Genetics is the study of traits that can be passed down from parent to child. A trait can be any feature of an individual such as height, hair color, blood type, or susceptibility to disease. For many traits, there is natural variation in the population. Some people are tall, some are short, and most are somewhere in between. This variation can be due to genes that are inherited, the environment, or some combination of genes and environment. For example, the language that a person speaks is due entirely to environment, while a person’s blood type is due entirely to inherited genes. On the other hand, a person’s chance of developing skin cancer is due to a combination of genes and environment. In this section, we will deal only with traits that have a strong genetic component (i.e. that are mostly due to inherited genes).

For humans, other animals, and plants, it’s long been observed that progeny (children) tend to resemble their parents. A person may have “her father’s eyes” or “her mother’s nose.” Yet for other features, an individual may seem like a blend of the two parents or may not resemble either parent. Here we will discuss the basic rules of genetic inheritance that cause progeny to resemble their parents in some respects, but not others.

Gregor Mendel, who was a monk and teacher in what is now the Czech republic, first described these rules of inheritance in 1865. However, his work was not widely recognized until the early 1900s. Mendel experimented with garden peas, using varieties that differed for traits such as seed shape, seed color, and flower color. Each trait had two possible observable forms, or phenotypes. The phenotype for seed shape could be round or wrinkled. The phenotype for seed color could be yellow or green. Flowers could be purple or white.

Like most familiar animals and plants, peas undergo sexual reproduction, where a sperm cell and an egg cell are required to produce offspring. Mendel set up carefully controlled crosses, or matings, between plants of different phenotypes. In a cross, a pollen grain (sperm) fertilizes an ovule (egg) in a flower to produce an embryo. The embryo develops into a pea, which is a seed that can be planted to produce a new pea plant. Each flower of a pea plant produces both pollen and ovules, which are enclosed together in a structure called a keel.

If left alone, pollen will fertilize an ovule from the same flower, resulting in self-fertilization. Cross-fertilization between different individuals can be achieved by cutting open the keel before pollen is formed, removing the structures that will produce pollen, and dusting the flower with pollen from a different individual. The ability to easily control crosses, without worrying about pollen carried by insects or wind, made pea plants well suited for genetic studies. From his experiments with garden peas, Mendel deduced the basic rules of genetic inheritance that apply to all sexually reproducing plants and animals.

Before he began his experiments, Mendel confirmed that his plants were true-breeding, meaning that self-fertilization always resulted in progeny with the same phenotype as the parent. True-breeding plants (and animals) are often referred to as “lines.” By using only true-breeding lines, any variation seen in the progeny of a cross can be attributed to the cross, and not some hidden variation in the lines.