Some animals, such as cows, normally produce only one offspring from each mating. If a cow showed a dominant phenotype, why would a typical testcross be a difficult way to determine the genotype of that animal?

Because of insufficient number of offsprings to determine the result of test cross.

Explanation:

Normally, in genetics, a test cross is a cross between a dominant phenotype (genotype unknown) and a homozygous recessive genotype in order to determine the genotype of that dominant organism.

The result of the test cross is obtained when a 1:1 phenotypic ratio is produced for each trait i.e. 1 dominant trait: 1 recessive trait showing that the parent is heterozygous while all the offsprings will show dominant traits if the dominant phenotype was homozygous. Hence, more than one offspring is required to be able to conclude

In this question, the cow only produce one offspring which may be DOMINANT for the observed trait making it difficult to know if other offsprings will all be dominant or some of them recessive. Hence, it is not a sufficient number needed to determine the genotype from the phenotypic ratio of offsprings.

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