Main articles: Growth hormone 1 and Growth hormone 2. Genes for human growth hormone, known as growth hormone 1 (somatotropin) and growth hormone 2, are localized in the q22-24 region of chromosome 17 and are closely related to human chorionic somatomammotropin (also known as placental lactogen) genes.
The U-shape of the marginal cost curve is directly attributable to increasing, then decreasing marginal returns (and the law of diminishing marginal returns). As marginal product (and marginal returns) increases for relatively small output quantities, marginal cost declines.
So long as the average cost curve is falling with the increase in output, the marginal cost curve lies below the average cost curve. When average total cost curve begins to rise, marginal cost curve also rises, passes through the minimum point of the average cost and then rises.