The answer is polygenic inheritance. Many physical characters (traits) depend on many different factors, each of which is determined by different genes. This is called polygenic inheritance.
For example, the color of the skin in humans. The color of the skin results from the interactions of several factors determined by different pairs of genes:-Certain genes could affect the metabolism of skin melanocytes.-Other genes can determine the distribution of melanin in the thickness of the skin.-Some genes could determine the relative amounts of each of the two possible types of melanin.-Others may affect the production of certain hormones involved in the activity of melanocytes.
A variable whose variation does not depend upon on any other variable in any manner is termed as independent variable.
An independent variable is the one that can only be actually varied by the experimenter who is using it to test the affect of this particular variable on other variables that depend on it and thus, change as the independent variable is changing.
In the given experiment the scientist is testing the effect of spraying chemicals on the apple yard, here<em> the chemical used is an independent variable as varying the amount of chemical that is sprayed will change the yield of the apples. </em>
