Suppose a male mouse is Bb and a female is bb. What percent of the mice offspring from these two mice would you expect to have black fur?

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sciftw | High School Teacher | (Level 1) Educator Emeritus

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50%.  

The question does not state which gene is dominant, so I do not know if B is black or b is black.  That does not matter though based on the parental genotypes that the question uses.  I will assume that BB and Bb are phenotypically black, and bb is phenotypically white.  

The initial cross is between a black (Bb) mouse and a white (bb) mouse.    

The genes will sort independently, so the black individual can donate B or b. The white individual can only donate b. 

Two potential offspring will be genotypically Bb.  Two potential offspring will be bb.  50% show the dominant trait, and 50% show the recessive trait.  That is why it does not matter if black is dominant or recessive.  Half of the F1 generation could be black, and half could be white. 

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