Final answer:
The gene affected by a missense mutation that can influence the size of the Shetland pony is the Myostatin gene (D). Missense mutations in the Myostatin gene lead to increased muscle mass, which affects the overall size of the animal.
Step-by-step explanation:
Among the options given, the gene affected by a missense mutation that can influence the size of the Shetland pony is the Myostatin gene (D). Myostatin is known to regulate muscle development, and mutations in this gene can lead to increased muscle mass and subsequently influence the overall size. Hormone receptors play a critical role in how tissues respond to hormones, and their mutation can lead to altered development or growth patterns. While the insulin receptor, growth hormone, and thyroid hormone receptor genes are all vital to growth and development, studies in various species, including dogs and horses, have shown that mutations in the myostatin gene specifically can lead to a significant increase in muscle mass, which would affect the overall size and phenotype of an animal, including the Shetland pony.
The hormone receptors, which include insulin receptors, growth hormone receptors as well as thyroid hormone receptors, bind to their respective hormones to initiate a cellular response. A missense mutation in any of these receptors can lead to altered signal transduction, affecting how cells and, ultimately, tissues respond to hormonal signals, thereby influencing growth and development. However, the Myostatin gene's direct influence on muscle mass makes it the most relevant to the question of size changes in the Shetland pony.