What is normal distribution? Why is it called normal? and why do genes often distribute themselves normally?

Question

What is normal distribution? Why is it called normal? and why do genes often distribute themselves normally?

Answer 1

The normal distribution, also called the Gaussian distribution, is an important family of continuous probability distributions.

The importance of the normal distribution as a model of quantitative phenomena in the natural and behavioral sciences is due in part to the CENTRAL LIMIT THEOREM. Many measurements, ranging from psychological to physical phenomena can be approximated, to varying degrees, by the normal distribution.

The normal distribution is graphically represented by a BELL-SHAPED CURVE.

For polygenic traits (i.e., traits that are determined by a very large number of genes), the normal distribution is found to have the following characteristics:

68% of individuals will have a measurement within one standard deviation of the mean.

95% of individuals will have a measurement within two standard deviations of the mean.

99.7% of individuals will have a measurement within three standard deviations of the mean.

For example, height is polygenically inherited and follows a normal distribution.  The mean height of men is about 70 inches, with a standard deviation of 4 inches.  Thus, 68% of males will be between 5'6" and 6'2", 95% of males will be between 5'2" and 6'6", and 99.7% will be between 4'10" and 6'10".  About 16% of males will be over 6'2", 16% will be under 5'6", 2.5% of men will be over 6'6", 2.5% will be under 5'2", and only about 15 of 10,000 men will be over 6'10" or less than 4'10".