Gregor Mendel

In 1843, at the age of 21, Gregor Mendel, shown in Figure 9-1, entered a monastery in Brunn, Austria. His task of tending the garden gave him time to think and to observe the growth of many plants. In 1851, he entered the University of Vienna to study science and mathematics. His mathematics courses included training in the then-new field of statistics. Mendel's knowledge of statistics later proved valuable in his research on heredity—the transmission of characteristics from parents to offspring. When Mendel returned to the monastery, he taught in a high school and also kept a garden plot. Although he studied many plants, he is remembered most for his experiments with Pisum sativum, a species of garden peas.

Mendel's Garden Peas

Mendel observed seven characteristics of pea plants. A characteristic is a heritable feature, such as flower color. Each characteristic occurred in two contrasting traits. A trait is a genetically determined variant of a characteristic, such as yellow flower color. The pea characteristics that Mendel observed were plant height (traits: long and short), flower position along stem (traits: axial and terminal), pod color (traits: green and yellow), pod appearance (traits: inflated and constricted), seed texture (traits: round and wrinkled), seed color (traits: yellow and green), and flower color (traits: purple and white). Mendel used his knowledge of statistics to analyze his observations of these seven characteristics.

figure 9-1

Gregor Johann Mendel lived from 1822 to 1884. Mendel's experiments with garden peas led to his discovery of the basic principles of genetics.

objectives

• Describe how Mendel was able to control how his pea plants were pollinated.

• Describe the steps in Mendel's experiments on true-breeding garden peas.

• Distinguish between dominant and recessive traits.

• State two laws of heredity that were developed from Mendel's work.

• Describe how Mendel's results can be explained by scientific knowledge of genes and chromosomes.

vocabulary genetics heredity trait pollination self-pollination cross-pollination true-breeding P generation F1 generation F2 generation dominant recessive law of segregation law of independent assortment molecular genetics allele figure 9-1

Gregor Johann Mendel lived from 1822 to 1884. Mendel's experiments with garden peas led to his discovery of the basic principles of genetics.

figure 9-2

Mendel controlled the breeding of his pea plants and tracked the inheritance of traits by transferring pollen from the anthers of one plant to the stigma of another plant.

www.scilinks.org Topic: Gregor Mendel Keyword: HM60698

www.scilinks.org Topic: Gregor Mendel Keyword: HM60698

Mendel collected seeds from his pea plants and carefully recorded each plant's traits and seeds. The next year, he planted the seeds. He observed that purple-flowering plants grew from most of the seeds obtained from purple-flowering plants but that white-flowering plants grew from some of the seeds of purple-flowering plants. And when experimenting with the characteristic of plant height, he observed that while tall plants grew from most of the seeds obtained from tall plants, short plants grew from some of the seeds obtained from tall plants. Mendel wanted to find an explanation for such variations.

Mendel's Methods

Mendel was able to observe how traits were passed from one generation to the next by carefully controlling how pea plants were pollinated. Pollination occurs when pollen grains produced in the male reproductive parts of a flower, called the anthers, are transferred to the female reproductive part of a flower, called the stigma.

Self-pollination occurs when pollen is transferred from the anthers of a flower to the stigma of either that flower or another flower on the same plant. Cross-pollination occurs between flowers of two plants. Pea plants normally reproduce through self-pollination.

Self-pollination can be prevented by removing all of the anthers from the flowers of a plant. Then, cross pollination can be performed by manually transferring pollen from the flower of a second plant to the stigma of the antherless plant, as Figure 9-2 shows. By preventing self pollination and manually cross-pollinating pea plants, Mendel selected parent plants that had specific traits and observed the traits that appeared in the offspring.

MENDEL'S EXPERIMENTS

Mendel initially studied each characteristic and its contrasting traits individually. He began by growing plants that were true-breeding for each trait. Plants that are true-breeding, or pure, for a trait always produce offspring with that trait when they self-pollinate. For example, pea plants that are true-breeding for the trait of yellow pods self-pollinate to produce offspring that have yellow pods. Mendel produced true-breeding plants by self-pollinating the pea plants for several generations, as Figure 9-3 shows. He eventually obtained 14 true-breeding plant types, one for each of the 14 traits observed.

Mendel cross-pollinated pairs of plants that were true-breeding for contrasting traits of a single characteristic. He called the true-breeding parents the P generation. He cross-pollinated by transferring pollen from the anthers of one plant to the stigma of another plant. For example, if he wanted to cross a plant that was true-breeding for the trait of yellow pods with one that was true-breeding for the trait of green pods, he first removed the anthers from the plant that produced green pods. Then, he dusted the pollen from a yellow-podded plant onto the stigma of a green-podded plant and allowed the seeds to develop.

When the plants matured, Mendel recorded the number of each type of offspring produced by each cross. He called the offspring of the P generation the first filial generation, or Fj generation. He then allowed the flowers from the F1 generation to self-pollinate and collected the seeds. Mendel called the plants in this generation the second filial generation, or F2 generation. Following this process, Mendel performed hundreds of crosses and documented the results of each by counting and recording the observed traits of every cross. Table 9-1 summarizes the results of many of Mendel's crosses.

THREE STEPS OF MENDEL'S EXPERIMENTS P generation

Word Roots and Origins filial from the Latin filialis, meaning "of a son or daughter"

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