Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 87
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The comparabledefects in green perception are called deuteranopia and deuteranomaly, respectively. Isolation of the red andgreen pigment genes and study of their organization in people with normal and defective color vision haveindicated quite clearly how the "-opias" and "-omalies" differ and have also explained why the frequency of colorblindness is so relatively high.The organization of the red and green pigment genes in men with normal vision is illustrated in Figure 7.21A.Unex-pectedly, a significant proportion of normal X chromosomes contain two or three green-pigment genes.
Howthese arise by unequal crossing-over is shown in Figure 7.21B. The red-pigment and green-pigment genes pair andthe crossover takes place in the region of homology between the genes. The result is a duplication of the greenpigment gene in one chromosome and a deletion of the green-pigment gene in the other.The recombinational origins of the defects in color vision are illustrated in Figure 7.22. The top chromosome inFigure 7.22A is the result of deletion of the green-Page 285Figure 7.22Genetic basis of absent or impaired red-green colorvision.
(A) Defects in green vision result fromunequal crossing-over between mispairedred-pigment and green-pigment genes, yieldinga green-red chimeric gene. If the green-pigmentgene is missing, or if the chimeric gene islargely "red" in its sequence, thendeuteranopia is the result. If the chimeric geneis largely "green" in its sequence, then deuteranomalyis the result. (B) Defects in redvision result from unequal crossing-over betweenmispaired red-pigment and green-pigmentgenes, again yielding a green-red chimericgene. If the chimeric gene is largely ''green,"then protanopia results; if it is largely "red," thenprotanomaly results .
Note that the redgene cannot be eliminated altogether (asthe green gene can), because thered gene is at the end of the region ofhomology between the chromosomespigment gene shown earlier in Figure 7.21B. Males with such an X chromosome have deuteranopia, or "greenblindness." Other types of abnormal pigments result when crossing-over takes place within mispaired red-pigmentand green-pigment genes. Crossing-over between the genes yields a chimeric gene, which is a composite gene:part of one joined with part of the other. The chimeric gene in Figure 7.22A joins the 5' end of the green-pigmentgene with the 3' end of the red-pigment gene.
If the crossover point is toward the 5' end (toward the left in thefigure), then the resulting chimeric gene is mostly "red" in sequence, so the chromosome causes deuteranopia, or"green-blindness." However, if the crossover point is near the 3' end (toward the right in the figure), then most ofthe green-pigment gene remains intact, and the chromosome causes deuteranomaly.Chromosomes associated with defects in red vision are illustrated in Figure 7.22B. The chimeric genes are thereciprocal products of the unequal crossovers that yield defects in green vision. In this case, the chimeric geneconsists of the red-pigment gene at the 5' end and the green-pigment gene at the 3' end.
If the crossover point isnear the 5' end, most of the red-pigment gene is replaced with the green-pigment gene. The result is protanopia, or"red-Figure 7.21(A) Organization of red-pigment and green-pigmentgenes in normal X chromosomes. Somechromosomes contain one copy of thegreen-pigment gene, others two, still others three. (B)Origin of multiple green-pigment genes byunequal crossing over in the region of DNA homologybetween the genes.
Note that one product ofunequal crossing-over is a chromosome containinga red-pigment gene but no green-pigment genePage 286Figure 7.23Partial genetic map of chromosome 2 of Drosophila melanogaster. (A) Normal order. (B) Genetic mapof the same region in a chromosome that has undergone an inversion in which the orderof vg and L is reversed.blindness." The same is true of the other chromosome indicated in Figure 7.22B. However, if the crossover point isnear the 3' end, then most of the red-pigment gene remains intact, and the result is protanomaly.InversionsAnother important type of chromosome abnormality is an inversion, a segment of a chromosome in which theorder of the genes is the reverse of the normal order. An example is shown in Figure 7.23.In an organism that is heterozygous for an inversion, one chromosome is structurally normal (wildtype), and theother carries an inversion.
These chromosomes pass through mitosis without difficulty because each chromosomeduplicates and its chromatids are separated into the daughter cells without regard to the other chromosome. Thereis a problem in meiosis, however. The problem is that the chromosomes are attracted gene for gene in the processof synapsis, as shown in Figure 7.24. In an inversion heterozygote, in order for gene-for-gene pairing to take placeeverywhere along the length of the chromosome, one or the other of the chromosomes must twist into a loop in theregion in which the gene order is inverted.
InFigure 7.24In an organism that carries a chromosome that is structurally normalalong with a homologous chromosome with an inversion,the gene-for-gene attraction between the chromosomes duringsynapsis causes one of the chromosomes to form into a loop in theregion in which the gene order is inverted. In this example, thestructurally normal chromosome forms the loop.Page 287Figure 7.24, it is the structurally normal chromosome that is shown as looped, but in other cells it may be theinverted chromosome that is looped. In either case, the loop is called an inversion loop.The loop itself does not create a problem. The looping apparently takes place without difficulty and can beobserved through the microscope. As long as there is no crossing-over within the inversion, the homologouschromosomes can separate normally at anaphase I, as illustrated in Figure 7.25.
On the other hand, when there iscrossing-over within the inversion loop, the chromatids involved in the crossing-over become physically joined,and the result is the formation of chromosomes containing large duplications and deletions. The products of thecrossing-over can be deduced from Figure 7.26A by tracing along the chromatids. The outer chromatids are theones not participating in the crossover.
One of these contains the inverted sequence and the other the normalsequence, as shown in Figure 7.26B. Because of the crossover, the inner chromatids, which did participate in thecrossover, are connected. If the centromere is not included in the inversion loop, as is the case here, then the resultis a dicentric chromosome. The reciprocal product of the crossover produces an acentric chromosome.
Neither thedicentric chromosome nor the acentric chromosome can be included in a normal gamete. The acentric chromosomeis usually lost because it lacks a centromere and, in any case, has a deletion of the a region and a duplication of thed region. The dicentric chromosome is also often lost because it is held on the meiotic spindle by the chromatidbridging between the centromeres; in any case, this chromosome is deleted for the d region and duplicated for the aregion.
Hence, when there is a crossover in the inversion loop, the only chromatids that can be recovered in thegametes are the chromatids that did not participate on the crossover. One of these carries the inversion, and theother does not. It is for this reason that inversions prevent the recovery of crossover products. In the early years ofgenetics, before their identity as inversions was discovered, inversion-bearing chromosomes were known as"crossover suppressors."Figure 7.25In an inversion heterozygote, if there is no crossingover within the inversion loop, then the homologouschromosomes disjoin without problems. Two of theresulting gametes carry the inverted chromosome,and two carry the structurally normal chromosome.The inversion shown in Figure 7.26, in which the centromere is not included in the inverted region, is known as aparacentric inversion, which means inverted "beside" (para-) the centromere.
As seen in the figure, the productsof crossing-over include a dicentric and an acentric chromosome.When the inversion does include the centromere, it is called a pericentric inversion, which means "around" (peri-)the centromere. Chromatids with duplications and deficiencies are also created by crossing-over within theinversion loop of a pericentric inversion, but in this case the crossover products are monocentric. The situation isillustrated in Figure 7.27A. The diagram is identical to that in Figure 7.26 except for the position of the centromere.The products of crossing-over can again be deduced by tracing the chromatids.
In this case, both products of thecrossover are monocentric, but one chromatid carries a duplication of a and a deletion of d, and the other carries aduplication of d and a deletion of a (Figure 7.27B). AlthoughPage 288Figure 7.26(A) Synapsis between homologous chromosomes, one of whichcontains an inversion. There is a crossing-over within the inversionloop. (B) Anaphase I configuration resulting from the crossover.Because the centromere is not included in the inverted region, oneof the crossover products is a dicentric chromosome, and the reciprocalproduct is an acentric chromosome. Among the two chromatidsnot involved in the crossover, one carries the inversion and theother the normal gene sequence.either of these chromosomes could be included in a gamete, the duplication and deficiency usually result ininviability.
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