Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 37
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The reason is that the X chromosome is transmitted in a different pattern by males and females.Figure 3.12B shows that a male transmits his X chromosome only to his daughters, whereas a female transmits oneof her X chromosomes to the offspring of both sexes. Figure 3.14 shows the normal chromosome complement ofDrosophila melanogaster. Females have an XX chromosome complement; the males are XY, and the Ychromosome does not contain a counterpart of the white gene.
A gene on the X chromosome is said to be X-linked.Figure 3.15 illustrates the chromosomal interpretation of the reciprocal crosses wildtype Sbi:male × whiteSbi:female (Cross A) and white Sbi:male × wildtype Sbi:female (Cross B). The symbols w and w+ denote themutant and wildtype forms of the white gene present in the X chromosome. The genotype of a white-eyed male iswY, and that of a wildtype male is w+Y. Because the w allele is recessive, white-eyed females are of genotype wwand wildtype females are eitherFigure 3.14The diploid chromosome complements ofa male and a female Drosophila melanogaster.The centromere of the X chromosomeis nearly terminal, but that of the Ychromosome divides the chromosomeinto two unequal arms.
The largeautosomes (chromosomes 2 and 3,shown in blue and green) are not easilydistinguishable in these types ofcells. The tiny autosome (chromosome4, shown in yellow) appears as a dot.Page 100Figure 3.15A chromosomal interpretation of the results obtained in F1 and F2 progenies in crosses of Drosophila. Cross A is a mating of awildtype (red-eyed) female with a white-eyed male. Cross B is the reciprocal mating of a white-eyed female witha red-eyed male.
In the X chromosome, the wildtype w+ allele is shown in red and the mutant w allele inwhite. The Y chromosome does not carry either allele of the w gene.heterozygous w+w or homozygous w+w+. The diagrams in Figure 3.15 account for the different phenotypic ratiosobserved in the F1 and F2 progeny from the crosses. Many other genes were later found in Drosophila that alsofollow the X-linked pattern of inheritance.The characteristics of X-linked inheritance can be summarized as follows:1. Reciprocal crosses resulting in different phenotypic ratios in the sexes often indicate X linkage; in the case ofwhite eyes in Drosophila, the cross of a red-eyed female with a white-eyed male yields all red-eyed progeny(Figure 3.15, Cross A), whereas the cross of a white-eyed female with a red-eyed male yields red-eyed femaleprogeny and white-eyed male progeny (Figure 3.15, Cross B).2.
Heterozygous females transmit each X-linked allele to approximately half their daughters and half their sons thisis illustrated in the F2 generation of Cross B in Figure 3.15.3. Males that inherit an X-linked recessive allele exhibit the recessive trait because the Y chromosome does notcontain aPage 101Connection The White-Eyed MaleThomas Hunt Morgan 1910Columbia University,New York, New YorkSex Limited Inheritance in DrosophilaMorgan's genetic analysis of the white-eye mutation marks the beginning of Drosophila genetics. It is inthe nature of science that as knowledge increases, the terms used to describe things change also. Thispaper affords an example, because the term sex limited inheritance is used today to mean somethingcompletely different from Morgan's usage. What Morgan was referring to is now called X-linkedinheritance or sex-linked inheritance.
To avoid confusion, we have taken the liberty of substituting themodern equivalent wherever appropriate. Morgan was also unaware that Drosophila males had a Ychromosome. He thought that females were XX and males X, as in grasshoppers (see the Carotherspaper). We have also supplied the missing Y chromosome. On the other hand, Morgan's gene symbolshave been retained as in the original. He uses R for the wildtype allele for red eyes and W for therecessive allele for white eyes. This is a curious departure from the convention, already introduced byMendel, that dominant and recessive alleles should be represented by the same symbol. Today we use wfor the recessive allele and w+ for the dominant allele.In a pedigree culture of Drosophila which had been running for nearly a year through a considerablenumber of generations, a male appeared with white eyes.
The normal flies have brilliant red eyes. Thewhite-eyed male, bred to his red-eyed sisters, produced 1,237 red-eyed offspring . . .. The F1 hybrids,inbred, produced2,4591,011782red-eyed femalesred-eyed maleswhite-eyed malesNo white-eyed females appeared.No white-eyed females appeared. The new character showed itself to be sex-linked in the sense that it wastransmitted only to the grandsons. But that the character is not incompatible with femaleness is shown bythe following experiment.
The white-eyed male (mutant) was later crossed with some of his daughters(F1), and produced1291328886red-eyed femalesred-eyed maleswhite-eyed femaleswhite-eyed malesThe results show that the new character, white eyes, can be carried over to the females by a suitable cross,and is in consequence in this sense not limited to one sex.
It will be noted that the four classes ofindividuals occur in approximately equal numbers (25 percent) . . .. The results just described can beaccounted for by the following hypothesis. Assume that all of the spermatozoa of the white-eyed malecarry the "factor" for white eyes "W"; that half of the spermatozoa carry a sex factor "X,'' the other halflack it, i.
e., the male is heterozygous for sex. [The male is actually XY.] Thus, the symbol for the male is"WXY", and for his two kinds of spermatozoa WX—Y. Assume that all of the eggs of the red-eyedfemale carry the red-eyed "factor" R; and that all of the eggs (after meiosis) carry one X each, the symbolfor the red-eyed female will be therefore RRXX and that for her eggs will be RX. . . . The hypothesis justutilized to explain these results first obtained can be tested in several ways.
[There follow four types ofcrosses, each yielding the expected result.] . . . In order to obtain these results it is necessary to assumethat, when the two classes of spermatozoa are formed in the RXY male, R and X go together. . . . The factis that this R and X are combined and have never existed apart.Source: Science 32: 120–122wildtype counterpart of the gene. Affected males transmit the recessive allele to all of their daughters but none oftheir sons; this principle is illustrated in the F1 generation of Cross A in Figure 3.15.
Any male that is not affectedcarries the wildtype allele in his X chromosome.An example of a human trait with an X-linked pattern of inheritance is hemophilia A, a severe disorder of bloodclotting determined by a recessive allele. Affected persons lack a blood-clotting protein called factor VIII neededfor normal clotting, and they suffer excessive, often life-threatening bleeding after injury. A famous pedigree ofhemophilia starts with Queen Victoria of England (Figure 3.16).
One of her sons, Leopold, was hemophilic, andtwo of her daughters were heterozygous carriers of the gene. Two of Victoria's granddaughters were also carriers,and by marriage they introduced the gene into the royal families of Russia and Spain. The heir to the Russianthrone of the Romanoffs, Tsarevich Alexis, was afflicted with the condition. He inherited the gene from his mother,the Tsarina Alexandra, one of Victoria's granddaughters.
The Tsar, the Tsarina, Alexis, and his four sisters were allexecuted by the Bolsheviks in the 1918 Russian revolution. Ironically, the present royal family of England isdescended from a normal son of Victoria and is free of the disease.Page 102Figure 3.16Genetic transmission of hemophilia A among thedescendants of Queen Victoria of England,including her granddaughter, Tsarina Alexandraof Russia, and Alexandra's five children.Thephotograph is that of Tsar Nicholas II, TsarinaAlexandra, and the Tsarevich Alexis, who wasafflicted with hemophilia.[Source: Culver Pictures.]Page 103In some organisms, the homogametic and heterogametic sexes are reversed; that is, the males are XX and thefemales are XY. This type of sex determination is found in birds, in some reptiles and fish, and in moths andbutterflies. The reversal of XX and XY in the sexes results in an opposite pattern of nonreciprocal inheritance of Xlinked genes.
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