X chromosome

“ 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. ”

“ McClung, Stevens and Wilson have discovered that the determination of sex is associated with the presence or absence of a particular chromosome, the X-chromosome, in the spermatozoon which fertilizes the egg. If an egg is fertilized by a sperm which lacks the X-chromosome a male is produced ”

“ In consequence there are two classes 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 sex linked character) in the fly, for the O sperm in man is equivalent to the Y sperm in the fly. ”

“ 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. ”

“ The noteworthy thing about this “chromosome x” is the fact that in every essential detail it corresponds to the accessory chromosome of the Orthoptera. It is a spermatogonial chromosome that comes over intact into the spermatocyte; it retains its form and staining power unchanged through the prophase of the spermatocyte; it divides in only one of the spermatocyte mitoses; and is a large and conspicuous element of the cell at all times. ”

“ 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 position of this lethal to the X chromosome was found as follows, by means of the cross-over value white lethal sa. Lethal-bearing females were mated to white males and the lethal-bearing daughters were again mated to white males. The white sons (894) were non-cross-overs and the red sons (256) were cross-overs. The percentage of crossing-over {65}is 22.2. A correction of 0.4 unit should be added for double crossing-over, indicating that the locus is 22.6 units from white, or at 23.7. ”

“ The males of Protenor possess thirteen chromosomes, and the qualitative division on gametogenesis results in the production of equal numbers of spermatozoa having six and seven chromosomes. The somatic number of chromosomes in the female is fourteen, and consequently all the mature ova have seven chromosomes. When a spermatozoon with seven chromosomes meets an ovum the resulting zygote has fourteen chromosomes and is a female; when a spermatozoon with six chromosomes meets an ovum the resulting zygote has thirteen chromosomes and is a male. ”

“ This mechanism resides in the so-called chromosomes of the nucleus. These exist for the most part in similar pairs in both sexes: but one pair is dissimilar in one sex. In mammals and man this sex is the male. Man possesses one chromosome less than woman. He possesses only one member of this pair of special sex-chromosomes, whereas she possesses two. All her ova are alike in possessing one, whereas half his sperms possess one, half possess none. Therefore, when the former kind of sperms fertilize an ovum, two sex-chromosomes [Pg 138] are present in the fertilized egg and a female results ”

“ 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. ”

“ 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. ”

“ At the close of the spermatogonial divisions, when the disruptive processes reduce the other chromosomes to masses of chromomeres in which chromosome identities are not apparent, the accessory chromosome, with apparently more cohesive vigor than the others, retains its general form and is at all times distinguishable. It is marked off from the others, not only by persistence of form, but also by the difference in staining reaction, this being such as is usually exhibited by chromatin when concentrated into homogeneous masses. ”

“ In some cases the female-producing spermatozoa contain one more chromosome than the male-producing ones, in others both classes have the same number of chromosomes but one of them is much smaller in the male-producing class. These initial differences in the spermatozoa lead to corresponding differences in the nuclei of the two sexes, the cells of females either containing one more chromosome than those of males or showing a greater quantity of chromatin in the greater size of one of the chromosomes. ”

“ Form, size, and number are different in different animals, but there is reason to believe that they are normally the same in all the individuals of a species. The fertilised nucleus, thus consisting of chromosomes from male and female, then divides by a complicated process known as karyokinesis, in which each chromosome splits longitudinally, one half passing to each daughter-nucleus. Throughout the whole process of embryonic and post-embryonic growth, the chromatin is gradually increasing in bulk, and being distributed by karyokinesis. ”

“ On a priori grounds there is no reason why several mutative changes might not take place in the same locus of a chromosome. If we think of a chromosome as made up of a chain of chemical particles, there may be a number of possible recombinations or rearrangements within each particle. Any change might make a difference in the end-product of the activity of the cell, and give rise to a new mutant type. It is only when one arbitrarily supposes that the only possible change in a factor is its loss that any serious difficulty arises in the interpretation of multiple allelomorphs. ”

“ In both cases the mitotic figure quickly reforms without an intervening rest stage in which the chromosomes lose their identity. There is a loosening up of the chromomeres in all the elements except the accessory chromosome, so that they have a structure and staining reaction similar to that of the first spermatocyte chromosomes just before they enter the metaphase. The dyads of the first spermatocyte telophase, and of the succeeding and greatly abbreviated second spermatocyte prophase, are quite as definite structures as are the chromosomes of the first spermatocyte prophase. ”

“ As the contraction and rod-like separation of the chromosomes serve to isnure the transmission of all hereditary units in the products of division of a nucleus, so, on the other hand, the reticular distension of each chromosome in the so-called resting nucleus may effect a separation of the carriers of hereditary units from each other and facilitate the specific activity of each of them. ”

“ This disproportion may be such that one chromosome will exceed another in the same cell by twenty or thirty times its volume. We have here, as is pointed out in another place, a strong proof concerning the individuality of the chromosomes, 203for in some species it is possible to distinguish a particular chromosome in all the spermatocytes. This is strikingly the case in Anabrus, where there is always one chromosome very much larger than any of the others. It exceeds in size even the accessory chromosome, and might be mistaken for it were it not for the difference in form. ”