Black fur in mice (B) is dominant to brown fur (b). Short tails (T) are dominant to long tails (t). What fraction of the offspring of crosses BbTt × BBtt will be expected to have black fur and long tails?

1/2 or 50%

Explanation:

B is dominant over b and T is dominant over t.

BbTt is crossed with BBtt

BbTt   x   BBtt  = 4 BBTt, 4 BBtt, 4 BbTt and 4 Bbtt

4 BBTt = black fur with short tail

4 BBtt = Black fur with long tail

4 BbTt = Black fur with short tail

4 Bbtt = Black fur with long tail.

8 of the offpring have black fur with long tail while 8 also have black fur with short tail.

The proportion of offspring having black fur with long tail is 1/2 or 50%.