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bacon and lard types of hogs or the mutton and wool breeds of sheep to secure some of the advantages of both parental races. Dent and flint varieties of corn when crossed usually give greater increases in yield than crosses within either type. In these diverse crosses many of the desirable features of both parental races are brought together. How this works is well illustrated in the cross of a "golden" type of corn which is deficient in chlorophyll with a "dwarf" as shown in figure 26. The plants resulting from this cross are tall, normally green and quite vigorous and productive. In this particular case one parent contributes normal stature and the other normal chlorophyll. Both these characters are dominant over the recessive condition so that all the hybrid plants

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Figure 25. It is the uniform production of a good ear on every planthat makes the first generation hybrids between inbred strains so product tive.

are alike in their tall stature and green color. Another case is shown in figure 27 of two dwarfs which are genetically different and which, when crossed, give a tall, vigorous hybrid. One of the dwarfs lacks something essential to normal height and all the plants are alike as long as they are not out-crossed. The other dwarf is lacking in some other essential factor present in normal corn. When these two small plants are combined each type supplies what the other lacks so that the result is normal stature in all the hybrid plants the first year after crossing. These illustrations of the result of crossing are extreme cases which show how conspicuous abnormalities are suppressed by crossing so that the hybrid offspring are able to make a greater growth than either parent. The same situation in principle exists in all crosses from

which hybrid vigor ensues. Different organisms possess different hereditary qualities. When brought together there is always a tendency for the hereditary factors which make for greater growth vigor to dominate the factors for lesser growth. The bringing

[graphic]

Figure 26. The result of crossing a golden, liguleless type, on the left, with a green dwarf on the right. The hybrid, in the center, has tall stature, normal foliage and green chlorophyll due to dominant factors contributed by each parent.

together of the best of both parents in this way gives the hybrid offspring a temporary advantage over either parent in the first generation following the cross. Recessive weaknesses are con

tinually occurring as mutations as shown by the many controlled observations on the fruit fly and other forms of life. In crossfertilized organisms, and particularly in domesticated animals and plants, crossing keeps these covered over and out of sight by combining them with normal factors. Many of these recessive weaknesses are not distinct and visible characters as are the

[graphic]

Figure 27.

Two genetically different dwarf types give tall plants when crossed, due to the fact that the normal growth factor which each lacks is supplied by the other.

chlorophyll deficiency or dwarfness in corn but nevertheless they weaken the organism in some way. When such crossbred races are inbred, the heterozygous combinations are reduced and the resulting individuals which are homozygous to a greater and

greater degree, as the inbreeding is continued, show the recessive weaknesses and are either unable to reproduce themselves or are reduced in size and rate of growth to a point below that of the original stock. The inbred individuals each receive some of the hereditary factors for vigorous growth. Some receive more than others as a chance allotment and are therefore better able to survive the inbreeding process. Others are so weakened that they perish. On account of the way in which the hereditary mechanism operates it is extremely improbable that any one individual will receive all the more favorable growth factors, and in actual practice inbred strains of corn are all reduced by inbreeding. It is theoretically possible to obtain individuals which possess an unusually large share of the more favorable growth factors or even all of them and for that reason show no reduction from inbreeding. Darwin obtained self-fertilized races of Ipomea and Mimulus which were more vigorous than the naturally cross-fertilized variety at the start. Cummings reports self-fertilized strains of squash that are as productive as the original variety and much more uniform in type. King has obtained inbred rats after long-continued brother and sister mating that are fully as vigorous as the material with which she started. The fact that no such result has been obtained with corn shows how dependent this plant has become upon cross-fertilization to maintain production.

THE TRANSITORY NATURE OF HYBRID VIGOR.

The increased growth resulting from crossing is quickly lost in the following generations when the hybrid individuals are bred among themselves or again inbred. In other words, hybrid vigor is a temporary manifestation which ordinarily cannot be fixed and made permanent in sexually reproduced offspring. The reason for this is readily appreciated when the illustrations previously given are followed into the later generations. The cross of the golden and dwarf corn gives all normal tall green plants in the first hybrid generation. Seed from these hybrid plants, either selfed or inter-crossed, always gives in the next generation all the possible combinations of characters that went into the cross. In this particular case the golden plants also lacked the ligule which is the small extension of the leaf sheath surrounding the stalk above the leaf blade. Liguleless plants hold their leaves in a characteristically upright position close to the stalk. In the second generation of this cross of liguleless golden by dwarf, eight different kinds of plants are produced. These are shown in figure 28. Due to the recombination of Mendelian units, this generation is extremely variable, and while some of the tall, green, liguled plants may be as vigorous and productive as the first crossed plants this generation as a whole averages much less productive. By further inbreeding, eight distinct pure-breeding combinations of these three characters

can be obtained and within each type still further minor differences could be established. Crossing any two of these types gives increased growth and restores the normal condition provided the factors for normal growth are all present in one or the other type. In the same way the vigorous and productive crosses between inbred strains of corn fall off in size and yield in the second generation and are much more variable. This always results whether the first crossed plants are self-fertilized or are inter-crossed among themselves. If the inbred strains are uniform and fixed in their type the first generation hybrid plants are germinally all alike so

[graphic]

Figure 28. The second generation offspring from the crossing of golden liguleless by dwarf. Eight different combinations of these three characters are obtained by Mendelian segregation and recombination.

that it is easily understood why self-fertilization and inter-crossing give the same result. To test this out two inbred strains were crossed after 14 generations of self-fertilization. A number of the hybrid plants were self-fertilized and an equal number were interpollinated. The seed of these two lots was planted in alternate rows, replicated three times. The self-fertilized plants averaged 76.2 .57 inches in height in comparison with the intercrossed plants which averaged 73.8+.70. In production of grain they stood respectively 22.2+1.2 and 22.0±2.4 bushels per acre. neither case are the differences significant.

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