Final answer:
An allele is a variant form of a gene. The wildtype allele is the standard variant found in nature, often providing the correct gene product dosage. LOF alleles reduce gene function and are often recessive, while GOF alleles enhance or alter gene function and can be dominant.
Step-by-step explanation:
An allele is a variant form of a gene. In the context of mutations, the wildtype allele is the version of a gene that is most commonly found in the natural population and is typically the functional one providing the correct dosage of gene product. For example, in rabbit fur color, the wildtype allele supplies the appropriate amount of fur pigment compared to mutant alleles.
Loss-of-function (LOF) alleles result in reduced or eliminated function of the gene product. They are often recessive because one wildtype allele can compensate. However, if an individual has two copies of the LOF allele, the phenotype can be more drastic and sometimes lethal. Gain-of-function (GOF) alleles enhance the function of the gene product or change its distribution, sometimes causing the phenotype to be dominant over the wildtype.
With respect to the genotype and phenotype of a tall, green plant that is homozygous for each trait, assuming T is the tall allele and G is the green allele, the genotype would be TTGG, and the phenotype would be tall and green.