It's not required to put the dominant allele before the recessive allele when writing a genotype, but it is conventional and makes it easier to understand.

Here's why:

* Clarity: Putting the dominant allele first visually emphasizes the trait that will be expressed in the phenotype. For example, "Bb" is more easily understood than "bB" because we know that the "B" allele will be dominant.

* Consistency: The convention of putting the dominant allele first is widely used in genetics, making it easier for everyone to understand and interpret genetic information.

* No biological difference: There's no biological difference between "Bb" and "bB". Both genotypes represent the same combination of alleles, and they will produce the same phenotype.

However, it's not wrong to write the recessive allele first. As long as you are clear about which allele is dominant and which is recessive, there's no problem.

Ultimately, the most important thing is to be consistent and clear in your notation.