A plantlike organism on the planet Pandora can have three recessive genetic traits: bluish leaves,due to an allele (a)of gene A;a feathered stem,due to an allele (b)of gene B;and hollow roots,due to an allele (c)of gene C.The three genes are linked and recombine.
A geneticist did a testcross with an organism that had been found to be heterozygous for the three recessive traits and was able to identify progeny of the phenotypic distribution (+ = wild type)shown in the figure below.
If recombination frequency is equal to distance in map units,what is the approximate distance between genes A and B?
A)1.5 map units
B)3 map units
C)6 map units
D)15 map units

Question

A plantlike organism on the planet Pandora can have three recessive genetic traits: bluish leaves,due to an allele (a)of gene A;a feathered stem,due to an allele (b)of gene B;and hollow roots,due to an allele (c)of gene C.The three genes are linked and recombine.
A geneticist did a testcross with an organism that had been found to be heterozygous for the three recessive traits and was able to identify progeny of the phenotypic distribution (+ = wild type)shown in the figure below. 
If recombination frequency is equal to distance in map units,what is the approximate distance between genes A and B?
A)1.5 map units 
B)3 map units 
C)6 map units 
D)15 map units

Quizplus · Accepted Answer

To determine the distance between genes A and B, we need to first identify the frequency of recombination between them. From the figure provided, we can see that of the total 500 offspring, 11 have the recombinant phenotype (bluish leaves and hollow roots but no feathered stem) while the other three possible combinations of traits are present in 246, 235, and 8 offspring each. 
The recombination frequency (RF) between genes A and B can be calculated as the ratio of the number of recombinants to the total number of offspring: 
RF = (11/500) × 100% = 2.2%
This means that 2.2% of the gametes produced by the heterozygous parent underwent crossing over between genes A and B during meiosis.
The next step is to use the fact that RF is equal to the distance between genes in map units (if we assume that no double crossovers occurred). We can convert RF to map units (centimorgans or cM) using the formula:
RF = (100 × # of recombinants) / total # of offspring
2.2 = (100 × 11) / total #, solve for total # gives us total # = 500
Then, 2.2 = (100 × 11) / 500 = 2.2 cM
Therefore, the approximate distance between genes A and B is 2.2 cM or 2.2 map units. 
Among the answer choices provided, only option B is closest to this value.

A Plantlike Organism on the Planet Pandora Can Have Three