Ne true-breeding line of mice is obese and dark, and the other true-breeding line is lean and light. Dark is dominant to light, but obese and lean exhibit incomplete dominance. The two genes involved are on separate chromosomes. What proportion of F2 offspring resulting from a cross of two F1 progeny from a cross of the two lines described above should be dark and intermediate between obese and lean?

Explanation:

Let the allele for dark color be represented by "D" and the allele for light color be represented by "d"

Also D is dominant over d

Similarly, let the allele for obese trait in mice be represented by "O" and the allele for lean trait in mice be represented by "L"

Obese and lean exhibit incomplete dominance

Genotype of true breeding dark and obese mice be DDOO

Genotype of true-breeding lean and light mice is ddLL

Offspring from F1 cross -  

DdOL

F2 cross is between DdOL * DdOL

DO         DL         dO          dL

DODDOODDOLDdOODdOL

DLDDOLDDLLDdOLDdLL

dODdOODdOLddOOddOL

dLDdOLDdLLddOLddLL

dark and intermediate between obese and lean offspring genotype –  

2 DDOL, 4 DdOL  

So total six out of sixteen offspring are dark and intermediate between obese and lean -  

car stops. he keeps going = he has inertia

the diagram which shows forces as either balanced = same speed or unbalanced = accelerarting