It's difficult to determine the genotype of an organism with a dominant phenotype because the dominant phenotype can be expressed by two different genotypes.

Let's break it down:

* Dominant allele: An allele that masks the expression of another allele (the recessive allele) when both are present.

* Recessive allele: An allele that is only expressed when two copies of it are present.

Here's an example using the classic pea plant color:

* Gene: Flower color

* Alleles: * Purple (P) - dominant

* White (p) - recessive

Possible Genotypes and Phenotypes:

* PP: Purple flowers (Homozygous dominant)

* Pp: Purple flowers (Heterozygous)

* pp: White flowers (Homozygous recessive)

The problem: An organism with purple flowers could have either the PP or Pp genotype. You can't tell just by looking at the flower color.

To determine the genotype, you need additional information, such as:

* Test cross: Cross the organism with a homozygous recessive individual (pp in our example). The offspring's phenotype will reveal the unknown parent's genotype.

* Family history: If you know the genotypes of the parents and other relatives, you can use this information to deduce the unknown individual's genotype.

In summary, the dominant phenotype can be expressed by multiple genotypes, making it challenging to determine the exact genetic makeup of an individual simply based on its outward appearance.