MENDELISM

WERE we transported back into the Middle Ages, we should not find the wide choice of different kinds of apples, pears and plums which we enjoy to-day. All cultivated fruit trees have orginated from wild types such as the crab apple and the sloe. When these wild types were brought under cultivation, it was noticed that new varieties sometimes appeared, and those which possessed more attractive features than those of the wild type were often selected by discerning growers and used to start new stocks of cultivated trees in place of the older, less attractive ones. All cultivated plants have undergone improvement by man ’s selection in this way.

There are two ways in which new varieties of plants may arise : Very occasionally a single seed of a particular plant will produce a new variety spontaneously. For example, in a field of red poppies a plant bearing white flowers may sometimes be seen. Such plants are known to gardeners as ’sports, ’ or, in scientific language, as ’mutations. ’ Accidental cross-pollination of two closely related types of plants may produce a new variety known as a hybrid, e.g. the primrose and the cowslip occasionally grow in the same locality, and by cross-pollination hybrid plants having larger flowers than the cowslip but borne in a stalked umbel unlike those of the primrose may be found.

Varieties in animals arise in similar ways. Albino blackbirds are sometimes observed, while black-coated rabbits may occur among wild ones. Such varieties are usually shunned or even killed off by the normal types, and so are rarely perpetuated in nature. By preserving these accidental varieties and by using them as parents, man has produced many so-called domesticated varieties in dogs, cats, rabbits, mice, cattle, horses and sheep, and also in tame birds such as budgerigars, fowls and pigeons. Cross-breeding between different varieties is often practiced to combine valuable characters. Those who keep poultry are familiar with hybrid birds such as ’crosses ’between Leghorns and Rhode Island Reds, combining the good egg-laying characters of the Leghorn breed with the ability to produce good ’table ’birds of the second breed.

Until comparatively recent times the production of new varieties of plants or animals by cross-breeding was a haphazard proceeding, and the results were often uncertain and useless. It was no doubt soon discovered that only closely related types of plants or animals could be sucoessfully ’crossed, ’ and then the hybrid produced was often sterile and incapable of reproduction. A familiar example of this is the sterile mule produced by crossing the horse and the donkey. In 1866 Gregor Mendel, Abbot of Brunn, published the results of certain experiments he had made in cross-breeding varieties of the common garden pea and the conclusions he had drawn from them. Though the value of his work was not recognized until 1900, the first half of this present century has seen great advances made in the knowledge of heredity based on the original discoveries he made, and the breeding of new varieties of plants and of animals is now a scientific process. One of the earliest examples of cross-breeding on Mendelian principles was the production by Professor Sir Rowland Biffen at Cambridge of a new type of wheat called Yeoman. This he obtained by crossing an English wheat variety with a Canadian one. The hybrid combined the good bread-making qualities of the Canadian wheat with the higher yield of the English wheat, but its grains when sown did not ’breed true. ’ However, guided by Mendelian principles, he was able at length to select from the progeny a true-breeding strain of the desired type which gave a high yield and good bread. This has enabled the English farmer to grow wheat sucoessfully in competition with the Canadian and American farmers, who possessed the advantage of large prairie farms with low rentals and smaller costs of production.

Mendel ’s Laws

In one of his experiments, Mendel cross-pollinated a variety of pea having round seeds when ripe with one having wrinkled seeds when ripe. This he effected by removing the unripe stamens from unopened flowers of one variety and then, when the stigmas were ready for pollination, he placed pollen on them from the other variety. When the ripe pods so produced were examined, all the peas in them were found to be round. This was the case whichever variety was used as the ’female ’parent, I.e. the one used to receive pollen from the other. These peas Mendel called the first filial generation. Mendel sowed these seeds the following year, and this time he allowed the flowers produced to be self-pollinated as they normally are in the garden pea, provided insects are excluded. The ripe pods this time contained both round and wrinkled peas. He called these peas the second filial generation, and on counting the two kinds found that there were 5,474 round and 1,850 wrinkled peas. The ratio of round peas to wrinkled was thus nearly 3:1. Mendel than proceeded to sow these peas in the next year, and again allowed the plants to be self-pollinated. The wrinkled peas produced plants which bore only wrinkled seeds, but only one-third of the round peas produced plants bearing round peas only ; the other two-thirds had pods containing both round and wrinkled peas in the former ratio of 3 round : 1 wrinkled.

Mendel explained these results in the following terms :—

Factors

The pollen grains and ovules bear character-producing factors. These are now called genes, and are known to be situated in the nuclei of the pollen and of the ovule. When fertilization occurs, one of the ’male ’nuclei of the pollen tube fuses with the ’female ’nucleus of the ovum in the ovule. From the zygote thus produced the new plant arises by cell division. The factors carried by the ’male ’nucleus are combined with those carried by the ’female ’nucleus, so that the new plant inherits half of its factors from the ’male ’parent and half from its ’female ’parent. These factors determine the characters formed in the new plant during its development. The germ cells of the ’round ’pea variety carry a factor for the production of the round character, while those of the ’wrinkled ’variety carry a factor for producing the wrinkled character. Since a pea cannot be both round and wrinkled, but only one or the other, Mendel called the two factors a pair of opposed factors.

Dominance

Mendel supposed that in any one pair of opposed factors, one factor was dominant and the other recessive. In the hybrid round pea both factors for ’roundness ’and ’wrinkledness ’are present, but the effect of the ’round ’factor only is seen since this factor is dominant, while the ’wrinkled ’factor is recessive. In this hybrid the ’round ’factor had been inherited from the ’round ’parent and the ’wrinkled ’factor from the ’wrinkled ’parent, so that both factors were present in the same plant.

Segregation

When the ’germ cells ’of the hybrid are formed, Mendel supposed that the opposed factors are separated, so that two kinds of pollen grains and of ovules are formed. These carry either one or the other of the two factors and not both. Thus the hybrid ’round ’pea plant produces some pollen grains carrying the ’round ’factor and some carrying the ’wrinkled ’factor and similarly the ovules. He supposed, moreover, that the two kinds of pollen grains and ovules were produced in approximately equal numbers, and that on self-pollination random combination of these two types of germ ’cells ’would produce four possible combinations, also in approximately equal numbers—just as when tossing two pennies four results may occur: head A and head B ; head A and tail B ; tail A and head B ; tail A and tail B.

Since the characters produced in the F2 plants are displayed by the cotyledons which are either plump, producing round seeds, or shrunken, producing wrinkled seeds, in the ripened stage, the effect of the factors is seen very early in the life cycle, namely, in the seed stage. The ripened pods of the Fx plants therefore show the effect of segregation and recombination of the factors. Of the four possible combinations,, and produce round seeds, since the dominant factor for ’roundness ’is present in each. Combination alone produces wrinkled peas, since only the recessive factor for ’wrinkledness ’is present. In F2 the homozygous combinations RR and zvw on self-pollination can only produce round and wrinkled peas respectively, while the heterozygous combinations Rw and zvR will show segregation, as in the case of the original Fx hybrid. This is what Mendel actually found by experiment. He also confirmed that in the segregation of the ’round ’factor from the ’wrinkled ’factor, equal numbers of the two types of pollen grains were formed, one carrying the ’round ’factor and the other the ’wrinkled ’ factor, by pollinating the ’wrinkled ’pea variety with pollen from the F1 hybrid plants. Since Fl pollen + egg cell produces a round pea and Fj pollen + egg cell produces a wrinkled pea, then if the two types of pollen grains and were produced in equal numbers, equal numbers of round and wrinkled peas should result. Again, the experimental result was that expected, equal numbers of round and wrinkled peas being found in the pods.

Mendel discovered that the same principles operated in the case of other opposed factors in peas. For example, when he crossed a tall variety with a dwarf one, all the hybrid F1 plants were tall, but in the F2 generation, after self-pollination, a ratio of three tall plants to one dwarf was obtained. Here the characters of tallness and dwarf-ness did not appear until later in the life cycle, I.e. until after the ripe seeds were sown.

Factor T is the dominant factor for producing tallncss ; factor t is the recessive factor for producing dwarfness.

Following the revival of Mendel ’s work in 1900, many breeding experiments have been made, and have shown that Mendel ’s principles are of general application both among plants and animals. A few cases have also been discovered in which the hybrid F1 plants have characters intermediate between those of the parents. A well-known example of this is found in a garden plant known as the ’four o ’clock. ’ Two varieties exist: one with red flowers and one with white flowers. The F1 hybrid has pink flowers, and when self-pollinated produces in F2 a ratio of one red-flowered plant to two pink-flowered plants to one white-flowered plant.

In fowls the ’blue ’Andalusian fowl is an Fx hybrid produced by mating a black Andalusian fowl with a white Andalusian variety. ’Blue ’Andalusian fowls have white plumage finely speckled with black, and when bred together produce black, white and ’blue ’progeny in the ratio of 1 black : 2 blue : 1 white.

Mendel also experimented with two pairs of opposed characters, taking for parents a round yellow pea and a wrinkled green pea. The Fx hybrid peas were all round and yellow, since round and yelfow are the dominant characters. When plants raised from these peas were self-pollinated four types of peas were obtained : round yellow and green wrinkled like the parent peas, and two new combinations, round green and wrinkled yellow. These four types were present in the following ratio :— 9 round yellow. 3 round green. 3 wrinkled yellow. 1 wrinkled green.

Mendel supposed that the factors in the hybrid Fx on segregation behaved independently apart from observing his principle of segregation that the two members of any particular pair must segregate. Thus four types of germ cell were possible, which on fertilization gave sixteen combinations.

To explain the foregoing example Mendel proposed another ’law ’called the Principle of Independent Assortment. This states that the members of different pairs of opposed factors segregate independently of one another. Factor R may segregate from r along with Y, but it may be accompanied by y. This often fails to occur, owing to what is known as linkage, in which members of different pairs appear to be linked together in inheritance, and do not segregate independently. In sweet peas the factors for purple flower colour and long pollen are linked together, and so are their allelomorphs red flower colour and round pollen. Following a cross between a plant with purple flowers and long pollen and one with red flowers and round pollen, the Fj plants have purple flowers and long pollen, since these are dominant characters. After self-pollination, the Fx plant produces seeds which develop into plants with either purple flowers and long pollen or red flowers and round pollen. Only in rare cases do purple flowers with round pollen and red flowers with long pollen appear.

Mendel ’s work has provided the basis for a new branch of science called genetics or the study of inheritance. Already outstanding achievements in the production of new varieties of plants with valuable properties such as disease resistance have been recorded. One of the latest examples is the tliornless cultivated blackberry produced at the John Innes Horticultural Institution at Merton in Surrey. Among poultry there has also been produced at the Cambridge University Poultry Station a new breed of fowls known as the Legbar, in which the sex of the chickens can be distinguished as soon as they are hatched so that the more profitable pullet chickens can be selected and reared.

Suggestions for Practical Work.

Mendel ’s original experiments can be repeated, care being taken to exclude insects by enclosing the flowers in cellophane bags tied to the stalks. Care and skill are needed for sucoessful results, both when removing the unripe stamens from flowers selected for subsequent pollination and in judging when the stigmas are ripe for pollination.

An easy experiment requiring no skill or special care can be performed wherever sweet maize can be grown sucoessfully. A few seeds of a yellow-seeded variety, e.g. Cuthbert ’s Dwarf Golden Spine, are sown alongside others of a white-seeded variety, e.g. Carter ’s Improved White. When the plants flower some of the pollen from the yellow variety will be blown on to the stigmas of the white variety, and the cobs produced on the latter will display a mixture of yellow and white seeds, thus showing the dominance of the yellow factor. Some of the pollen of the white variety will pollinate certain ovules of the yellow variety, but all the seeds produced will be yellow, showing that the white factor is recessive. If the yellow hybrid seeds on the cobs of the white variety are sown in the following year, the F2 plants will produce cobs containing yellow and white seeds in an approximate ratio of three yellow to one white, thus showing segregation. The cobs in all cases should be examined at the unripe stage when ready for eating, since when allowed to ripen the seeds become translucent and their colours less easy to distinguish.

The fruit fly Drosophila has winged and ’wingless ’varieties. When a winged male is mated with a ’wingless ’female or vice versa, the hybrid F flies are all winged, but when these are mated together the F2 progeny are either winged or ’wingless, ’ and appear in a ratio approaching three winged to one ’wingless. ’ Details of the culture and handling of these flies can be found in various textbooks, and especially in ’Drosophila for Practical Work in Genetics, ’ Dr G. Pugh Smith, School Science Review, vol. xviii, No. 70.

Tame mice may be utilized, e.g. by mating a black variety with an albino, provided pure strains can be obtained.

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