Variation in Chromosome Morphology Help
A translocation can change the structure of the chromosome both genetically and morphologically. The length of the chromosome may be longer or shorter, depending upon the size of the translocated piece. An inversion does not normally change the length of the chromosome, but, if the inversion includes the centromere (pericentric), the position of the centromere may be changed considerably. Deletions or duplications, if viable, may sometimes be detected cytologically by a change in the size of the chromosome (or banding pattern in the case of the giant chromosomes of Drosophila), or by the presence of "bulges" in the pairing figure. Chromosomes with unequal arm lengths may be changed to isochromosomes, having arms of equal length and genetically homologous with each other, by an abnormal transverse division of the centromere. The telocentric X chromosome of Drosophila may be changed to an "attached-X" form by a misdivision of the centromere (Fig. 7.5).
The shape of a chromosome may change at each division once it has broken. Following replication of a broken chromosome, the broken ends of the sister chromatids may be fused by DNA repair mechanisms. Such broken ends are said to be "sticky." When the chromatids move to opposite poles, a bridge is formed. The bridge will break somewhere along its length and the cycle repeats at the next division. This sequence of events is called the bridge-breakage-fusion-bridge cycle. Mosaic tissue appearing as irregular patches of an unexpected phenotype on a background of normal tissue (variegation) can be produced by such a cycle. The size of the unusual tissue generally bears an inverse relationship to the period of development at which the original break occurred; i.e., the earlier the break occurs, the larger will be the size of the abnormal tissue.
Chromosomes are not always rod-shaped. Occasionally, ring chromosomes are encountered in plants or animals. If breaks occur at each end of a chromosome, the broken ends may become joined to form a ring chromosome (Fig. 7.6).
If an acentric fragment is formed by union of the end pieces, it will soon be lost. The phenotypic consequences of these deletions vary, depending on the specific genes involved.
Robertsonian Translocation (Centric Fusion)
A whole arm fusion is called a Robertsonian translocation, named after W. R. B. Robertson, and is an eucentric, reciprocal translocation between two acrocentric chromosomes where the break in one chromosome is near the front of the centromere and the break in the other chromosome is immediately behind its centromere. The smaller chromosome thus formed consists of largely inert heterochromatic material near the centromeres; it usually carries no essential genes and tends to become lost. A Robertsonian translocation thus results in a reduction of the chromosome number (Fig. 7.7).
EXAMPLE 7.16 Humans have 46 chromosomes whereas the great apes (chimpanzees, gorillas, and orangutans) have 48. It seems likely that humans evolved from a common human/ape ancestor by (among other structural changes) centric fusion of two acrocentrics to produce a single large chromosome (2) containing the combined genetic content of the two acrocentrics. Structural rearrangements of chromosomes may lead to reproductive isolation and the formation of new species. The mule is a hybrid from crossing the horse (2n = 64) and the ass or donkey (2n = 62). The mule is sterile because there is insufficient homology between the two sets of chromosomes to pair successfully at meiosis.
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