Question

A homozygous tomato plant with red fruit and yellow flowers was crossed with a homozygous tomato plant with golden fruit and white flowers. The F1 all had red fruit and yellow flowers. The F1 were testcrossed by crossing them to homozygous recessive individuals and the following offspring were obtained:Red fruit and yellow flowers—41Red fruit and white flowers—7Golden fruit and yellow flowers—8Golden fruit and white flowers—44
How many map units separate these genes?

Answer 1

Answer:

15 m.u.

Explanation:

The genes for fruit and flower color are located in the same chromosome and we have to determine the distance between them.

The initial cross between two homozygous individuals with different traits resulted in an homogeneous F1 with red fruit and yellow flowers. Because of Mendel's Law of Uniformity, we know that the red fruits and yellow flowers are the dominant alelles.

I will call:

R_ = red fruit

rr   = golden fruit

Y_ = yellow flowers

yy  = white flowers

Initial cross:

P      RY/RY    x    ry/ry

F1              RY/ry

All F1 tomato plants have red fruit and yellow flowers because they have the R and Y alelle.

Test cross:

RY/ry     x    ry/ry

The gametes produced by the homozygous individual can only be ry.

The gametes produced by the heterozygous individual can be:

RY - Parental

ry - Parental

Ry - Crossover

rY - Crossover

F2 progeny:

41 RY/ry (Parental, Red fruit and yellow flowers)

44 ry/ry (Parental, Golden fruit and white flowers)

7 Ry/ry (CO, Red fruit and white flowers)

8 rY/ry (CO, Golden fruit and yellow flowers)

Total= 100.

The formula to calculate genetic distance is:  

Genetic Distance (m.u.)= Recombination Frequency X 100.

The recombination frequency is calculated as the number of crossover individuals divided by the total number of individuals.

Replacing the results in the formula, we have=

Distance = (7 + 8)/100 x 100

Distance = 15 map units.