Introduction to Genetics/Chapter 1. Introduction

Modern genetics can be divided into four main fields: quantitative genetics, classical genetics, population genetics, and molecular genetics. The following sections are a brief introduction to each of these fields, not that they are not integrated with each other and with other fields within and outside of biology. The concept of heritability is a good place to start in quantitative genetics. The connection between genotype and phenotype is used in classical genetics. Hardy-Weinberg genotype proportions introduce population genetics. And the flow of information from DNA to RNA to protein is mentioned in molecular genetics. Finally, the fields of statistics and genetics are closely connected, especially in a historical sense, and this is introduced with binomial probabilities. 

Quantitative Genetics Introduction

A species is characterized by a range of traits such as height, length, pigmentation, weight, and growth rates. The genetic component of these traits can be quantified. These traits are called phenotypes and the amount of variation in the trait that is determined by genetic differences is called heritability. A classical example is human height. Stature is partially influenced by genetic variation and partially influenced by environmental factors such as health and nutrition.  We tend to be more similar in height to our own parents than to unrelated people, but we cannot predict someone's height with perfect accuracy from their parents' height. Another overt example in humans is skin color. There is genetic variation that influences skin pigmentation. Again, children tend to be more similar to their parents in comparison to randomly selected people; however, there are also clear non-genetic environmental influences that influence skin color, such as exposure to ultraviolet light.

Let's switch gears and talk about something with no heritabilty to illustrate. We can roll two six-sided dice in a game of craps and add up the total from two (snake eyes) to 12 (boxcars). If we have fair dice the outcome of the first roll has no influence on the outcome of the second roll. The outcomes are only a result of physics: the speed, rotation, air drag, starting orientation, and friction with a surface. Dice have zero heritability (influence from previous rolls) and are only influenced by environmental effects. If we plotted the results from a first and second roll we do not expect a significant correlation in the sum of two dice.

A counter example is eye color in humans. Iris pigmentation due to the amount of melanin is almost completely explained by genetic variation inherited from our parents and has essentially no influence from the environment. If we scored the eye color of a large number of parents and offspring we expect a significant correlation because eye color is heritable and due to these genetic effects.

There are a wide range of traits in various species that have a range of heritability. Domesticated plant varieties and animal breeds are good examples. Phenotype variation selected by humans has increased the underlying genetic variation. Breeds have certain sets of unique traits because of a high heritability of these traits. 

However, keep in mind that not all variation is heritable and due to genetic variation. American redstarts have variation in some of the feathers, from lighter yellow to darker orange-red, that is due at least in part to the availability of carotenoids in their diet and may indicate relative nutrition levels (Reudink et al., 2014).  

Later I will write about how to quantify heritability, different types of heritability, how heritability can be used to make predictions in artificial selection experiments and some additional related topics.