Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 36
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There is no mention ofthe Y chromosome because in the grasshopper she studied, the females have the sex chromosomeconstitution XX, whereas the males have the sex chromosome constitution X. In the males she examined,therefore, the X chromosome did not have a pairing partner. The instrument referred to as a cameralucida was at that time in widespread use for studying chromosomes and other microscopic objects. It isan optical instrument containing a prism or an arrangement of mirrors that, when mounted on amicroscope, reflects an image of the microscopic object onto a piece of paper where it may be traced.The aim of this paper is to describe the behavior of an unequal bivalent in the primary spermatocytes ofcertain grasshoppers.
The distribution of the chromosomes of this bivalent, in relation to the Xchromosome, follows the laws of chance; and, therefore, affords direct cytological support of Mendel'slaws. This distribution is easily traced on account of a very distinct difference in size of the homologouschromosomes. Thus another link is added to the already long chain of evidence that the chromosomes aredistinct morphological individuals continuous from generation to generation, and, asAnother link is added to the already long chain of evidence that the chomosomes aredistinct morphological individuals continuous from generation to generation, and,as such, are the bearers of the hereditary qualities.such, are the bearers of the hereditary qualities. . .
. This work is based chiefly on Brachystola magna [ashort-horned grasshopper]. . . . The entire complex of chromosomes can be separated into two groups, onecontaining six small chromosomes and the other seventeen larger ones. [One of the larger ones is the Xchromosome.] Examination shows that this group of six small chromosomes is composed of five of aboutequal size and one decidedly larger. [One of the small ones is the homolog of the decidedly larger one,making this pair of chromosomes unequal in size.] . .
.In early metaphases the chromosomes appear astwelve separate individuals [the bivalents]. Side views show the X chromosome in its characteristicposition near one pole. . . . Three hundred cells were drawn under the camera lucida to determine thedistribution of the chromosomes in the asymmetrical bivalent in relation to the X chromosome. . . . In 228cells the bivalent and the X chromosome were in the same section [the cells had been embedded in waxand thinly sliced].
In 107 cells the smaller chromosome was going to the same pole as the X chromosome,and in the remaining 121 the larger chromosome occupied this position. In the other 72 cells the Xchromosome and the bivalent were in different sections, but great care was used to make sure that therewas no mistake in identifying the cell or in labeling the drawings. The smaller chromosome isaccompanying the X chromosome in 39 of the cells, and the larger in 33. As a net result, then, in the 300cells drawn, the smaller chromosome would have gone to the same nucleus as the X chromosome 146times, or in 48.7 percent of the cases; and the larger one, 154 times, or in 51.3 percent of the cases.
. . .Aconsideration of the limited number of chromosomes and the large number of characters in any animal orplant will make it evident that each chromosome must control numerous different characters. . . . Since therediscovery of Mendel's laws, increased knowledge has been constantly bringing into line facts that at firstseemed utterly incompatible with them. There is no cytological explanation of any other form ofinheritance.
. . . It seems to me probable that all inheritance is, in reality, Mendelian.Source: Journal of Morphology 24:487–5113.4—Chromosomes and HeredityShortly after the rediscovery of Mendel's paper, it became widely assumed that genes were physically located inthe chromosomes. The strongest evidence was that Mendel's principles of segregation and independent assortmentparalleled the behavior of chromosomes in meiosis. But the first undisputable proof that genes are parts ofchromosomes was obtained in experiments concerned with the pattern of transmission of the sex chromosomes,the chromosomes responsible for the determination of the separate sexes in some plantsPage 97and in almost all animals.
We will examine these results in this section.Chromosomal Determination of SexThe sex chromosomes are an exception to the rule that all chromosomes of diploid organisms are present in pairs ofmorphologically similar homologs. As early as 1891, microscopic analysis had shown that one of the chromosomesin males of some insect species does not have a homolog. This unpaired chromosome was called the Xchromosome, and it was present in all somatic cells of the males but in only half the sperm cells. The biologicalsignificance of these observations became clear when females of the same species were shown to have two Xchromosomes.In other species in which the females have two X chromosomes, the male has one X chromosome along with amorphologically different chromosome.
This different chromosome is referred to as the Y chromosome, and itpairs with the X chromosome during meiosis in males because the X and Y share a small region of homology. Thedifference in chromosomal constitution between males and females is a chromosomal mechanism for determiningsex at the time of fertilization. Whereas every egg cell contains an X chromosome, half the sperm cells contain anX chromosome and the rest contain a Y chromosome.
Fertilization of an X-bearing egg by an X-bearing spermresults in an XX zygote, which normally develops into a female; and fertilization by a Y-bearing sperm results inan XY zygote, which normally develops into a male (Figure 3.12). The result is a criss-cross pattern of inheritanceof the X chromosome in which a male receives his X chromosome from his mother and transmits it only to hisdaughters.The XX-XY type of chromosomal sex determination is found in mammals, including human beings, many insects,and other animals, as well as in some flowering plants.
The female is called the homogametic sex because onlyone type of gamete (X-bearing) is produced, and the male is called the heterogametic sex because two differenttypes of gametes (X-bearing and Y-bearing) are produced. When the union of gametes in fertilization is random, asex ratio at fertilization of 1:1 is expected because males produce equal numbers of X-bearing and Y-bearingsperm.The X and Y chromosomes together constitute the sex chromosomes; this term distinguishes them from other pairsof chromosomes, which are called autosomes. Although the sex chromosomes control the developmental switchthat determines the earliest stages of female or male development, the developmental process itself requires manygenes scattered throughout the chromosome complement, including genes on the autosomes.
The X chromosomealso contains many genes with functions unrelated to sexual differentiation, as will be seen in the next section. Inmost organisms, including human beings, the Y chromosome carries few genes other than those related to maledetermination.X-linked InheritanceThe compelling evidence that genes are in chromosomes came from the study of a Drosophila gene for white eyes,which proved to be present in the X chromosome. Recall that in Mendel's crosses, it did not matter which trait waspresent in the male parent and which in the female parent. Reciprocal crosses gave the same result.
One of theearliest exceptions to this rule was found by Thomas Hunt Morgan in 1910, in an early study of a mutant in thefruit fly Drosophila melanogaster that had white eyes. The wildtype eye color is a brick-red combination of redand brown pigments (Figure 3.13). Although white eyes can result from certain combinations of autosomal genesthat eliminate the pigments individually, the white-eye mutation that Morgan studied results in a metabolic blockthat knocks out both pigments simultaneously.Morgan's study started with a single male with white eyes that appeared in a wildtype laboratory population thathad been maintained for many generations.
In a mating of this male with wildtype females, all of the F1 progeny ofboth sexes had red eyes, which showed that the allele for white eyes is recessive. In the F2 progeny from the matingof F1 males and females, Morgan observed 2459 red-eyed females, 1011 red-eyed males, and 782 white-eyedmales. The white-eyed phenotype was somehow connected with sex because all of the white-eyed flies were males.Page 98Figure 3.12The chromosomal basis of sex determination in mammals, many insects, and other animals. (A) In females the X chromosomessegregate from each other; in males the X and Y segregate.
In both sexes, each pair of autosomes segregates as well,so a females gamete contains one X chromosome and a complete set of autosomes, whereas a male gamete carrieseither an X chromosome or a Y chromosome along with a complete set of autosomes.(B) Punnett square format showing only the sex chromosomes.
Note that each songets his X chromosome from his mother and his Y chromosome from his father.Page 99Figure 3.13Drawings of a male and a female fruit fly, Drosophila melanogaster. The photographs show the eyes ofa wildtype red-eyed male and a mutant white-eyed male.[Drawings courtesy of Carolina Biological Supply Company; photographs courtesy of E. R. Lozovskaya.]On the other hand, white eyes were not restricted to males. For example, when redeyed F1 females from the crossof wildtype; were backcrossed with their white-eyed fathers, the progeny consisted of both red× whiteeyed and white-eyed females and red-eyed and white-eyed males in approximately equal numbers.A key observation came from the mating of white-eyed females with wildtype males.
All the female progeny hadwildtype eyes, but all the male progeny had white eyes. This is the reciprocal of the original cross of wildtypewhich had given only wildtype females and wildtype males, so the reciprocal crosses gave different× whiteresults.Morgan realized that reciprocal crosses would yield different results if the allele for white eyes were present in theX chromosome.
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