Publisher Summary

This chapter describes some of the experiments that first gave an understanding of heredity and laid the foundation of the modern science of genetics. The importance of Mendel’s work was recognized, and his discovery was accepted as one of the most fundamental and important that has been made in biology. Mendel liked growing plants in the monastery gardens, and he was interested in how they grew. He planned some experiments to see if he could discover laws governing the appearance of these different characters. The pollen is usually carried from one plant to another by bees or butterflies. This is known as cross-fertilization. A plant in which the female organs are fertilized by the male organs of the same flower is said to be self-fertilizing. A plant in which the female organs are fertilized by the male organs of another plant is said to be cross fertilizing.

Mendel

Mendel liked growing plants in the monastery gardens, and he was interested in how they grew. He studied his pea plants and wondered why some were tall and some dwarf, why some had red flowers and some white, why some seeds in a pod were wrinkled and some smooth.

He wanted to find out how these variations happened, and how they were inherited, or passed from parent to offspring when one generation followed another. He planned some experiments to see if he could discover laws governing the appearance of these different characters.

variations

Mendel’s basic work was with one plant, the garden pea (the scientific name is Pisum sativum), and was successful because he introduced some new principles into his work which had not been used before (also, he was fortunate in picking the garden pea to work with, as we shall see later).

new principles

The first of Mendel’s new principles was that he worked with pure lines. That is, plants that had been inbred for long enough to make sure that they hardly varied in the characteristics he was investigating when they were bred amongst themselves.

We shall define pure lines more precisely later.

pure lines (or strains)

Another reason for his success was that he chose a plant, the garden pea, which is normally self fertilising, and so would prevent cross fertilisation unless he desired it.

Stamens producing pollen stigma and ovary containing ovules

Fertilisation occurs when ripe pollen is deposited in the ripe stigma. A tube grows from the pollen grain, down through the style until it meets an ovule in the ovary. The male contribution passes down the tube and joins with the female contribution present in the ovule. The result develops into a seed.

A seed is the product of contributions from the … and the … organs of the flower.

male female

It is not necessary to go into any more botanical detail. There are many different arrangements for the production of pollen and for fertilisation. We have illustrated what might be termed a typical flower.

The point we wish to emphasise is that the seed contains contributions from the male and female sides, whether they are from the same flower, or on entirely different plants.

stamens

In other plants it is arranged that self fertilisation does not take place. This is done in several ways, such as:

(i) The pollen does not ripen at the same time as the stigma and ovules. (ii) Only one set of sex organs grows in each flower. The marrow plant has male flowers with stamens producing pollen, and female flowers having only stigmas and styles leading to the ovary and ovules. Other plants, the willow for example, have the male flowers on one plant and the female flowers on another.

Self. Mendel wanted a true breeding stock as he was going to cross fertilise plants deliberately to see what happened.

Another reason for Mendel’s success was that he looked only for character differences that were simple and clear-cut, not characters in which there is a continuous variation—like the different shades in roses, for example.

Mendel used varieties of peas that differed only in one or more of seven pairs of characters. He paid attention only to these character pairs, e.g. tall/dwarf, yellow/green seeds, round/wrinkled seeds, etc., and not to the appearance of the whole plant.

Another reason for Mendel’s success was that he took notice only of distinct pairs of …

characters

Finally, he kept full statistical records of each individual cross and its progeny in each generation.

This was important for two reasons. First, he knew what had happened as a result of each cross. Secondly, he was able to combine large numbers of results, and to express the proportions in which different characters appeared as simple ratios.

Can you recall other reasons for Mendel’s success?