Variation in the Arrangement of Chromosome Segments Help
Chromosomes occasionally undergo spontaneous rupture, or can be induced to rupture in high frequency by ionizing radiation. The broken ends of such chromosomes behave as though they were "sticky" and may rejoin into nonhomologous combinations (translocations). A reciprocal translocation involves the exchange of segments between two nonhomologous chromosomes. During meiosis, an individual that is structurally heterozygous for a reciprocal translocation (i.e., two structually normal chromosomes and two chromosomes that are attached to non-homologous pieces, as shown in Example 7.5) must form a cross-shaped configuration in order to effect pairing or synapsis of all homologous segments. In many of the following diagrams, only chromosomes (not chromatids) are shown and centromeres are omitted for the sake of simplicity.
The only way that functional gametes can be formed from a translocation heterozygote is by the alternate disjunction of chromosomes.
Translocation heterozygotes have several distinctive manifestations. (1) If an organism produces gametes with equal facility by either segregation of adjacent chromosomes (Example 7.6) or by alternate chromosomes (Example 7.7), semisterility occurs because only the latter mechanism produces functional gametes. (2) Some genes that formerly were on nonhomologous chromosomes will no longer appear to be assorting independently. (3) The phenotypic expression of a gene may be modified when it is translocated to a new position in the genome. Position effects are particularly evident when genes in euchromatin are shifted near heterochromatic regions, areas of active genes, areas devoid of active genes.
Assume that the normal order of segments within a chromosome is (1-2-3-4-5-6) and that breaks occur in regions 2-3 and 5-6, and that the broken piece is reinserted in reverse order. The inverted chromosome now has segments (1-2-5-4-3-6). One way in which inversions might arise is shown in Fig. 7.3.
An inversion heterozygote has one chromosome in the inverted order and its homologue in the normal order. During meiosis the synaptic configuration attempts to maximize the pairing between homologous regions in the two chromosomes. This is usually accomplished by a loop in one of the chromosomes. Crossing over within the inverted segment gives rise to crossover gametes that are inviable because of duplications and deficiencies. Chromatids that are not involved in crossing over will be viable. Thus, as we have seen with translocations, inversions produce semisterility and altered linkage relationships. Inversions are sometimes called "crossover suppressors." Actually they do not prevent crossovers from occurring but they do prevent the crossover products from functioning. Genes within the inverted segment are thus held together and transmitted as one large linked group. Balanced lethal systems involve either a translocation or an inversion to prevent the recovery of crossover products and thus maintain heterozygosity generation after generation. In some organisms, these "inversions" have a selective advantage under certain environmental conditions and become more prevalent in the population than the standard chromosome order. Two types of inversion heterozygotes will be considered in which crossing over occurs within the inverted segment.
- Pericentric Inversion. The centromere lies within the inverted region. First meiotic anaphase figures appear normal unless crossing over occurs within the inversion. If a single two-strand crossover occurs within the inversion, the two chromatids of each chromosome will usually have arms of unequal length (unless there are chromosome segments of equal length on opposite sides of the inversion). Half of the meiotic products in this case (resulting from crossing over) contain duplications and deficiencies and are nonfunctional. The other half of the gametes (noncrossovers) are functional; one-quarter have the normal segmental order, one-quarter have the inverted arrangement.
- Paracentric Inversion. The centromere lies outside the inverted segment. Crossing over within the inverted segment produces a dicentric chromosome (possessing two centromeres) that forms a bridge from one pole to the other during first anaphase. The bridge will rupture somewhere along its length and the resulting fragments will contain duplications and/or deficiencies. Also, an acentric fragment (without a centromere) will be formed; and since it usually fails to move to either pole, it will not be included in the meiotic products. Again, half of the products are nonfunctional, one-quarter are functional with a normal chromosome, and one-quarter are functional with an inverted chromosome.
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