frequencies indicate close linkage.
Data from the Test Cross and Recombinant Calculation
Given in the progeny:
-
Wild type (non-recombinant parental type): 200
-
Brown eyes, curled wings (non-recombinant parental type): 150
-
Brown eyes, normal wings (recombinant type): 30
-
Normal eyes, curled wings (recombinant type): 20
Total progeny = 400
Recombinant offspring = 30 + 20 = 50
Calculating Genetic Distance Between Genes
The recombination frequency (RF) is calculated as:
RF=Number of recombinant offspringTotal number of offspring=50400=0.125=12.5%
Therefore, the genetic distance between the brown eye and curled wing genes is 12.5 cM.
Explanation of Options
(1) 12.5 cM – Correct: Matches the recombination frequency calculated from progeny data.
(2) 50 cM – Incorrect: This would mean half of the offspring were recombinant, which is not the case.
(3) 150 cM – Incorrect: Genetic distances over 100 cM are not possible because recombination frequencies max out at 50%.
(4) 25 cM – Incorrect: Double the actual recombinant frequency in this scenario.
This calculation shows the genes for brown eyes and curled wings in Drosophila are linked with a genetic distance of 12.5 cM, derived from observed recombination in test cross progeny.
