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

requires a sizeable number of offspring to give reliable results
requires sometimes rarer, fully recessive individuals
the maturation time in order to observe the phenotypes expressed may be lengthy.

Explanation:

In a test cross, the homozygous recessive genes for a trait are crossed with those the individual with an unknown genotype. The homozygous alleles are the same. The unknown individual is thus, dominant, and may be either heterozygous (different alleles) or homozygous.

In sexual reproduction, gametes are formed from the independent assortment of genes. The parental genes are packaged together in new combinations. Thus, to determine an unknown, the test crosses determine which is the parent genotype through its ratios:

a homozygous dominant parent produces all heterozygous offspring
a heterozygous dominant parent produces homozygous and heterozygous offspring

However, the phenotypes must be observed to give results of this cross.