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Inversion chromosome Figure 8.3
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Deletion chromosome
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Two possible consequences of a double break (top arrows) in the same chromosome.
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II. Mendelism and the Chromosomal Theory
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Eight
Cytogenetics
f e a b c d g h i j
Normal chromatids Deletion chromatids
Heterochromatin
A bulge can occur in a meiotic tetrad if a large deletion has occurred.
suppressed, but rather that the products of recombination within a loop are usually lost. (Suppression can also occur in small inversions where loops don t form.) Figure 8.8 shows a crossover within a loop. The two nonsister chromatids not involved in a crossover in the loop will end up in normal gametes (carrying either the normal chromosome or the intact inverted chromosome). The products of the crossover, rather than being a simple recombination of alleles, are a dicentric and an acentric chromatid. The acentric chromatid is not incorporated into a gamete nucleus, whereas the dicentric chromatid begins a breakage-fusion-bridge cycle that creates a genetic imbalance in the gametes. The gametes thus carry chromosomes with duplications and de ciencies. The inversion pictured in gure 8.8 is a paracentric inversion, one in which the centromere is outside the inversion loop. A pericentric inversion is one in which the inverted section contains the centromere. It, too, suppresses crossovers, but for slightly different reasons
Heterochromatin Figure 8.5 An inversion in the X chromosome of Drosophila produces a variegation in eye color in a female if her other chromosome is normal and carries the white-eye allele (Xw ).
( g. 8.9). All four chromatid products of a single crossover within the loop have centromeres and are thus incorporated into the nuclei of gametes. However, the two recombinant chromatids are unbalanced they both have duplications and de ciencies. One has a duplication for
a b c a b c
h h f
Synapsis occurs
f e e abc d g h i j g
h i j
Tetrad at meiosis showing the loop characteristic of an inversion heterozygote.
Tamarin: Principles of Genetics, Seventh Edition
II. Mendelism and the Chromosomal Theory
8. Cytogenetics
The McGraw Hill Companies, 2001
Variation in Chromosomal Structure
A Drosophila heterozygous for an inversion will show a loop in the salivary gland chromosomes. (Compare with gure 8.6.)
a-b-c-d and is de cient for h-i-j, whereas the other is the reciprocal de cient for a-b-c-d and duplicated for h-i-j (in g. 8.9). These duplication-deletion gametes tend to form inviable zygotes. The result, as with the paracentric inversion, is the apparent suppression of crossing over.
Results of Inversion
Crossing over within inversion loops results in semisterility. Almost all gametes that contain dicentric or imbalanced chromosomes form inviable zygotes. Thus, a certain proportion of the progeny of inversion heterozygotes are not viable. Inversions have several evolutionary rami cations. Those alleles originally together in the noninversion chromosome and those found together within the inverf
sion loop tend to stay together because of the low rate of successful recombination within the inverted region. If several loci affect the same trait, the alleles are referred to as a supergene. Until careful genetic analysis is done, the loci in a supergene could be mistaken for a single locus; they affect the same trait and are inherited apparently as a single unit. Examples include shell color and pattern in land snails and mimicry in butter ies (see chapter 21). Supergenes can be bene cial when they involve favorable gene combinations. However, at the same time, their inversion structure prevents the formation of new complexes. Supergenes, therefore, have evolutionary advantages and disadvantages. 21 discusses these evolutionary topics in more detail. Sometimes the inversion process produces a record of the evolutionary history of a group of species. As species evolve, inversions can occur on preexisting inversions. This leads to very complex arrangements of loci. We can readily study these patterns in Diptera by noting the changed patterns of bands in salivary gland chromosomes. Since certain arrangements can only come about by a speci c sequence of inversions, it is possible to know which species evolved from which. The same series of events can occur within the same species (box 8.1). In summary then, inversions result in suppressed crossing over, semisterility, variegation position effects, and new linkage arrangements. All of these events have evolutionary consequences.
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