Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 38
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For example, some breeds of chickens have feathers with alternating transverse bands of light anddark color, resulting in a phenotype referred to as barred. The feathers are uniformly colored in the nonbarredphenotypes of other breeds. Reciprocal crosses between truebreeding barred and nonbarred types give thefollowing outcomes:These results indicate that the gene that determines barring is on the chicken X chromosome and is dominant.
Todistinguish sex determination in birds, butterflies, and moths from the usual XX-XY mechanism, in theseorganisms the sex chromosome constitution in the homogametic sex is sometimes designated WW and that in theheterogametic sex as WZ. Hence in birds, butterflies, and moths, males are chromosomally WW and females arechromosomally WZ.Nondisjunction As Proof of the Chromosome Theory of HeredityThe parallelism between the inheritance of the Drosophila white mutation and the genetic transmission of the Xchromosome supported the chromosome theory of heredity that genes are parts of chromosomes.
Otherexperiments with Drosophila provided the definitive proof.One of Morgan's students, Calvin Bridges, discovered rare exceptions to the expected pattern of inheritance incrosses with several X-linked genes. For example, when white-eyed Drosophila females were mated with red-eyedmales, most of the progeny consisted of the expected red-eyed females and white-eyed males.
However, about onein every 2000 F1 flies was an exception, either a white-eyed female or a red-eyed male. Bridges showed that theserare exceptional offspring resulted from occasional failure of the two X chromosomes in the mother to separatefrom each other during meiosis—a phenomenon called nondisjunction. The consequence of nondisjunction of theX chromosome is the formation of some eggs with two X chromosomes and others with none.
Four classes ofzygotes are expected from the fertilization of these abnormal eggs (Figure 3.17). Animals with no X chromosomeare not detected because embryos that lack an X are not viable; likewise, most progeny with three X chromosomesdie early in development. Microscopic examination of the chromosomes of the exceptional progeny from the crosswhiteshowed that the exceptional white-eyed females had two X chromosomes plus a Y× wildtypechromosome, and thePage 104Figure 3.17The results of meiotic nondisjunction of the X chromosomes in a female Drosophila.exceptional red-eyed males had a single X but were lacking a Y.
The latter, with a sex-chromosome constitutiondenoted XO, were sterile males.These and related experiments demonstrated conclusively the validity of the chromosome theory of heredity.Chromosome theory of heredity: Genes are contained in the chromosomes.Bridges's evidence for the chromosome theory was that exceptional behavior on the part of chromosomes isprecisely paralleled by exceptional inheritance of their genes.
This proof of the chromosome theory ranks amongthe most important and elegant experiments in genetics.Sex Determination in DrosophilaIn the XX-XY mechanism of sex determination, the Y chromosome is associated with the male. In someorganisms, including human beings, this association occurs because the presence of the Y chromosome triggersevents in embryonic developmentPage 105that result in the male sexual characteristics. Drosophila is unusual among organisms with an XX-XY type of sexdetermination because the Y chromosome, although associated with maleness, is not maledetermining.
This isdemonstrated by the finding, shown in Figure 3.17, that in Drosophila, XXY embryos develop intomorphologically normal, fertile females, whereas XO embryos develop into morphologically normal, but sterile,males. (The "O" is written in the formula XO to emphasize that a sex chromosome is missing.) The sterility of XOmales shows that the Y chromosome, though not necessary for male development, is essential for male fertility; infact, the Drosophila Y chromosome contains six genes required for the formation of normal sperm.The genetic determination of sex in Drosophila depends on the number of X chromosomes present in an individualfly compared with the number of sets of autosomes.
In Drosophila, a haploid set of autosomes consists of one copyeach of chromosomes 2, 3, and 4 (the autosomes). Normal diploid flies have two haploid sets of autosomes (ahomologous pair each of chromosomes 2, 3, and 4) plus either two X chromosomes (in a female) or one X and oneY chromosome (in a male). We will use A to represent a complete haploid complement of autosomes; henceIn these terms, a normal male has the chromosomal complement XYAA, and the ratio of X chromosomes to sets ofautosomes (the X/A ratio) equals 1 X : 2 A, or 1 : 2. Normal females have the chromosomal complement XXAA,and in this sex the X/A ratio is 2 X : 2 A, or 1 : 1.
Flies with X/A ratios smaller than 1 : 2 (for example, XAAA—one X chromosome and three sets of autosomes) are male; those with X/A ratios greater than 1 : 1 (for example,XXXAA—three X chromosomes and two sets of autosomes) are female. Intermediate X/A ratios such as 2 : 3 (forexample, XXAAA—two X chromosomes and three sets of autosomes) develop as intersexes with somecharacteristics of each sex.Sexual differentiation in Drosophila is controlled by a gene called Sex-lethal (Sxl).
The Sxl gene codes for twosomewhat different proteins, depending on whether a male-specific coding region is included in the messengerRNA. Furthermore, the amount of Sxl protein present in the early embryo regulates the expression of the Sxl geneby a feedback mechanism. At low levels of Sxl protein, the male-specific form of the protein is made and shuts offfurther expression of the gene. At higher levels of the Sxl protein, the female-specific form of the protein is madeand the gene continues to be expressed. In some unknown manner, the products of certain genes are sensitive to theX/A ratio and determine the amount of Sxl protein available to regulate the Sxl gene. The genes for sensing thenumber of X chromosomes are called numerator genes because they determine the "numerator" of the ratio X/A,and the genes for sensing the number of sets of autosomes are known as denominator genes because theydetermine the "denominator" in the ratio X/A.
In normal males (X/A = 1 : 2), there is too little Sxl protein and theSxl gene shuts down; in the absence of Sxl expression, sexual differentiation follows the male pathway, which isthe "default" pathway. In normal females (X/A = 1 : 1), there is enough Sxl protein that the Sxl gene continues to beexpressed. Continued expression of the Sxl gene initiates a cascade of genetic events, each gene in the cascadecontrolling one or more other genes downstream, and results in the expression of female-specific gene products andthe repression of male-specific gene products.
In intermediate situations when the X/A ratio is between 1 : 2 and 1 :1, some genes specific to each sex are expressed, and the resulting sexual phenotype is ambiguous—an intersex.The Sxl protein is an RNA-binding protein that determines the type of mRNA produced by some of thesexdetermining genes. An outline of the genetic control of sex determination in Drosophila is shown in Figure3.18.Page 106Figure 3.18Early steps in the genetic control of sex determination in Drosophila through the activity of theSexlethal protein and ultimately through the "numerator" and "denominator" genes that signal theratio of X chromosomes to sets of autosomes (the X/A ratio).3.5—Probability in Prediction and Analysis of Genetic DataGenetic transmission includes a large component of chance. A particular gamete from an Aa organism might ormight not include the A allele, depending on chance. A particular gamete from an Aa Bb organism might or mightnot include both the A and B alleles, depending on the chance orientation of the chromosomes on the metaphase Iplate.
Genetic ratios result not only from the chance assortment of genes into gametes, but also from the chancecombination of gametes into zygotes. Although exact predictions are not possible for any particular event it ispossible to determine the probability that a particular event might be realized, as we have seen in Chapter 2. In thissection, we consider some of the probability methods used in interpreting genetic data.Using the Binomial Distribution in GeneticsThe addition rule of probability deals with outcomes of a genetic cross that are mutually exclusive.
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