Question

The three-spined stickleback (gasterosteus aculeatus) is a small fish found in both marine and freshwater environments. Marine stickleback populations consist mainly of individuals with amor-like plates covering most of their body surface (completely plated.) approximately 10,000 years ago, some marine sticklebacks colonized freshwater environments. After many generations in the freshwater environments, the freshwater stickleback populations lacked the armor plating (low plated) typical of marine stickleback populations. A single gene, ectodysplasin (eda), is thought to be responsible for the variation in the number of armor plates in sticklebacks.
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 F2 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 1

Answer:

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

Explanation:

The results from the breeding experiment showed that the completely-plated phenotype is controlled by a dominant allele of a single gene because it followed Mendelian pattern of inheritance.

According to Mendel, in a dominant/recessive gene situation, the recessive gene do not manifest physically in F1 generation and at F2, the phenotypic  ratio of dominant to recessive gene is usually 3:1.

For example, assuming the completely-plated gene is represented by P and the alternate form (low-plated) by p.

At F1:    PP   x   pp

Progeny: all Pp, completely-plated stickleback in appearance.

At F2:     Pp   x   Pp

Progeny: PP, Pp, Pp, and pp

3/4 of the progeny have completely-plated appearance while 1/4 have low-plated appearance. The ratio of completely-plated to low-plated is 3:1.

The correct option is B.