Hartl, Jones - Genetics. Principlers and analysis - 1998 (522927), страница 88
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Thus, as with the paracentric inversion, the products of recombination are not recovered, but for adifferent reason. Among the chromatids not participating in the crossing-over in Figure 7.27A, one carries thepericentric inversion, and the other has the normal sequence.The looping of one chromosome in an inversion heterozygote creates a rather complex structure, but it can bedepicted in simplified form as in Figure 7.28. In this diagram, only two of the four chromatids are shown (the twothat participate in the crossing-over), and the location of the centromere is not indicated. This kind of diagram isconvenient because it shows that the essential result of the crossing-over is the production of the duplication anddeletion products.
These will also be dicentric or acentric if the centromere is located outside of the inverted region(a paracentric inversion).Reciprocal TranslocationsA chromosomal aberration resulting from the interchange of parts between nonhomologous chromosomes is calleda translocation. In Figure 7.29, organism A is homozygous for two pairs of structurally normal chromosomes.Organism B contains one structurally normal pair of chromosomes and another pair of chromosomes that haveundergone an interchange of terminal parts.
The organism is said to be heterozygous for the translocation. Thetranslocation is properly called a reciprocal translocation because it consists of two reciprocally interchangedparts. As indicated in Figure 7.29C. an organism can also be homozygous for a translocation if both pairs ofhomologous chromosomes undergo an interchange of parts.An organism that is heterozygous for a reciprocal translocation usually produces only about half as many offspringas normal—a condition that is called semisterility. The reason for the semisterility is difficulty in chromosomesegregation in meiosis.
When meiosis takes place in a translocation heterozygote, the normal and translocatedchromosomes must undergo synapsis, as shown in Figure 7.30. Ordinarily, there would also be chiasmata betweennonsister chromatids in the arms of the homologous chromosomes, but these are not shown, as though thetranslocation were present in an organism with no crossing-over, such as a male Drosophila. Segregation from thisconfiguration can take place in any of three ways. In the list that follows, the symbolmeans thatat the first meiotic anaphase, the chromosomes in Figure 7.30 labeled 1 and 2 go to one pole and those labeled 3and 4 go to the opposite pole. The redPage 289Figure 7.27(A) Synapsis between homologous chromosomes, one ofwhich carries a pericentric inversion. A crossing-over within theinversion loop is shown. (B) Anaphase I configuration resultingfrom the crossover.
One of the crossover products is duplicated fora and deficient for d; the other is duplicated for d and deficientfor a. Among the two chromatids not involved in thecrossover, one carries the inversion and the other is normal.numbers indicate the two parts of the reciprocal translocation. The three types of segregation are•This mode is called adjacent-1 segregation. Homologous centromeres go to opposite poles, buteach normal chromosome goes with one part of the reciprocal translocation.
All gametes formed from adjacent-1segregation have a large duplication and deficiency for the distal part of the translocated chromosomes. (The distalpart of a chromosome is the part farthest from the centromere.) The pair of gametes originating from the 1 + 2 poleare duplicated for the distal part of the blue chromosome and deficient for the distal part of the red chromosome;the pair of gametes from the 3 + 4 pole have the reciprocal deficiency and duplication.Page 290Figure 7.28Simplified diagram of an inversion loop showing the consequenceof crossing-over within the loop. Only the two chromatids participatingin the crossover are shown, and the location of the centromeresis not indicated.•This mode is adjacent-2 segregation, in which homologous centromeres go to the same pole atanaphase I.
In this case, all gametes have a large duplication and deficiency of the proximal part of the translocatedchromosome. (The proximal part of a chromosome is the part closest to the centromere.) The pair of gametes fromthe 1 + 3 pole have a duplication of the proximal part of the red chromosome and a deficiency of the proximal partof the blue chromosome; the pair of gametes from the 2 + 4 pole have the reciprocal deficiency and duplication.Figure 7.29(A) Two pairs of nonhomologous chromosomes in a diploid organism. (B) Heterozygous reciprocaltranslocation, in which two nonhomologous chromosomes (the two at the top) have interchangedterminal segments.
(C) Homozygous reciprocal translocation.Page 291Figure 7.30Quadrivalent formed in the synapsis of a heterozygous reciprocal translocation. The translocated chromosomes arenumbered in red, their normal homologs in black. No chiasmata are shown. (A) Adjacent-1 segregation, in whichhomologous centromeres separate at anaphase I; all of the resulting gametes have a duplication of one terminal segment and adeficiency of the other. (B) Adjacent-2 segregation, in which homologous centromeres go together at anaphase I;all of the resulting gametes have a duplication of one proximal segment and a deficiency of the other. (C) Alternatesegregation, in which half of the gametes receive both parts of the reciprocal translocation and the other half receive bothnormal chromosomes.Page 292•In this type of segregation, which is called alternate segregation, the gametes are all balanced(euploid), which means that none has a duplication or deficiency.
The gametes from the 1 + 4 pole have both partsof the reciprocal translocation; those from the 2 + 3 pole have both normal chromosomes.The semisterility of genotypes that are heterozygous for a reciprocal translocation results from lethality due to theduplication and deficiency gametes produced by adjacent-1 and adjacent-2 segregation. The frequency with whichthese types of segregation take place is strongly influenced by the position of the translocation breakpoints, by thenumber and distribution of chiasmata in the interstitial region between the centromere and each breakpoint, and bywhether the quadrivalent tends to open out into a ring-shaped structure on the metaphase plate.
A ring often formsif the breakpoints are in the middle region of the arms and there is no crossing-over in the interstitial regions; inthis case, the frequencies of adjacent-1: adjacent-2: alternate segregation are approximately 1: 1: 2. If there iscrossing-over in the interstitial regions, or if one breakpoint is near a telomere, then the orientation of thequadrivalent at metaphase tends to discourage adjacent-2 segregation. In some cases, adjacent-2 segregation iseliminated, and the ratio of adjacent-1: alternate segregation is approximately 1: 1. Adjacent-1 segregation is quitecommon in any event, which means that semisterility is to be expected from virtually all translocationheterozygotes.Translocation semisterility is manifested in different life-history stages in plants and animals.
Plants have anelaborate gametophyte phase of the life cycle, a haploid phase in which complex metabolic and developmentalprocesses are necessary. In plants, large duplications and deficiencies are usually lethal in the gametophyte stage.Because the gametophyte produces the gametes, in higher plants the semisterility is manifested as pollen or seedlethality. In animals, by contrast, only minimal gene activity is necessary in the gametes, which function in spite ofvery large duplications and deficiencies. In animals, therefore, the semisterility is usually manifested as zygoticlethality.Certain groups of plants have reciprocal translocations present in natural populations without the semisterilityusually expected.
Among these are the evening primroses in the genus Oenothera, of which there are about 100wild species native to North America and many cultivated varieties. Oenothera escapes the translocationsemisterility because segregation is always in the alternate mode. In terms of Figure 7.30, the mode of segregationis always. In some species of Oenothera, entire sets of chromosomes are interconnected througha chain of reciprocal translocations of the type illustrated in Figure 7.31A. The chromosomes at the top left are allnormal; those at the top right are translocated in such a way that each chromosome has exchanged an arm with thenext in line. When the chromosomes in the complex translocation heterozygote undergo synapsis, the result is aring of chromosomes (Figure 7.31B). The astonishing feature of meiosis in such Oenothera heterozygotes is thatthe segregation is exclusively of the alternate type, so the only gametes formed contain either the entire set ofnormal chromosomes or the entire set of translocated chromosomes.
This is not the end of the surprises. InOenothera, one of the gametic types is inviable in the pollen and the other is inviable in the ovule, so fertilizationrestores the karyotype of the complex translocation heterozygote!In species in which translocation heterozygotes exhibit semisterility, the semisterility can be used as the phenotypeto map the breakpoint of the translocation just as though it were a normal gene. The mapping procedure can bemade clear by means of an example.
Translocation TB-10LI is a translocation with one breakpoint in the long armof chromosome 10 in maize, and it results in semisterility when heterozygous. A cross is made between atranslocation heterozygote and a genotype homozygous for both zn1 (zebra necrotic 1) and tp2 (teopod 2), andsemisterile progeny are testcrossed with zn1 tp2 homozygotes. (The phenotypes of zn1 and tp2 are shown in Figure4.8.) The parental genotype isPage 293Figure 7.31(A) Complex translocation heterozygote of the type found in somespecies of Oenothera.
The chromosomes at the top left are not rearranged.Those at the top right are connected by a chain of translocations, eachchromosome having exchanged an arm with the next chromosome inline. (B) At metaphase I in meiosis, the pairing configuration of thetranslocation heterozygote is a ring of chromosomes in which each armis paired with its proper partner; note that each chromosome consistsof two chromatids. Alternate segregation from the metaphase ring yields,after the second meiotic division, two types of gametes: thosecontaining all normal chromosomes and those containing alltranslocated chromosomes.therefore Zn1 Tp2 TB-10L1/zn1 tp2 +, where the + denotes the position of the translocation breakpoint in thehomologous chromosome.
The progeny phenotypes are as follows:nonzebrastripenonteopodsemisterile392nonzebrastripenonteopodfertilenonzebrastripeteopodsemisterile42nonzebrastripeteopodfertile73zebrastripenonteopodsemisterile83zebrastripenonteopodfertile34zebrastripeteopodsemisterile31zebrastripeteopodfertile372These data are analyzed exactly like the three-point crosses in Section 4.3.
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