Answer:
When at least one allele contains instructions for a long tail
Explanation:
Gregor Mendel discovered that an organism receives two forms of a gene called ALLELE for a particular trait. Each allele encodes or represent a different trait in that same gene. However, according to Mendel, one allele called DOMINANT allele is capable of masking the expression of the other allele, called RECESSIVE allele. This means that in a heterozygous state i.e. combination of the two different alleles, the dominant trait encoded by the dominant allele will always be expressed.
In this case, the dominant characteristics or trait is long tail. This means that the allele for long tail will always mask the phenotypic expression of its allele pair in the tail length gene. Therefore, the characteristics of a long tail will definitely be expressed in two ways:
1. If both alleles in that gene encodes for the dominant trait i.e. long tail. This means both alleles will be dominant and the same, a term called HOMOZYGOUS DOMINANT.
2. If one of the two alleles that make up the tail length gene is dominant (long tail allele) and the other is recessive. Since the dominant allele will mask its phenotypic expression, the long tail (dominant trait) will be expressed. This is called HETEROZYGOUS DOMINANT.
Therefore, the long tail characteristics will be expressed if AT LEAST one of the two alleles contains information for a long tail (dominant characteristics)
