X. Texts for discussion:

What is intelligence?

a) Although the question has n6t yet been settled many students of heredity believe that mental ability is an ininherited trait. They base their claims on studies of certain families in which a high degree of intelligence appears repeatedly in the offspring. Other families have been studied that indicate that low intelligence may be inherited as well. It is true, that identical twins seem to have about the same level of mental ability. This is what we would expect if intelligence were controlled by genes, since identical twins have the same gene combinations.

Interesting studies have been made of identical twins who were separated after birth and raised in different homes. It was found that they sometimes showed greater differences in intelligence than would be expected if they had been raised in the same home. They also showed considerable differences in personality.

Scientists agree that you can do nothing to change your genes, but you can do a great deal to improve the traits controlled by your genes. Education and training will develop the mental traits that you have inherited. You can even improve your physical traits.

b) Actually there is no precise definition for this trait of intelligence. To most of us it means the capacity, for learning or simply the ability to learn. We know some things about intelligence. We know there is extreme variation in mental capacity among human beings. It ranges from idiocy at one extreme to genius at the other extreme, with most people having average or near average intelligence. From this we may conclude that multiple genes are involved. Also it seems from evidence accumulated that the extreme variations in intelligence among human beings are partly hereditary and partly environmental.

Education and training play an Important part in bringing out intellectual potentialities. Yet even among persons with similar training there are great variations in general intelligence.

It has been observed that when children of the same family differ from each other in mental,capacity, they usually continue to differ despite the fact that they live in the same school. The important thing: is for each one of us to apply ourselves and to get the. most out of our inherited potentiates. Very few of us do it.

Can we conclude that heredity sets certain limfts as to the results that can be obtained from study or training?

Discuss the problem.

XI. Read the text “genetics and the essence of life” from “texts for home-reading”. Lesson ten

Improvement of plants

All varieties of crops have some desirable characteristics or they would not be used. Nevertheless, each of these varieties is known to possess one or more undesirable traits which, if eliminated, would result in higher yields and better quality. The aim of the plant breeder is to develop superior varieties by eliminating the undesirable characteristics and combining the desirable ones in the same variety.

Plant improvement is based on the principles or laws of heredity which are included in the science known as genetics. Many of the principles and techniques used in plant breeding are complex and to understand them fully

intensive study and training are required. Selection is a simple, but important method of improving plants. As the name suggests this method consists of selecting the outstanding types and discarding those that are undesirable because of certain characteristics being possessed by them. For example, in small grains, plants resistant to lodging may be selected; and with alfalfa those capable of surviving in severe winters are to be retained. After a period of testing, during which plants are selected for certain desired traits or characteristics, a superior strain may be developed. Improvement by selection cannot be accomplished, however, unless the variety from which the selections are being made possesses some plants containing the characteristics desired.

Selection is not a new method of improving plants. Actually this process is as old as plants themselves. For many thousands of years plants have been subjected to the stern and relentless forces of nature, and only the fittest is left entirely to nature, the process is extremely slow. Man cannot wait for nature alone to improve plants for him. By selecting superior plants, he is able to bring about improvements in a few years that would require thousands of years of time if left to nature alone. Two procedures are commonly used when new varieties are developed by the process of selection. They are referred to as mass selection and individual selection. . Mass selection consists of selecting a fairly large number of individual plants possessing the desired characteristics. The seed from such plants is then mixed and sown together, and the better individuals are again selected or the poorer ones discarded. This process of selection is to be repeated for a few years until the plants prove to be reasonably uniform for the qualities desired Individual plant selection is commonly referred to as pedigree or pure-line selection. When this method is used, individual plants are selected that are superior for certain characters but instead of mixing the seed as in mass selection, the seed from each head or individual is planted in a row of its own in such a manner as to keep the progeny of each parent separate. The progeny of each plant are then carefully observed, a record being made of their appearance and performance. Comparisons between the different progenies are made, those with undesirable characters being discarded.

Records of performance are carefully checked and compared each year with those of standard varieties which are also grown under the same conditions. If after a testing for a number of years, the strain proves to be superior to the standard varieties, it is then grown in larger plots to increase the supply of seed.

A period of several years may be required for sufficient seed to be obtained for general distribution to farmers. As a rule, 8 to 14 years are usually required for making the selection, testing it and increasing it to the point where the new variety can be released to farmers.

Notes to the text:

1. as the name suggests—как указывает само название

2. whichever is most convenient — что является наиболее удобным

3. in a row of its own — на своем ряду

4. ...that are superior for certain characteristics — которые являются лучшими в отношении опреде ленных качеств

5. he used to work — он имел обыкновение работать

EXERCISES