Inheritance and Mendelian Genetics

Gregor Mendel

Mendel's Discoveries

Gregor Mendel, the father of modern genetics, discovered the mechanism of inheritance.
Today, scientists know that organisms have many units of heritable material that code for particular traits, each called a gene. Each trait, or characteristic, is determined by an organism's genes. Two hundred years ago, however, the mechanisms of inheritance were not known. Gregor Mendel (1822–84) is an Austrian monk credited with discovering the principles underlying genes and inheritance. Mendel performed a series of experiments with pea plants in the 19th century. These led to the discovery of the genetic basis of inheritance (the traits that are received by offspring from parents) and resulted in him being posthumously considered the father of modern genetics.

The Pea Plant Experiments

Mendel used pea plants to learn the genetic basis of inheritance.
Since childhood, Mendel had been a gardener. In the monastery, he set up a series of experiments using pea plants. Mendel observed that pea plants could vary in terms of seven different characteristics:
  • flower color (purple or white)
  • flower position (axial, on the shoot where a branch begins; or terminal, at the end of the shoot)
  • pea color (green or yellow)
  • pea shape (round or wrinkled)
  • plant size (tall or short)
  • pod color (green or yellow)
  • pod shape (constricted or inflated)

In his experiments Mendel cross-fertilized plants with different characteristics and observed the resulting offspring. He noticed that patterns emerged; some of these patterns were immediately evident and others required multiple generations to emerge. Mendel concluded that each plant had factors that determined how the plant would look. Today, Mendel's factors are known as genes.

To better understand Mendel's experiments, one can follow an example using pea color as the selected characteristic. Mendel observed that peas could be green or yellow. He began by crossing a plant with green peas and a plant with yellow peas. The original two organisms that are crossed to produce offspring are called the parent generation (P). The first offspring resulting from a parental cross are called the first filial generation (F1). Mendel's first generation plants all had yellow peas. He then pollinated one of these offspring plants with itself. The offspring of a cross between individuals of the first generation are called the second filial generation (F2). Mendel's second generation plants had a mixture of green and yellow peas. In fact, the ratio of plants with yellow peas to plants with green peas was 3:1. Mendel then pollinated each plant in the second generation with itself, and he found that one plant with yellow peas gave only plants with yellow peas, while others continued to give plants showing the 3:1 ratio. The plant with green peas gave only plants with green peas. This is the basis for Mendel's law of segregation.

Mendel's Pea Plant Experiments

"Mendel crossed a plant with yellow peas and a plant with green peas in the parent generation (P). This produced all yellow peas (F1). One of these plants was self-pollinated, giving a 3:1 ratio of yellow to green peas in the F2 generation.