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?

Answer 1

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