_It is important to note that mutations in the first chromosome 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 special relation between a given chromosome and special regions of the body. It can not 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 the 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 will be shown presently, 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 location of the parts of the body to each other.

LOCALIZATION OF FACTORS IN THE CHROMOSOMES

_The Evidence from s.e.x Linked Inheritance_

When we follow the history of pairs of chromosomes we find that their distribution in successive generations is paralleled by the inheritance of Mendelian characters. This is best shown in the s.e.x chromosomes (fig. 57).

In the female there are two of these chromosomes that we call the X chromosomes; in the male there are also two but one differs from those of the female in its shape, and in the fact that it carries none of the normal allelomorphs of the mutant factors. It is called the Y chromosome.

The course followed by the s.e.x chromosomes and that by the characters in the case of s.e.x linked inheritance are shown in the next diagram of Drosophila ill.u.s.trating a cross between a white eyed male and a red eyed female.

[Ill.u.s.tration: FIG. 57. Scheme of s.e.x determination in Drosophila type.

Each _mature_ egg contains one X, each mature sperm contains one X, or a Y chromosome. Chance union of any egg with any sperm will give either XX (female) or XY (male).]

[Ill.u.s.tration: FIG. 58. Cross between white eyed male of D. ampelophila and red eyed female. The s.e.x chromosomes are indicated by the rods. A black rod indicates that the chromosome carries the factor for red; the open chromosome the factor for white eye color.]

The first of these represents a cross between a white eyed male and a red eyed female (fig. 58, top row). The X chromosome in the male is represented by an open bar, the Y chromosome is bent. In the female the two X chromosomes are black. Each egg of such a female will contain one "black" X after the polar bodies have been thrown off. In the male there will be two cla.s.ses of sperm--the female-producing, carrying the (open) X, and the male-producing, carrying the Y chromosome. Any egg fertilized by an X bearing sperm will produce a female that will have red eyes because the X (black) chromosome it gets from the mother carries the dominant factor for red. Any egg fertilized by a Y-bearing sperm will produce a male that will also have red eyes because he gets his (black) X chromosome from his mother.

When, then, these two F_1 flies (second row) are inbred the following combinations are expected. Each egg will contain a black X (red eye producing) or a white X (white eye producing) after the polar bodies have been extruded. The male will produce two kinds of sperms, of which the female producing will contain a black X (red eye producing). Since any egg may by chance be fertilized by any sperm there will result the four cla.s.ses of individuals shown on the bottom row of the diagram. All the females will have red eyes, because irrespective of the two kinds of eggs involved all the female-producing sperm carry a black X. Half of the males have red eyes because half of the eggs have had each a red-producing X chromosome. The other half of the males have white eyes, because the other half of the eggs had each a white-producing X chromosome. Other evidence has shown that the Y chromosome of the male is indifferent, so far as these Mendelian factors are concerned.

[Ill.u.s.tration: FIG. 59. Cross between red eyed male and white eyed female; reciprocal cross of Fig. 58.]

The reciprocal experiment is ill.u.s.trated in figure 59. A white eyed female is mated to a red eyed male (top row). All the mature eggs of such a female contain one white-producing X chromosome represented by the open bar in the diagram. The red eyed male contains female-producing X-bearing sperm that carry the factor for red eye color, and male-producing Y chromosomes. Any egg fertilized by an X-bearing sperm will become a red eyed female because the X chromosome that comes from the father carries the dominant factor for red eye color. Any egg fertilized by a Y-bearing sperm will become a male with white eyes because the only X chromosome that the male contains comes from his mother and is white producing.

When these two F_1 flies are inbred (middle row) the following combinations are expected. Half the eggs will contain each a white producing X chromosome and half red producing. The female-producing sperms will each contain a white X and the male-producing sperms will each contain an indifferent Y chromosome. Chance meetings of egg and sperm will give the four F_2 cla.s.ses (bottom row). These consist of white eyed and red eyed females and white eyed and red eyed males. The ratio here is 1:1 and not three to one (3:1) as in other Mendelian cases. But Mendel's law of segregation is not transgressed, as the preceding a.n.a.lysis has shown; for, the chromosomes have followed strictly the course laid down on Mendel's principle for the distribution of factors. The peculiar result in this case is due to the fact that the F_1 male gets his single factor for eye color from his mother only and it is linked to or contained in a body (the X chromosome) that is involved in producing the females, while the mate of this body--the Y chromosome--is indifferent with regard to these factors, yet active as a mate to X in synapsis.

[Ill.u.s.tration: FIG. 60. Diagram of s.e.x determination in type with XX female and XO male (after Wilson).]

In man there are several characters that show exactly this same kind of inheritance. Color blindness, or at least certain kinds of color blindness, appear to follow the same scheme. A color blind father transmits through his daughters his peculiarity to half of his grandsons, but to none of his grand-daughters (fig. 38A). The result is the same as in the case of the white eyed male of Drosophila. Color blind women are rather unusual, which is expected from the method of inheritance of this character, but in the few known cases where such color blind women have married normal husbands the sons have inherited the peculiarity from the mother (fig. 38B). Here again the result is the same as for the similar combination in Drosophila.

[Ill.u.s.tration: FIG. 61. Spermatogenesis in man. There are 47 chromosomes (diploid) in the male. After reduction half of the sperm carry 24 chromosomes (one of which is X) and half carry 23 chromosomes (no X).]

In man the s.e.x formula appears to be XX for the female and XO for the male (fig. 60), and since the relation is essentially the same as that in Drosophila the chromosome explanation is the same. According to von Winiwarter there are 48 chromosomes in the female and 47 in the male (fig.

61). After the extrusion of the polar bodies there are 24 chromosomes in the egg. In the male at one of the two maturation divisions the X chromosome pa.s.ses to one pole undivided (fig. 61, C). In consequence there are two cla.s.ses of sperms in man; female producing containing 24 chromosomes, and male producing containing 23 chromosomes. If the factor for color blindness is carried by the X chromosome its inheritance in man works out on the same chromosome scheme and in the same way as does white eye color (or any other s.e.x linked character) in the fly, for the O sperm in man is equivalent to the Y sperm in the fly.

In these cases we have been dealing with a single pair of characters. Let us now take a case where two pairs of s.e.x linked characters enter the cross at the same time, and preferably a case where the two recessives enter the cross from the same parent.

If a female with white eyes and yellow wings is crossed to a wild male with red eyes and gray wings (fig. 62), the sons are yellow and have white eyes and the daughters are gray and have red eyes. If two F_1 flies are mated they will produce the following cla.s.ses.

[Ill.u.s.tration: FIG. 62. Cross between a white eyed, yellow winged female of D. ampelophila and a red eyed, gray winged male. Two pairs of s.e.x linked characters, viz., white-red and yellow-gray are involved. (See text.)]

Yellow Gray Yellow Gray White Red Red White ------------ -------------

99.% 1.%

Not only have the two grandparental combinations reappeared, but in addition two new combinations, viz., grey white and yellow red. The two original combinations far exceed in numbers the new or exchange combinations. If we follow the history of the X chromosomes we discover that the _larger cla.s.ses_ of grandchildren appear in accord with the way in which the X chromosomes are transmitted from one generation to the next.

The _smaller cla.s.ses_ of grandchildren, the exchange combinations or cross-overs, as we call them, can be explained by the a.s.sumption that at some stage in their history an interchange of parts has taken place between the chromosomes. This is indicated in the diagrams.

The most important fact brought out by the experiment is that the factors that went in together tend to stick together. It makes no difference in what combination the members of the two pairs of characters enter, they tend to remain in that combination.

If one admits that the s.e.x chromosomes carry these factors for the s.e.x-linked characters--and the evidence is certainly very strong in favor of this view--it follows necessarily from these facts that at some time in their history there has been an interchange between the two s.e.x chromosomes in the female.

There are several stages in the conjugation of the chromosomes at which such an interchange between the members of a pair might occur. There is further a small amount of direct evidence, unfortunately very meagre at present, showing that an interchange does actually occur.

At the ripening period of the germ cell the members of each pair of chromosomes come together (fig. 49, e). In several forms they have been described as meeting at one end and then progressively coming to lie side by side as shown in fig. 63, e, f, g, h, i. At the end of the process they appear to have completely united along their length (fig. 63, j, k, l). It is always a maternal and a paternal chromosome that meet in this way and always two of the same kind. It has been observed that as the members of a pair come together they occasionally twist around each other (fig. 63, g, l, and 64, and 65). In consequence a part of one chromosome comes to be now on one side and now on the other side of its mate.

[Ill.u.s.tration: FIG. 63. Conjugation of chromosomes (side to side union) in the spermatogenesis of Batracoseps. (After Janssens.)]

When the chromosomes separate at the next division of the germ cell the part on one side pa.s.ses to one pole, the part on the other to the opposite pole, (figs. 64 and 65). Whenever the chromosomes do not untwist at this time there must result an interchange of pieces where they were crossed over each other.

[Ill.u.s.tration: FIG. 64. Scheme to ill.u.s.trate a method of crossing over of the chromosomes.]

Janssens has found at the time of separation evidence in favor of the view that some such interchange probably takes place.

We find this same process of interchange of characters taking place in each of the other three groups of Drosophila. An example will show this for the Group II.

[Ill.u.s.tration: FIG. 65. Scheme to ill.u.s.trate double crossing over.]

If a black vestigial male is crossed to a gray long-winged female (fig. 66) the offspring are gray long. If an F_1 female is back-crossed to a black vestigial male the following kinds of flies are produced:

Black Gray Black Gray vestigial long long vestigial ----------------- -----------------

83% 17%

The combinations that entered are more common in the F_2 generations than the cross-over cla.s.ses, showing that there is linkage of the factors that entered together.

Another curious fact is brought out if instead of back-crossing the F_1 female we back-cross the F_1 male to a black vestigial female. Their offspring are now of only two kinds, black vestigial and gray long. This means that in the male there is no crossing-over or interchange of pieces.

This relation holds not only for the Group II but for all the other groups as well.

Why interchange takes place in the female of Drosophila and not in the male we do not know at present. We might surmise that when in the male the members of a pair come together they do not twist around each other, hence no crossing-over results.

[Ill.u.s.tration: FIG. 66. Cross between black vestigial and gray long flies.

Two pairs of factors involved in the second group. The F_1 female is back crossed (to right) to black vestigial male; and the F_1 male is back crossed to black vestigial female (to left). Crossing over takes place in the F_1 female but not in the F_1 male.]

Crossing-over took place between white and yellow only once in a hundred times. Other characters show different values, but the same value under the same conditions is obtained from the same pair of characters.

[Ill.u.s.tration: FIG. 67. Map of four chromosomes of D. ampelophila locating those factors in each group that have been most fully studied.]

If we a.s.sume that the nearer together the factors lie in the chromosome the less likely is a twist to occur between them, and conversely the farther apart they lie the more likely is a twist to occur between them, we can understand how the linkage is different for different pairs of factors.

On this basis we have made out chromosomal maps for each chromosome (fig.

67). The diagram indicates those loci that have been most accurately placed.

_The Evidence from Interference_

There is a considerable body of information that we have obtained that corroborates the location of the factors in the chromosome. This evidence is too technical to take up in any detail, but there is one result that is so important that I must attempt to explain it. If, as I a.s.sume, crossing over is brought about by twisting of the chromosomes, and if owing to the material of the chromosomes there is a most frequent distance of internode, then, when crossing over between nodes takes place at same level at a-b in figure 68, the region on each side of that point, a to A and b to B, should be protected, so to speak, from further crossing over. This in fact we have found to be the case. No other explanation so far proposed will account for this extraordinary relation.

[Ill.u.s.tration: FIG. 68. Scheme to indicate that when the members of a pair of chromosomes cross (at a-b) the region on each side is protected inversely to the distance from a-b.]

What advantage, may be asked, is there in obtaining numerical data of this kind? It is this:--whenever a new character appears we need only determine in which of the four groups it lies and its distance from two members within that group. With this information we can predict with a high degree of probability what results it will give with any other member of any group. Thus we can do on paper what would require many months of labor by making the actual experiment. In a word we can predict what will happen in a situation where prediction is impossible without this numerical information.