A Maryland farmer has been trying to produce a new flower to sell to customers.

Question

A Maryland farmer has been trying to produce a new flower to sell to customers. She has identified three dominant mutant traits: glowing leaves (G), big and beautiful flowers (B), rose aroma (R) that are more desirable than the wild type traits of non-glowing leaves (g), ugly flowers (b), no aroma (r). From her original crosses he determines that the three genes are linked. To map the genes she performs a heterozygous testcross and obtains the following results: Draw a map of these genes showing the proper gene order and distances between each gene Define AND determine the coefficient of coincidence and interference

Explanation / Answer

1. Determining parental genotype:

The most abundant genotypes are the parental genotypes.

Here, gbr and GBR with 364 and 353 are the parental genotypes.

2. Gene order:

Least frequent genotypes are gBR and Bbr with 11 and 9 progeny respectively

Double-crossover event moves the middle allele from one sister chromatid to the other

We can see from the table that the G gene must be in the middle because the recessive g allele is now on the same chromosome as the B and R alleles, and the dominant G allele is on the same chromosome as the recessive b and r alleles

Therefore the gene order is BGR

3. Linkage distance:

B-G distance = [100 x (54+62+9+11)] / 1085 = 12.53 cM

G-R distance = [100 x (111+121+9+11)] / 1085 = 23.22 cM

4. Gene map:

12.53 23.22

I_________I_________________I

B G R

5. Co-efficient of coincidence:

= observed double recombinants / expected double recombinants

expected double recombinants = 100 x (0.1253 x 0.2322) = 0.029 = 2.9%

In 1085 progeny, the expected double recombinants is 1085 x 0.029 = 31

co-efficient of coincidence = observed double recombinants / expected double recombinants

= (20) / (31)

= 0.645

= 64.5%

6. Co-efficient of incidence:

Co-efficient of incidence = 1 - co-efficient of coincidence

= 1 - 0.645

= 0.355

35.5%

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