A breeder of orange trees begins to assemble a genetic map based on four recessi
ID: 148422 • Letter: A
Question
A breeder of orange trees begins to assemble a genetic map based on four recessive loci- thickskin, yellowish, sour and petite – named after the fruit phenotypes of homozygotes. After identifying two true-breeding trees that are either completely wild type or are mutant for all four loci, the breeder crosses them and plants an orchard of the resultant F2 trees. Based on the following frequencies of mutant classes, determine which loci are likely to be on the same chromosome and which are the most closely linked
Phenotypes
Number of trees
Normal
402
Petite
127
Sour
115
Thick and yellow
108
Thick, yellow, and petite
42
Thick, yellow, and sour
41
Sour and petite
38
Yellow and sour
24
Thick
18
Yellow, sour and petite
12
Yellow
11
Thick and petite
10
Thick, yellow, sour and petite
8
Thick and sour
7
Yellow and petite
3
Thick, sour and petite
2
Phenotypes
Number of trees
Normal
402
Petite
127
Sour
115
Thick and yellow
108
Thick, yellow, and petite
42
Thick, yellow, and sour
41
Sour and petite
38
Yellow and sour
24
Thick
18
Yellow, sour and petite
12
Yellow
11
Thick and petite
10
Thick, yellow, sour and petite
8
Thick and sour
7
Yellow and petite
3
Thick, sour and petite
2
Explanation / Answer
first add all the phenotypes to confirm that mutant gene segregates with mendelian ratios.so based on four recessives now we should calculate so total trees are 968 and one quarter of them is 242i.e (968/4).so 242 shows recessive phenotype.now their are 242 peptite ,249 reddish ,247 sour and 236 thick skin orange producing trees.based on this completely linked loci is nearly 240 double mutants.hence by adding the double mutants: A.Thick and red(199) and red and peptite(65) B.thickand peptite(62) and red and sour (85) C.thick and sour(85) and sour and peptite(60). this shows that thickskin and reddish loci are closely linked,reddish and sour loci are also linked but less than thickkskin and reddish.thik skin and the sour appear independently (the reddish loci might be present between them) and the peptite maps to different linkage group.
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