If we allow “B” to represent the dominant allele for black fur and “b” to represent the recessive allele for fur color, then “BB” could represent the genotype of a rabbit that is homozygous dominant for black fur.
Alleles are varieties of genes. Alleles can be dominant or recessive. A dominant allele is the “stronger” variety of a gene. Dominant alleles are indicated by a capital letter. The capital letter used is often the first letter of the dominant trait that is being represented. A recessive allele is the “weaker” variety of the gene. Recessive alleles are represented by lowercase letters. The lowercase letter that is used is usually the first letter of the dominant allele that is being represented.
A genotype is the genetic combination of alleles that an individual has for a trait. Phenotypes are the physical representation of a genotype. Because dominant alleles are “stronger” than recessive alleles, only one dominant allele is needed to produce a dominant phenotype.
Genotypes can be homozygous dominant, heterozygous, or homozygous recessive. Homozygous dominant genotypes have two dominant alleles and will show the dominant phenotype. A heterozygous genotype has one dominant and one recessive allele. Because a heterozygous genotype contains a dominant allele, it will also show the dominant phenotype. Homozygous recessive genotypes contain two recessive alleles. Having a homozygous recessive genotype is the only way an individual can show the recessive phenotype.