A completely-plated stickleback from a marine population was mated to a low-plated stickleback from a freshwater population. The resulting F1 hybrids all displayed a completely plated phenotype. When the F1 hybrids were allowed to interbreed, the resulting F2 generation included completely plated offspring and low-plated offspring in an approximate 3:1 ratio. Which of the following conclusions is best supported by the results of the breeding experiments?

(A) Phenotypic variation in the F sub two generation suggests that armor morphology is controlled by many alleles of a single gene.

(B) The completely-plated phenotype is controlled by a dominant allele of a single gene.

(C) Armor loss is an acquired characteristic that is affected by one or more environmental factors.

(D) Patterns of armor plating in stickleback populations are regulated by sex-specific signals.

Answer:

(B) The completely-plated phenotype is controlled by a dominant allele of a single gene.

Explanation:

In the given genetic cross, all the F1 progeny expressed a "completely plated" phenotype. This means that this phenotype is dominant. Interbreeding of F1 hybrids obtained the "completely plated" and "low plated" progeny in a 3: 1 ratio. This means that both these phenotypes are controlled by two alleles of a gene.

The F1 hybrids were heterozygous dominant for the trait and carried both recessive and dominant alleles of the gene. Segregation of these alleles during gamete formation formed 50% alleles with the dominant allele and rest 50% with the recessive allele. The random fusion of these gametes gave the F2 progeny in 3 dominant: 1 recessive ratio.