Black fur in mice (
b) is dominant to brown fur (b). short tails (t) are dominant to long tails (t). what fraction of the progeny of crosses bbtt × bbtt will be expected to have black fur and long tails?
a) 1/16
b) 3/16
c) 3/8
d) 1/2
e) 9/16

The answer is D) 1/2.

Let's imagine that two genes of the genotype can be analysed separately.

Black fur is determined by dominant allele B. In this case, parents Bb and BB will give offspring with only brown fur:
Parents:          Bb     x     BB 
Offspring:     BB   BB   Bb   Bb
Both homozygous (BB) and heterozygous (Bb) offspring will have black fur.

On the other hand, long tails are determined by recessive allele t. Parents Tt and tt will have 50% offspring with short tail and 50% offspring with long tail:
Parents:         Tt         x       tt
Offspring:    Tt      Tt        tt      tt
Heterozygous offspring (Tt) will have short tail, and homozygous offspring (tt) will have a long tail.

So, the offspring will definitely have black fur and will not affect the fraction of the offspring with black fur and long tails from the cross of BbTt × BBtt. But, there will be 2 out of 4 offspring with short tails which is 50% or 1/2.