What could be some weakness with using a test cross to determine Fred's genotype?

Test-cross is often used to determine the genotype of parent that is dominant as we cannot determine that the dominant parent is homozygous or heterozygous for the trait just be looking at it.

In a test-cross, the dominant trait organism will be crossed with a recessive type organism. The results will predict the genotype of the dominant parent.

Some of the drawbacks of the test cross are:

a) It might be difficult to find a recessive type of the particular trait.

b) The result prediction can take a lot of time. This is because enough offspring have to be developed to predict the result which will take time.