Question

Priscilla Lane and Margaret Green studied the linkage relations of three genes affecting coat color in mice: mahogany (mg), agouti (a), and ragged (Rg). They carried out a series of three-point crosses, mating mice that were heterozygous at all three loci with mice that were homozygous for the recessive alleles at these loci (P W. Lane and M. C. Green. 1960. Journal of Heredity 51:228-230). The following table lists the results of the testcrosses. Determine the order of the loci that encode mahogany, agouti, and ragged on the chromosome, the map distances between them, and the interference and coefficient of coincidence for these genes. Draw a picture of the two chromosomes in the triply heterozygous mice used in the testcrosses,

Answer 1

Answer:

A).

Order of genes = Rg—a---mg

Gene map:

Rg----------25.35mu--------a-----12.71mu-----mg

Coefficient of Coincidence (COC) = 0.29

Interference = 0.71

B). Triple heterozygote = + a mg / Rg + +

Explanation:

Hint: Always parental combinations are more in number than the any type of recombinants. Triple heterozygote = a + mg / + Rg +

1).

If single crossover occurs between a & rg

Normal combination: a + / + rg

After crossover: a rg / + +

a rg progeny= 1+36=37

+ + progeny = 1+16 = 17

Total this progeny = 54

Total progeny = 213

The recombination frequency between a & rg = (number of recombinants/Total progeny) 100

RF = (54/213)100 = 25.35%

2).

If single crossover occurs between rg & mg

Normal combination: + mg / Rg +

After crossover: + + / Rg mg

+ + progeny= 15+16=31

Rg mg progeny = 9+36= 45

Total this progeny = 76

The recombination frequency between rg & mg = (number of recombinants/Total progeny) 100

RF = (76/213)100 = 35.68%

3).

If single crossover occurs between a & mg

Normal combination: a mg / + +

After crossover: a + / + mg

a + progeny= 15+1=16

+ mg progeny = 1+9=10

Total this progeny = 26

The recombination frequency between a & mg = (number of recombinants/Total progeny) 100

RF = (26/213)100 = 12.71%

Recombination frequency (%) = Distance between the genes (mu)

Rg----------25.35mu--------a-----12.71mu-----mg

Expected double crossover frequency = (RF between Rg & a) * (RF between a & mg)

= 25.35% * 12.71% = 0.0322

The observed double crossover frequency = 1+1/213 = 0.0094

Coefficient of Coincidence (COC) = Observed double crossover frequency / Expected double crossover frequency

= 0.0094 / 0.0322

= 0.29

Interference = 1-COC

= 1-0.29 = 0.71