Final answer:
The puppies' fur colors are likely the result of a Mendelian monohybrid cross, where brown fur is the dominant allele and gray fur is the recessive allele. Two heterozygous parents (Bb x Bb) would produce a 3:1 phenotypic ratio of dominant to recessive fur colors among their offspring.
Step-by-step explanation:
Explanation of Puppies' Fur Colors
The most likely reason for the puppies' fur colors, with three having brown fur and one having gray fur, can be understood through Mendelian inheritance patterns. If we assume that brown fur is the dominant allele and gray fur is the recessive allele, the presence of both phenotypes among the puppies suggests that each parent dog was heterozygous for the fur color trait. That is, each parent carried one allele for brown fur and one allele for gray fur (Bb).
When two heterozygous individuals breed (Bb x Bb), the expected phenotypic ratio of their offspring is 3:1, following a typical Mendelian monohybrid cross. In this ratio, there will be approximately three offspring with the dominant phenotype for every one offspring with the recessive phenotype. In the case of the puppies, this indicates that three would likely have dominant brown fur (BB or Bb) and one would have the recessive gray fur (bb).
The Labrador retriever example illustrates a more complex interaction of alleles known as epistasis, where one gene can mask the expression of another gene. However, this concept is not necessary to explain the fur colors in the scenario described by the student since a simple dominance relationship appears to be at play.