Part 3 (2/2)
A factor that causes duplication in the legs has also been found. Here the interesting fact was discovered (Hoge) that duplication takes place only in the cold. At ordinary temperatures the legs are normal.
[Ill.u.s.tration: FIG. 30. Mutant race of fruit fly, called eyeless; a, a'
normal eye.]
In contrast to the last case, where a character is doubled, is the next one in which the eyes are lost (fig. 30). This change also took place at a single step. All the flies of this stock however, cannot be said to be eyeless, since many of them show pieces of the eye--indeed the variation is so wide that the eye may even appear like a normal eye unless carefully examined. Formerly we were taught that eyeless animals arose in caves. This case shows that they may also arise suddenly in gla.s.s milk bottles, by a change in a single factor.
I may recall in this connection that wingless flies (fig. 5 f) also arose in our cultures by a single mutation. We used to be told that wingless insects occurred on desert islands because those insects that had the best developed wings had been blown out to sea. Whether this is true or not, I will not pretend to say, but at any rate wingless insects may also arise, not through a slow process of elimination, but at a single step.
The preceding examples have all related to recessive characters. The next one is dominant.
[Ill.u.s.tration: FIG. 31. Mutant race of fruit fly called bar to the right (normal to the left). The eye is a narrow vertical bar, the outline of the original eye is indicated.]
A single male appeared with a narrow vertical red bar (fig. 31) instead of the broad red oval eye. Bred to wild females the new character was found to dominate, at least to the extent that the eyes of all its offspring were narrower than the normal eye, although not so narrow as the eye of the pure stock. Around the bar there is a wide border that corresponds to the region occupied by the rest of the eye of the wild fly. It lacks however the elements of the eye. It is therefore to be looked upon as a rudimentary organ, which is, so to speak, a by-product of the dominant mutation.
The preceding cases have all involved rather great changes in some one organ of the body. The following three cases involve slight changes, and yet follow the same laws of inheritance as do the larger changes.
[Ill.u.s.tration: FIG. 32. Mutant race of fruit fly, called speck. There is a minute black speck at base of wing.]
At the base of the wings a minute black speck appeared (fig. 32). It was found to be a Mendelian character. In another case the spines on the thorax became forked or kinky (fig. 52b). This stock breeds true, and the character is inherited in strictly Mendelian fas.h.i.+on.
[Ill.u.s.tration: FIG. 33. Mutant race of fruit fly called club. The wings often remain unexpanded and two bristles present in wild fly (b) are absent on side of thorax (c).]
In a certain stock a number of flies appeared in which the wing pads did not expand (fig. 33). It was found that this peculiarity is shown in only about twenty per cent of the individuals supposed to inherit it. Later it was found that this stock lacked two bristles on the sides of the thorax.
By means of this knowledge the heredity of the character was easily determined. It appears that while the expansion of the wing pads fails to occur once in five times--probably because it is an environmental effect peculiar to this stock,--yet the minute difference of the presence or absence of the two lateral bristles is a constant feature of the flies that carry this particular factor.
In the preceding cases I have spoken as though a factor influenced only one part of the body. It would have been more accurate to have stated that the _chief_ effect of the factor was observed in a particular part of the body.
Most students of genetics realize that a factor difference usually affects more than a single character. For example, a mutant stock called rudimentary wings has as its principle characteristic very short wings (fig. 34). But the factor for rudimentary wings also produces other effects as well. The females are almost completely sterile, while the males are fertile. The viability of the stock is poor. When flies with rudimentary wings are put into compet.i.tion with wild flies relatively few of the rudimentary flies come through, especially if the culture is crowded. The hind legs are also shortened. All of these effects are the results of a single factor-difference.
[Ill.u.s.tration: FIG. 34. Mutant race of fruit fly, called rudimentary.]
One may venture the guess that some of the specific and varietal differences that are characteristic of wild types and which at the same time appear to have no survival value, are only by-products of factors whose most important effect is on another part of the organism where their influence is of vital importance.
It is well known that systematists make use of characters that are constant for groups of species, but which do not appear in themselves to have an adaptive significance. If we may suppose that the constancy of such characters may be only an index of the presence of a factor whose _chief_ influence is in some other direction or directions, some physiological influence, for example, we can give at least a reasonable explanation of the constancy of such characters.
I am inclined to think that an overstatement to the effect that each factor may affect the entire body, is less likely to do harm than to state that each factor affects only a particular character. The reckless use of the phrase ”unit character” has done much to mislead the uninitiated as to the effects that a single change in the germ plasm may produce on the organism.
Fortunately, the expression ”unit character” is being less used by those students of genetics who are more careful in regard to the implications of their terminology.
There is a cla.s.s of cases of inheritance, due to the XY chromosomes, that is called s.e.x linked inheritance. It is shown both by mutant characters and characters of wild species.
For instance, white eye color in Drosophila shows s.e.x linked inheritance.
If a white eyed male is mated to a wild red eyed female (fig. 35) all the offspring have red eyes. If these are inbred, there are three red to one white eyed offspring, but white eyes occur only in the males. The grandfather has transmitted his peculiarity to half of his grandsons, but to none of his granddaughters.
[Ill.u.s.tration: FIG. 35. Diagram showing a cross between a white eyed male and a red eyed female of the fruit fly. s.e.x linked inheritance.]
The reciprocal cross (fig. 36) is also interesting. If a white eyed female is bred to a red eyed male, all of the daughters have red eyes and all of the sons have white eyes. We call this criss-cross inheritance. If these offspring are inbred, they produce equal numbers of red eyed and white eyed females and equal numbers of red eyed and white eyed males. The ratio is 1: 1: 1: 1, or ignoring s.e.x, 2 reds to 2 whites, and not the usual 3:1 Mendelian ratio. Yet, as will be shown later, the result is in entire accord with Mendel's principle of segregation.
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