Three genes with recessive mutant phenotypes are located on the same chromosome
ID: 203263 • Letter: T
Question
Three genes with recessive mutant phenotypes are located on the same chromosome in maize: short stem (ss), purple aleurone (pr), and knotted leaf (kn). A plant with all three mutant phenotypes is crossed with a true-breeding, wild-type plant. Resulting F1 plants are then backcrossed to the parental mutant line to produce the following progeny: (7 points)
Short Purple Knotted Number
ss + kn 2342
+ pr + 2297
ss pr kn 4095
+ + + 3963
+ + kn 979
ss pr + 992
ss + + 77
+ pr kn 75
A. How many non-recombinant progeny were identified?
B. How many of the F2 plants contained a crossover between the short stem and knotted leaf genes?
C. Draw a chromosome map with the correct distances (rounded to one decimal point) noted between adjacent pairs of genes.
D. What is the gene order and arrangement of alleles for the P and F1 plants used in these crosses?
Explanation / Answer
Answer:
A). Non-recombinant progeny = 4095+3963 = 8058
B). Short stem and knotted leaf gene progeny = 6437
ss + kn----2342
ss pr kn---4095
Tottal----6437
C). pr--------32.3cM-----ss-----14.3cM-------kn
D).
pr ss kn / pr ss kn x +++ / +++ -------P1
pr ss kn / + + + --------------F1
Explanation:
Hint: Always recombinant genotypes are smaller than the non-recombinant genotypes.
Hence, the parental (non-recombinant) genotypes is ss pr kn / + + +
1).
If single crossover occurs between ss(first allele) & pr
Normal combination: ss pr/ + +
After crossover: ss + / + pr
ss + progeny= 2342+77= 2419
pr + progeny = 2297+75=2372
Total this progeny =4791
Total progeny = 14820
The recombination frequency between ss&pr = (number of recombinants/Total progeny) 100
RF = (4791/14820)100 = 32.3%
2).
If single crossover occurs between pr (second allele) & kn.
Normal combination: pr kn / + +
After crossover: pr + /+ kn
pr + progeny= 2297+992=3089
+ kn progeny = 2342+979=3321
Total this progeny = 6410
The recombination frequency between pr & kn = (number of recombinants/Total progeny) 100
RF = (6410/14820)100 = 43.3%
3).
If single crossover occurs between ss (first allele) & kn.
Normal combination: ss kn / + +
After crossover: ss + /+ kn
ss + progeny= 77+992=1069
+ kn progeny = 75+979=1054
Total this progeny = 2123
The recombination frequency between ss & kn = (number of recombinants/Total progeny) 100
RF = (2123/14820)100 = 14.3%
Recombination frequency (%) = Distance between the genes (cM)
pr--------32.3cM-----ss-----14.3cM-------kn
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