Question

Black fur in mice (B) is dominant to brown fur (b). Short tails (T) are dominant to long tails (t). Of 32 progeny of the cross BbTt x BbTt, approximately how many will have brown fur and short tails? (assume the genes are not linked)

a) 16
b) 6
c) 0
d) 32
e) 18

Answer 1

Approximately 6 out of 32 mice are expected to have brown fur and short tails in a cross between two BbTt heterozygous mice, as deduced from the 9:3:3:1 phenotypic ratio.

Step-by-step explanation:

The direct answer to how many mice will have brown fur and short tails in a cross between BbTt x BbTt is b) 6.

An explanation of this involves understanding the genetic cross and the expected phenotypic ratio. In a dihybrid cross where two heterozygotes (BbTt) are crossed, the typical phenotypic ratio for the F₂ generation is 9:3:3:1. This ratio represents the phenotypes of dominant-dominant, dominant-recessive, recessive-dominant, and recessive-recessive traits respectively.

Since brown fur (b) is recessive and short tails (T) are dominant, we are looking for the proportion that is recessive for fur color and dominant for tail length, which is represented by the 3:3 part of the ratio. But since the total number of expected progeny is 32, we must calculate how many will have the specific phenotype of brown fur and short tails. To find this, we take 3/16 (the fraction that represents brown fur and short tails) of the total progeny, 32, which gives us 3/16 x 32 = 6.