Mammal? Is this for science or?
        
                    
             
        
        
        
Dominant' traits will actually disappear faster if they are disadvantageous.
Think about it: if everyone who has even a single copy of a particular allele is at a disadvantage (manifests the phenotype, in this case six fingers), then even single copies are selected against.
In the case of recessive traits, selection occurs only against homozygous carriers, who may be very rare if the allele itself is rare.
A concrete example would be something like Tay-Sachs disease. If the allele that causes this were dominant, every carrier would die before adulthood, and it would occur only as a very rare de novo mutation. But because it is recessive, it persists for now; heterozygous carriers have no disadvantage.
        
             
        
        
        
Answer:
Please find how this is possible in the explanation below
Explanation:
This question is regarding a gene in rabbit coding for fur color. Brown fur (B) in rabbits is dominant over white fur (b). However, according to the question, a white female gives birth to all white rabbits, even though the father had brown fur. This is possible because the father is heterozygous for the brown fur i.e Bb. 
Hence, in a cross between a heterozygous father (Bb) and a white mother (bb), the following gametes will be produced by the following parent:
Bb - B and b
bb - b and b
Using these gametes in a punnet square (see attached image), the probability of 1/2 of the offsprings will possess white fur (bb). Based on this result, the white female is able to give birth to all white rabbits because the male rabbit contributed the white allele (b).