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An Introduction to Mendelian Genetics
From: The Natural History Museum
| By:
Colin Patterson |
EDITOR'S INTRODUCTION |
When Charles Darwin published his ideas on evolution and heredity in 1868, he was completely unaware of the mechanisms by which the instructions for the development of each organism are passed down from generation to generation. However, two years previously, Gregor Mendel, an Austrian monk, published an account of his breeding experiments with pea plants. Within this work lay the answers to Darwin's problems. Mendelian theory was to become an essential basis for the study of genetics. Colin Patterson, of The Natural History Museum, London, tells the story of Mendel's experiments and explains what Mendel realised when he analysed their results. |
 n most species that breed sexually the only material that passes from the parents to the next generation is the minute amount contained in the egg and sperm (the human egg, for example, is about 0.1 mm across and the sperm is very much smaller). The fertilized eggs of the vast majority of species are left to take care of themselves. Since such eggs develop into copies of the parents, they must contain instructions for doing so. |
Darwin was completely ignorant of the way in which these instructions are passed from generation to generation, despite his experience in breeding animals and plants. He knew that inheritance is often 'blending'--the offspring being 'of mixed blood', intermediate between the parents--but he also knew of many exceptions to that rule. He believed that characteristics acquired during the life of an organism, such as parts or organs enlarged by use or weakened by disuse, were transmitted to the offspring, and in 1868 published a theory of heredity that seemed to explain this. He proposed that all parts of an organism secrete minute granules throughout life, which circulate around the body and accumulate in the reproductive organs, ready to be passed on to the next generation. However, two years before, in 1866, Gregor Mendel, an Austrian monk, published an account of a long series of breeding experiments with pea plants, from which he developed a theory that explained many of Darwin's difficulties (and eventually showed Darwin's own theory of heredity to be completely false). Unfortunately, the significance of Mendel's work was not noticed by Darwin--or any other scientist--until 1900, by which time both Mendel and Darwin had died. |
Mendel's theory
Mendel experimented with varieties of garden peas (Pisum sativum). He found varieties with clear-cut differences in features such as flower colour (white versus purple), seed colour (green versus yellow) and form of seed (round versus wrinkled), and grew each variety for two years to make sure that it was pure--that it bred true. He chose pea plants because the flower is normally self-fertilized, but can be cross-fertilized experimentally if the stamens are removed before they ripen. |
In a typical experiment Mendel cross-fertilized the flowers of plants grown from green and yellow seeds and found that the resulting hybrid seeds (peas) were all yellow. He then planted these hybrids, allowed them to self-fertilize and examined the seeds produced. He found that the hybrid plants, grown from yellow seed, did not breed true. Out of 8,023 peas in this second generation, 6,022 were yellow and 2,001 green--a ratio very close to three yellow to one green. Again, he planted these peas and allowed the second-generation plants to self-fertilize. He found that the green peas always bred true, but that the yellow ones did not. Out of 519 second-generation yellow peas, 166 bred true but 353 did not: the latter group again produced yellow and green seeds in the ratio of three to one. When a plant yielded yellow and green seeds, the different types were randomly distributed in the pod: a pod might contain five or six yellow and two or three green peas. |
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| In the Mendel square above, hybrid (Aa) yellow peas are bred together, resulting in a three-to-one ratio of yellow to green peas. Green peas are produced because the hybrid yellow peas have both a dominant gene (A) and a recessive gene (a). When the recessive genes combine (aa), the peas appear green. | |
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Mendel's fame is due to the theory he devised to explain these facts. First, since a cross between the pure-bred yellow line and the pure-bred green produced all yellow peas, he supposed that the character 'yellow' was dominant over 'green', which he called recessive. Since a hybrid yellow pea can produce both green and yellow offspring, it must carry the 'green' character in latent form. To explain this, he proposed that each pea and plant carries a double dose of characters or hereditary factors (now called genes), so that pure-bred yellow could be symbolized by AA, pure-bred green by aa. The pollen and egg cells would carry only one factor, A or a. The first-generation peas in his experiment (F1), all yellow, would be Aa, having received 'A' (in pollen or egg) from one parent and 'a' from the other. The second generation (F2), produced by self-fertilization of the first, should then have an equal chance of receiving either 'A' or 'a' in both the pollen and the egg cell, and so should be 25 per cent AA (pure-breeding yellow), 50 per cent Aa (like their parents, impure breeding yellow) and 25 per cent aa (pure-breeding green). Thus the second generation should contain three-quarters yellow (1/4 AA + 1/2 Aa) and one-quarter green (aa), which explains the 3:1 ratio that Mendel observed. |
By a series of crosses between plants of each constitution, Mendel was able to test this theory, and to show that it accounted for all the available facts. He also found that reciprocal crosses (pollen from green crossed with egg from yellow; yellow pollen with green egg) proved that the allocation of the genetic factors (A or a) to the pollen and egg (male and female sex cells, or gametes) is random. Each gamete has an equal chance of receiving either factor. Fertilization is also random; an egg cell with A having an equal chance of being fertilized by pollen with A or a. |
Most subsequent work in genetics has been based on Mendel's principles and practices. The great advances that Mendel made were:
- his ideas of dominant and recessive factors (genes)
- the double dose of factors in an organism, so that an organism can be hybrid
- the random segregation of single factors to the gametes, so that a gamete is always pure, carrying one factor or the other
the transmission of the factors, unchanged and uncontaminated, from generation to generation.
Mendel showed that inheritance is not blending, as Darwin supposed, but particulate (Mendel's particles, the genes, are nothing to do with the mythical 'granules' that Darwin supposed were given off by all parts of the organism). |
One other point of great importance that follows from Mendel's theory is the clear distinction between the physical appearance of an organism (its phenotype) and genetic constitution (genotype). The same phenotype may have different genotypes (e.g. peas with a yellow phenotype may have the genotype AA or Aa), or one genotype may have several phenotypes (e.g. the caterpillar, the chrysalis and butterfly are three different phenotypes, all expressing the same genotype). |
Mendel also conducted more complicated experiments involving two pairs of factors, for example crossing pea plants grown from pure-bred yellow, wrinkled seeds with plants grown from green, smooth seeds. He found that the yellow/green pair of factors and the wrinkled/smooth pair were inherited independently. |
Subsequent experiments have confirmed all these aspects of Mendel's work, and shown that his principles can be applied in all sexual organisms. However, we now know that clear-cut dominant/recessive systems involving a single pair of factors are not all that common--in many attributes, such as height in human beings, the offspring are roughly intermediate between their parents. But analysis shows this apparent blending of parental features is not a different type of inheritance. Instead it involves attributes that are controlled by several or many Mendelian genes, with additive effects. Mendel's rule, that different pairs of factors are inherited independently, also has many exceptions. Factors tend to be transmitted in groups, called linkage groups. These, and other apparent were anomalies, were not explained until the cells of organisms were first studied under a microscope. |
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