Sex chromosome

“ If the accessory chromosome is a sex-determinant, then when an egg is fertilized by a sperm lacking this element, the egg itself must carry the factor that determines that the sex of the resulting organism of 35 chromosomes shall be a male. In the event of the other possible combination the egg selected must have been one carrying the female determinant and, since the accessory chromosome is a male-determinant and the union of the two produces a sex-hybrid, femaleness must dominate in order to yield a female organism of 36 chromosomes. ”

“ It is evident from these data that there must be present in the sex-chromosome a gen that causes the death of every male that receives this chromosome, and that this lethal factor lies very close to the factor for white eyes. ”

“ Diagram illustrating the inheritance of a sex-limited character such as color-blindness in man on the assumption that the factor in question is located in the sex-chromosome (from Loeb after Wilson) . The normal sex-chromosome is indicated by a black X, the one lacking the factor for color perception, by a light X. It is assumed that a normal female is mated with a color-blind male. ”

“ It has been objected that this use of the term implies a knowledge of a factor for sex in the X chromosome to which the other factors in that chromosome are linked; but in fact we have as much knowledge in regard to the occurrence of a sex factor or sex factors in the X chromosome as we have for other factors. It is true we do not know whether there is more than one sex-factor, because there is no crossing-over in the male (the heterozygous sex) , and crossing-over in the female does not influence the distribution of sex, since like parts are simply interchanged. ”

“ CROSSING-OVER.
When two or more sex-linked factors are present in a male they are always transmitted together to his daughters, as must necessarily be the case if they are carried by the unpaired X chromosome. If such a male carrying, let us say, two sex-linked factors, is mated to a wild female, his daughters will have one X chromosome containing the factors for both characters, derived from the father, and another X chromosome that contains the factors that are normal for these two factors (the normal allelomorphs) . ”

“ The result is that the gametes (sperm and eggs) they respectively produce in maturation are not exactly alike as to chromosome composition. All the eggs contain what is known as the "X" type of sex chromosome. But only half the male sperm have this type—in the other half is found one of somewhat different type, known as "Y." (This, again, is for the human species—in some animals the mechanism and arrangement is somewhat different.) If a sperm and egg both carrying the X-type of chromosome unite in fertilization, the resulting embryo is a female. If an X unites with a Y, the result is a male. ”

“ The Mendelian theory therefore is that when an ovum has two X sex-chromosomes it can only after a number of cell-divisions, at the following reduction division, give rise to ova, while an ovum containing one X sex-chromosome, or two different, XY, chromosomes, at the next reduction division gives rise to spermatozoa. The X sex-chromosome is not in itself either female or male, since, as we have seen, either ovum or spermatozoon may contain a single X chromosome. The ovum then with one X chromosome or one X and one Y changes its sex at the next reduction division and becomes male. ”

“ It is therefore evident that an egg fertilized by a spermatozoön (1) containing the small member of an unequal pair or (2) lacking one chromosome, must develop into a male, while an egg fertilized by a spermatozoön containing the larger element of an unequal pair of heterochromosomes or the odd chromosome must produce a female.
Whether these heterochromosomes are to be regarded as sex chromosomes in the sense that they both represent sex characters and determine sex, one can not decide without further evidence. ”

“ The Sex Chromosome.—The evidence centers about a special chromosome or chromosome-group commonly designated as the sex-chromosome or X-element, which has been found in various species of animals, including man. ”

“ Both the X and the Y sperm have been shown to produce the sex opposite to that which they normally produce when they fertilize eggs that are normal in every respect, except that of their X chromosome content. These facts establish experimentally that sex is determined by the combinations of the X chromosomes, and that the male and female combinations are the causes of sex differentiation and are not simply the results of maleness and femaleness already determined by some other agent. ”

“ The X-chromosome is only one factor in the determination of sex, but if it is a factor which differs in the ease of the two sexes it is a "sex-determining factor." There are many parts of a germ cell, all of which may be concerned in heredity and development, but the chromosomes appear to be the seat of the differential factors for Mendelian characters. ”

“ This crossing over of determiners from one chromosome to another takes place only among such as are actually in contact at times within the nucleus as seen under the microscope, which confines it to the members of corresponding pairs.
In man, and in many of the lower animals, sex is a hereditary character. That means that there is a determiner for it which is grouped with other determiners in one of the chromosomes. In man the determiner is for femaleness; there is no special determiner for producing the male sex ”

“ Therefore we have made use of a trick we call “marking the chromosome,” which means that we use a male whose sex chromosome carries a known gene near the tumor locus. By using this type of male in successive generations we get two types of daughters: one type like their surviving brothers in eye color that do not carry the tumor-gene and the other daughter with normal eyes that carries it. ”

“ When the female producing sperm fertilizes either kind of egg it brings in the two dominant factors GR; hence all the females are gray in body color and have red eyes. Since the male-producing sperm does not bring into the egg
Fig. 5. The two upper lines represent the two sex chromosomes of the female, showing their original composition, viz., GR and YW. The lower line shows "crossing-over" between the two pairs. The numerals to the left give the F2 males, whose composition is the same as that of the chromosome that stands next to them. ”

“ It seems, therefore, as if the accessory chromosome was a sex-determinant in some sense—probably only in the sense of a visible accompaniment of some hidden underlying physiological cause of sex—perhaps as the carrier of a specific enzyme or "hormone."
To bring these facts into line with Mendelian principles it becomes necessary to postulate (1) two kinds of eggs, just as there are two kinds of spermatozoa, (2) selective fertilization, and (3) dominance of femaleness, and the observed facts can be explained. ”

“ When the general principles arrived at are borne in mind, it must be confessed that the prospects of our ever attaining this power of control or even of prediction are not very hopeful, but the possibility of it cannot be yet regarded as entirely excluded. The general conclusions arrived at are that sex is determined by a physiological condition of the embryonic cells, that this condition is induced, at least in the absence of disturbing causes, by the presence of a particular sex-chromosome. ”

“ Extensive statistics show that in many animals including man more males are born than females, whereas according to the chromosome theory of sex determination as many female-producing sperm are formed as male-producing. It is possible to explain such departure from the 1:1 ratio of males and females in conformity with the chromosome theory if one class of spermatozoa are more active or have greater vitality than the other class, or if after fertilization one sex is more likely to live than the other. ”

“ This conclusion leaves the Y chromosome without any observed relation to sex-determination, despite the fact that the Y is normally present in every male and is confined to the male line. The question may be asked, and in fact has been asked, why may not the presence of the Y chromosome determine that a male develop and its absence that a female appear? The only answer that has yet been given, outside of the work on Drosophila, is that since in some insects there is no Y chromosome, there is no need to make such an assumption. ”

“ The presence of a female of this kind will be at once noticed by the fact that she produces no sons, or very rarely one, giving in consequence extraordinary sex-ratios. The rare appearance of a son from such a female can be accounted for in the following way: If crossing-over occurs between her X chromosomes the result will be that one X will sometimes contain two lethals, the other none. The latter, if it passes into a male, will lead to the development of a normal individual. The number of such males depends on the distance apart of the two lethals in the chromosome. ”

“ These observations show why it is impossible to influence the sex of a developing embryo by external influences. If, for example, in the human a spermatozoon without an X-chromosome enters into an egg, the egg will give rise to a boy, but if a spermatozoon with an X-chromosome gets into the egg the latter will give rise to a girl. Since always both kinds of spermatozoa are given off by the male it is a mere matter of chance whether a boy or a girl originates ”

“ As the cultures dry out the abnormal males are the first to change over to normal, then the heterozygous females, and lastly the homozygous females. It is doubtful if any far-reaching conclusion can be drawn from this series, because the first and second classes differ from each other not only in the presence of one or of two factors for abnormal, but also by the absence in the first case (male) of an entire X chromosome with its contained factors. ”

“ The heterozygous lethal-bearing female, fertilized by a normal male, will give rise to two kinds of daughters; one normal in both X's, the other with a normal X and a lethal-bearing X chromosome. The former are always normal in behavior, and the latter repeat in their descendants the 2:1 sex-ratio.
Whether a female bearing the same lethal twice (i.e., one homozygous for a given lethal) would die, can not be stated, for no such females are obtainable, because the lethal males, which alone could bring about {15}such a condition, do not exist. ”