Final answer:
The phenotype of a dog with the genotype 'BB' is brown fur. 'BB' indicates that the dog is homozygous for the dominant allele resulting in the expression of brown fur as the observable characteristic.
Step-by-step explanation:
The question 'A dog has brown fur. It is shown by writing the two letters BB. What is the phenotype for the dog?' involves understanding the genetic basis of inheritance in biology, specifically the concept of genotype and phenotype. The letters 'BB' indicate the genotype of the dog, which corresponds to the alleles the dog has inherited for a particular trait - in this case, fur color. The term phenotype refers to the observable characteristics or traits of an organism that are expressed as a result of the genotype. Here, a dog with brown fur corresponds to the dominant allele 'B', and because the dog has two copies (homozygous dominant), its phenotype is brown fur.
In this situation, the dog's genotype 'BB' results in the physical expression of the dominant trait, therefore the phenotype of the dog is brown fur. This is consistent with the patterns of inheritance that follow Mendelian genetics where dominant alleles mask the effects of recessive ones. As 'BB' represents two copies of the dominant allele, no alternative (recessive) phenotype is expressed.