Book - Evolution and Genetics 7
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Chapter 7 The Linkage Groups and the Chromosomes
If the hereditary elements, the genes, are carried by the chromosomes and if the chromosomes are persisting structures, there should be as many groups of hereditary characters as there are kinds of chromosomes. In only a few cases has a sufficient number of characters been studied to show whether there is any correspondence between the number of hereditary groups of characters and the number of chromosomes. In the yinegar fly, Drosophila, there are about four hundred characters that fall into four groups. On page 88 (fig. 38) some of these are given arranged according to groups. The characters are arranged in four grou2)s, Group I, II, III and IV. Three of these groups are equally large or nearly so; Group IV contains only three characters. The characters are put into these groups because, in heredity, the members of each group tend to be inherited together, i.e., if two or more enter the cross together they tend to remain together through subsequent generations. On the other hand, any member of one group is inherited entirely independently of any member of the other groups ; in the same way as Mendel's yellow-green pair of characters is inherited independently of the round-wrinkled pair.
Fig. 38. Chart of the genes of the chromosomes of Drosophila. The genes are arranged in the four linkage groups, I, II, III, IV. The name of the gene is given to the right of its locus, and the distance of the loci from one end of the chromosome is indicated by the numbers to the left of each locus. The "distance" gives the cross-over value for the genes corrected for double crossing-over.
In the chromosome group of Drosophila melanogaster (fig. 39) there are four pairs of chromosomes, three of nearly the same size and one much smaller. Xot only is there agreement between the number of hereditary groups and the number of the chromosomes, but even the size relations are the same, for there are three large groups of characters and three pairs of large chromosomes, and one small group of characters and one pair of small chromosomes.
Fig. 39. Female group and male group of chromosomes of Drosophila melanogaster.
The Four Linkage Groups of Drosophila Melanogaster
The following description of the characters of the wild fly may be useful in connection with the account of the modifications of these characters that appear in the mutants.
The head and thorax of the wild fly are grayish yellowy the abdomen is banded w4th alternate stripes of yellow and black. In the male (fig. 6, left) , there are three narrow^ bands and a black tip. In the female there are five black bands (fig. 6, right) . The wings are gray with a sin*face texture of such a kind that at certain angles they are iridescent. The eves are a deep, brick-red. The minute hairs that cover the body have a characteristic arrangement that is most obvious on the head and thorax. There is a definite number of larger hairs called bristles which have a characteristic position and are used for diagnostic purposes in classifying the species. On the foreleg of the male there is a comb-like organ formed by a row of bristles ; it is absent in the female. The comb is a secondary sexual character.
Some of the characters of the mutant types are shown in figures 40, 41, 42, 43. The drawing of a single fly is often used here to illustrate more than one character. This is done to economize space, but of course there would be no difficulty in actually bringing together in the same individual any two or more characters belonging to the same group (or to different groups) . Without colored figures it is not possible to show many of the most striking differences of these mutant races ; at most, dark and light coloring can be indicated by the shading of the body, wings, or eyes.
The hereditary elements of this group are carried by the A^- chromosomes. The characters are said to be sex-linked.
111 the six flies drawn in figure 40 there are shown five different wing characters. The first of these types (a) is called cut, because the ends of the wings look as though they had been cut to a point. The antennae are displaced downward and appressed and their bristle-like aristae are crumpled.
The second figure (h) represents a fly with a notch in the ends of the wings. This character is dominant, but the same factor that produces the notch in the wings is also a recessive lethal factor; because of this latter effect of the character, no males of this race exist, and the females of the race are never pure but hybrid. This same figure (b) is used here to show two other sex-linked characters. The spines on the thorax are twisted or kinky, which is due to a factor called "forked." The effect is best seen on the thorax, but all spines on the body are similarly modified; even the minute hairs are also affected. The lighter color of the body and antennae is intended to indicate that the character tan is also present. The tan flies are interesting because they have lost the positive heliotropism. As this peculiarity of the tan flies is inherited like all the other sexlinked characters, it follows that when a tan female is bred to a wild male all the sons inherit the recessive tan color and indifference to light, while the daughters show the dominant sex-linked character of their father, i.e., they are "gray," and go to the light. Hence when such a brood is disturbed the females fly to the light, but the males remain behind.
Fig. 40. Some of the characters of the first chromosome of Drosophila melanogaster. See text.
One of the first mutants that appeared was called rudimentary on account of the condition of the wings (c) . The same mutation has apj^eared independently several times. In the drawing (c) the dark body color is intended to indicate "sable."
In the fourth figure (d) the third and fourth longitudinal veins of the wing are fused into one vein from the base of the wing to the level of the first cross-vein and in addition converge and meet near their outer ends.
In the fifth figure (e) the wings are shorter and more pointed than in the wild fly. This character is called miniature. The light color of the drawing may be taken to represent yellow body color.
In the last figure (/) of "club," the wings are pads, essentially in the same condition that they are in when the fly emerges from the pupa case. Not all the flies of this stock have the wings in this condition ; some have fully expanded wings that ap23ear normal in all respects. Nevertheless, about the same percentage of offspring show the pads irrespective of whether the parents had pads or expanded wings. The flies of this stock show, however, another character, which is a product of the same factor, and which is constant, i.e., repeated in all individuals. The two bristles on the sides of the thorax are constantly absent in this race.
There are many different eye colors in Group I, ranging from a pure white eye to vermilion, which could be shown onlv by the use of colored drawings.
The hereditary elements of members of Group II are carried by one pair of the two large bent chromosomes.
In the first drawing (a) of figure 41 which contains members of Group II, the wings are almost entirely absent or "vestigial." This condition arose at a single step and breeds true, although it appears to be influenced to some extent by temperature, also by modifiers that sometimes appear in the stock.
In the second figure ( b ) the wings turn up at the end. The mutant is called jaunty.
In the third figure (c) the wing is long and narrow and sometimes bent back on itself, as shown here. In several respects the wing resembles strap (d) but seems to be due to another factor, called antlered.
In the fourth figure (d) the wings are long and narrow and several of the veins are unrepresented. This character, "strap," is very variable. On the thorax there is a deep black mark called trefoil. In the wild fly there is a three-pronged mark on the thorax present in many individuals. Trefoil is a further development and modification of this mark and is due to a special factor.
Fig. 41. Some of the characters of the second chromosome of Drosophila melanogaster. See text.
The fifth figure (e) is called dachsoid; the body is shortened and the wings are broad and held out. The legs are short. The absence of cross-veins in the wings is characteristic.
In the sixth figure (/), apterous, the wings are entirely absent, not even the base remaining as in vestigial. The apterous flies are almost completely sterile.
The seventh figure (g) shows the wings "curved." In addition there is j^resent a minute black speck at the base of each wing, due to another factor called speck.
In the eighth figure (k) the wings are arched. The factor is called arc. The dark color of the body, and especially of the wings, indicates the factor for black.
There are also a number of different eye colors in this group — one of which, brown, is darker in old flies than the red of the wild type.
The hereditary elements of Group III are carried by the other pair of large bent chromosomes.
In figure 42 (a) , a mutant type called bithorax is shown. The old metathorax is replaced by another mesothorax thrust in between the normal mesothorax and the abdomen. It carries a pair of wings that do not completely unfold. On this new mesothorax the characteristic arrangement of the bristles is shown. Thus at a single step a typical region of the body has doubled. The character is recessive.
In the second figure ( h ) the dark color of the fly is due to a factor called ebony.
The size of adult flies varies according to the amount of nourishment obtained by the larva. After the fly emerges its size remains nearly constant, as in many insects. Two races have, however, been separated that are different in size as a result of a genetic factor. The first of these, called dwarf, is represented by figure 42 (c). The race is small but variable in size, depending on food and other conditions. The same figure shows the presence of another factor, "sooty," that makes the fly dark.
In the fourth figure (d) another mutation in size is shown. It is called "giant." The flies are twice the size of wild flies.
In the fifth figure (e) the mutant dichaete is shown. It is characterized by the absence of two of the bristles on the thorax. Other bristles may also be absent, but not so constantly as the two just mentioned. Another effect of the same factor is the spread-out condition of the wings.
In the sixth figure (/) the wings are curled up over the back ; the character is called curled.
In the seventh figure (g) the wings are beaded, i.e., the margin is defective at intervals, giving a beaded-like outline to the wings. This condition is very variable and much affected by other factors that influence the shape of the wings.
There are many eye colors in this Group — one of these, sepia, becomes very dark in old flies. Pink and peach eye colors are modifications of the same gene (a case of multiple allelomorphs). Two other eye colors in this group, scarlet and cardinal, are almost indistinguishable, but the genes for these characters lie in quite different parts of the chromosome.
Fig. 42. Some of the characters of the third chromosome of Drosophila melanngaster. S€e text.
The hereditary elements of Group IV are carried by the pair of very small chromosomes. Only three mutants have been obtained. One of these, called "eyeless" (fig. 43 a, a"), is variable — the eyes are often entirely absent or represented by a few or by several ommatidia (h, h') . On the sides of the head where the normal eve lies there is, in "eyeless," a corresponding empty area in the more extreme condition (a, a^) , and even when a piece of the eye is present it lies in this area but failing to fill all of it, the outline of the full sized eye is, so to speak, still present. These parts of eyes (b, h') might be spoken of as rudimentary organs.
Fig. 43. The three characters of the foiirth chromosome of Drosophila melanogaster. See text.
Drawing (c) in figure 43, represents "bent," so called from the shape of the wings. This mutant is likewise very variable, often indistinguishable from the wild type, yet when well developed strikingly different from any other mutant.
The third mutant (d) is called shaven. The bristles and hairs are extremely short, and the thorax especially, appears as though shaven.
This brief account of a few of the mutant races that can be most easily represented by uncolored figures will serve to show how all parts of the body may change, some of the changes being so slight that they would be overlooked except by an ex^^eii:, others so great that the characters affected depart far from the original one.
It is important to note that the mutant genes in the X -chromosomes are not limited to any part of the body, nor do they affect more frequently a particular part. The same statement holds equally for all of the other chromosomes. In fact, since each factor may affect visibly several parts of the body at the same time there are no grounds for expecting any specific relation between a given chromosome and special regions of the body. It cannot too insistently be urged that when we say a character is the product of a particular factor we mean no more than that it is the most conspicuous effect of that factor.
If, then, as these and other results to be described point to the chromosomes as the bearers of the Mendelian factors, and if, as has been shown, these factors have a definite location in the chromosomes, it is clear that the location of the factors in the chromosomes bears no spatial relation to the architecture of the body.
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