Page images
PDF
EPUB
[blocks in formation]

3. Purity of the Germ-cells.-The great discovery of Mendel is this: The hybrid, whatever its own character, produces ripe germ-cells which bear only the pure character of one parent or the other. Thus, when one parent has the character A, and the other the character B, the hybrid will have the character AB, or in cases of simple dominance, A(B) or B(A). But whatever the character of the hybrid may be, its germ-cells, when mature, will bear either the character A or the character B, but not both; and As and Bs will be produced in equal numbers. This perfectly simple principle is known as the law of 'segregation,' or the law of the 'purity of the germ-cells.' It bids fair to prove as fundamental to a right understanding of the facts of heredity as is the law of definite proportions in chemistry. From it follow many important consequences.

A first consequence of the law of purity of the germ-cells is polymorphism of the second and later hybrid generations. The individuals of the first hybrid generation are all of one type, provided the parent individuals were pure. Each has a character resulting from the combination of an A with a B, let us say AB. [In cases of dominance it would more properly be expressed by A (B) or B (A).] But in the next generation three sorts of combinations

* The parenthesis is used to indicate a recessive character not visible in the individual.

are possible, since each parent will furnish As and Bs in equal numbers. The possible combinations are AA, AB and BB. The first sort will consist of pure As and will breed true to that character ever afterward, unless crossed with individuals having a different character. Similarly. the third sort will consist of pure Bs and will breed true to that character. But the second sort, AB, will consist of hybrid individuals, like those of which the first hybrid generation was exclusively composed. If, as supposed, germ-cells, A and B, are produced in equal numbers by hybrids of both sexes, and unite at random in fertilization, combinations AA, AB and BB should occur in the frequencies, 1:2:1. For in unions between two sets of gametes, each A+B, there is one chance each for the combinations AA and BB, but two chances for the combination AB.

If the three forms AA (or simply A), AB and B are all different in appearance, it will be a very simple matter in an experiment to count those of each class and determine whether they occur in the theoretical proportions, 1: 2: 1. One such case has been observed by Bateson (:02, p. 183) among Chinese primroses (Primula sinensis). An unfixable hybrid variety known as 'giant lavender,' bearing flowers of a lavender color, was produced by crossing

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][subsumed][merged small][merged small][subsumed][merged small][merged small][subsumed][merged small][merged small][subsumed][merged small][merged small][merged small][ocr errors][merged small]
[blocks in formation]

preceding generation. These are precisely the theoretical proportions, A+ 2 A(B) + B.

In the case of mice, it has been shown independently by Cuenot (:02) and by the writer's pupil, Mr. G. M. Allen, that the second hybrid generation, obtained by crossing gray with white mice, consists of gray mice and white mice approximately in the ratio 3: 1. (See Table III.) The white are pure recessives, producing only white offspring, when bred inter se. What portion of the grays are pure dominants has not yet been determined with precision, but we may confidently expect that it will prove to be not far from 1 in 3.

TABLE III.

HEREDITY OF COAT-COLOR AMONG CROSS-BRED MICE OBTAINED BY MATING WHITE MICE (W) WITH GRAY MICE (G).

one parental character, only two categories of offspring will be recognizable and these will be numerically as 3:1. But further breeding will allow the separation of the larger group into two subordinate classes-first, individuals bearing only the dominant character; secondly, hybrids; that is, into groups A and A(B), which will be numerically as 1:2.

Observed results are in this case also very close to theory. Mendel, by crossing yellow with green peas, obtained, as we have seen, only yellow (hybrid) seed. Plants raised from this seed bore in the same pods both yellow seed and green seed in the ratio 3:1. (See Table II.) Under self-fertilization, the green seed produced in later generations green seed only. It bore only the recessive character. Of the yellow seeds, one in three produced only yellow offspring, i. e., contained only the dominant character; but two out of three proved to be hybrid, producing both green and yellow seed, as did the hybrids of the

[blocks in formation]

A further test of the correctness of Mendel's hypothesis of the purity of the germ-cells and of their production in equal numbers, is afforded by back-crossing of a hybrid with one of the parental forms. For example, take a case of simple dominance, as of cotyledon-color in peas or coat-color in mice. We have here characters D (dominant) and R (recessive). The first generation hybrids will all be D(R). Any one of them back-crossed with the recessive parent will produce fifty per cent. of pure recessive offspring and fifty per cent. of hybrids.

For the hybrid produces germ-cells The recessive parent produces germcells ....

The possible combinations are....

D+R

R+R 2D (R) + 2R

This case has been tested for peas and for mice and found to be substantially as stated.

We have thus far considered only cases of cross-breeding between parents differing in a single character. We have seen that in such cases, no new forms, except the unstable hybrid form, are produced. But when the parent forms crossed differ in two or more characters, there will be produced in the second and later hybrid generations individuals possessing new combinations of the characters found in the parents; indeed, all possible combinations of those characters will be formed, and in the proportions demanded by chance. Thus when parents are crossed which differ in two respects, A and B, let us designate the dominant phase of these characters by A, B, the recessive phase by a, b. The immediate offspring resulting from the cross will all be alike, AB (ab), but the second and later generations of hybrids will contain the stable, i. e., pure classes, AB, Ab, aB, ab, in addition to other (unstable or still hybrid) forms, namely, AB (ab), AB(b), A(a)B, A(a)b and aB(b). In every sixteen second-generation offspring there will be, on the average, one representing each of the stable combinations. Two of the stable combinations will be identical with the parent forms, the other two will be new. The remaining twelve individuals will be hybrid in one or both characters.

An illustration may help to make this case clear. Among domesticated guineapigs, as among mice and rabbits, albinism is recessive with respect to pigmented coat. Further, there occur among guinea-pigs individuals known as 'Abyssinians,' whose

*This is Mendel's use of lower-case letters to designate recessive characters, with which I have combined the use of a parenthesis when a character by nature recessive is not visible in the individual.

coat presents a curious rough appearance, for the reason that the hair stands out stiffly from the body in a number of 'cowlicks' or rosettes. In crosses the Abyssinian or rough coat regularly dominates over the normal or smooth coat. Now let us consider what happens when a cross is made involving both these pairs of Mendelian characters, albinism vs. pigmented coat, and smooth vs. rough coat. If a white Abyssinian is bred to a pigmented smooth guinea-pig, the young are without exception pigmented and rough, these being the dominant members of the two pairs of characters. But the young of

course contain in a latent condition the two recessive characters, white coat and smooth coat, which fact may be indicated by designating them as already suggested, AB (ab) [A, a referring to the rough or smooth character of the coat and B, b to its color].

A

These primary hybrids, if bred inter se, will produce young of four different sorts, viz., rough pigmented, rough rough white, smooth pigmented and smooth white. certain number of the animals of each sort will breed true, i. e., will produce only their own sort when mated to animals like themselves. Theoretically there should be one pure individual of each of the four sorts in a total of sixteen young. four pure individuals answer to the classes AB, Ab, aB, ab already mentioned.

The

But, besides these pure individuals, there will occur in three of the four classes impure or hybrid individuals, which will transmit to some of their young the dominant character or characters which they themselves possess, but to others of their young the corresponding recessive character or characters. Only the class of smooth white animals (of which there should be one in sixteen young) contains none but pure individuals, for they bear

the two recessive characters (ab), and so conceal no hidden recessives. They may at once be set aside as pure. But in the other three classes nothing but actual breeding tests will serve to show which individuals are pure and which impure or hybrid. To each pure individual possessing one dominant and one recessive character there will be two others, exactly like it in appearance, but hybrid in one pair of characters. This statement applies to the two classes, rough-white and smooth-pigmented, in which the impure individuals would be designated A(a)b and aB(b) respectively. Such impure animals bred inter se would produce, in the case of rough-white parents, both rough-white and smooth-white offspring, and in the case of smooth-pigmented parents, both smoothpigmented and smooth-white offspring.

In the class of rough-pigmented secondgeneration offspring, which combines the two dominant characters, there will be to each pure individual (AB) eight which are impure in one or both characters. Two of the eight will be hybrid in one character only, as in the rough vs. smooth character they form the class A(a)B; two other individuals will be hybrid in the other character, albino vs. pigmented, forming the class AB(b); while the remaining four will be hybrid in both characters, exactly like the entire first generation of offspring, AB(ab).

[ocr errors][merged small]

confidence in the outcome, whereas hitherto his work, important and fascinating as it is, has consisted largely of groping for a treasure in the dark.

The greater the number of separately variable characters involved in a cross, the greater will be the number of new combinations obtainable; the greater, too, will be the number of individuals which it will be necessary to raise in order to secure all the possible combinations; and the greater, again, will be the difficulty of isolating the pure, i. e., stable forms from such as are similar to them in appearance but still hybrid in one or more characters. Mendel has generalized these statements substantially as follows: In cases of complete dominance, when the number of differences between the parents is n, the number of different classes into which the second generation of offspring fall will be 3", of which 2" will be pure (stable); the remainder will be hybrid, though indistinguishable from pure individuals. The smallest number of individuals which in the second hybrid generation will allow of one pure individual to each visibly different class will be 4". (See Table IV.)

[merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

The law of Mendel reduces to an exact science the art of breeding in the case most carefully studied by him, that of entire

dominance. It gives to the breeder a new conception of 'purity.' No animal or plant is 'pure' simply because it is descended from a long line of ancestors possessing a desired combination of characters; but any animal or plant is pure if it produces gametes of only one sort, even though its grandparents may among themselves have possessed opposite characters. The existence of purity can be established with certainty only by suitable breeding tests (especially by crossing with recessives), but it may be safely assumed for any animal or plant descended from parents which were like each other and had been shown by breeding tests to be pure.

Special Cases under the Law of Mendel. -It remains to speak of some special cases under the law of Mendel, which apparently are exceptions to one or another of the principles already stated, and which probably result from exceptional conditions known to us imperfectly. These special cases have come to light in part through Mendel's own work, in part through that of others.

The

1. Mosaic Inheritance.-It occasionally happens that in crosses which bring together a pair of characters commonly related as dominant and recessive, the two characters appear in the offspring in patches side by side, as in piebald animals and parti-colored flowers and fruits. normal dominance apparently gives place in such cases to a balanced relationship between the alternative characters. What conditions give rise to such relationships is unknown, but when they are once secured they often prove to possess great stability, breeding true inter se. This, for example, is the case in spotted mice, which usually produce a large majority of spotted off spring. The balanced relationship of characters possessed by the parents is transmitted to the germ-cells, which are, not as

in ordinary hybrid individuals D or R, but DR. This has been shown to be the case in spotted mice by Mr. Allen and myself, in a paper published elsewhere. (Castle and Allen, :03.)

2. Stable Hybrid Forms.-This is a case, in some respects similar to the last, which was familiar to Mendel (:70) himself. It sometimes happens, as we have seen, that the hybrid has a form of its own different from that of either parent. To such cases the law of dominance evidently does not apply. In a few cases-Hieracium hybrids (Mendel), Salix hybrids (Wichura) --it has been found that the hybrid form does not break up in the second generation and produce individuals like the grandparents, but breeds true to its own hybrid character. This can be explained only on one of two assumptions. Either the germcells bear the two characters in the balanced relationship, AB, as do those of spotted mice, or, of the two gametes which unite in fertilization, one invariably bears the character A, the other the character B. Of the two explanations, the former seems at present much the more probable. 3. Coupled Characters.-This is the phenomenon of correlation of characters in heredity. It is sometimes found that, in cross-breeding, two characters can not be separated. When one is inherited, the other is inherited also. Thus, in crossing different sorts of Datura (the Jamestown weed) it has been found that purple color of stem invariably goes with blue color of flowers, whereas green stems are constantly associated with white flowers. Again in mice, rabbits and most other mammals, white hair and pink eyes commonly occur together and may not be separated in heredity. Very rarely, however, as I have observed, an otherwise perfectly white guinea-pig has dark eyes; further the ordinary albino guinea-pig

« EelmineJätka »