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CYTOGENETICS; CHROMOSOMAL ABERRATIONS PART II: Structural Changes in Chromosomes There are 4 common types of structural aberrations; duplications, deletions (also called deficiencies), inversions, and translocations. Along with definitions and descriptions, it is important to understand the consequences of each type of aberration with respect to chromosome pairing at synapsis in meiosis (especially in a cell that is heterozygous for the aberration), on fertility, and their potential roles in evolution. Deletions; loss of chromosomal material Breaks in chromosomes can result from a variety of factors, including X-rays or stray cosmic radiation. While a single break leading to the loss of the ends of chromosomes should be most common, such 'terminal deletions' are actually rare. Unless telomeres are present to protect and preserve the ends of the chromosome, exonucleases and cell division/DNA replication will lead to the loss of broken chromosomes. Thus, most deletions that are detected are 'internal' (interstitial) deletions. If the genes of a normal chromosome fall in the order 1-7, a deletion can be represented as: Normal: 1 2 3 4 5 6 7 Deletion: 1 2 3 6 7 + 4 5 (lost) There are DNA repair mechanisms that recognize broken ends of chromsomes ('sticky ends') and attempt to make repairs. In this example the repair incorrectly attached gene 3 to gene 6 and healed genes 4 and 5 into a small circle, which will be lost during cell division since it does not have a centromere. Consequences: Missing genes generally behave as recessive 'null' alleles meaning no mRNA or protein can be made from the missing gene(s). Heterozygotes (one normal chromosome and one deleted chromosome are generally OK so long as the deletion is not too large; if it is large, it is liable to overlap with a defective gene on the normal chromosome. The result is a mutant phenotype which may be lethality. If a gene needed for gamete function is deleted, the individual may have lowered fertility.
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Homozygozity for a large deletion is often lethal, since many genes are vital to normal development. Chromosome pairing in a heterozygote can be detected by the presence of a looped out region on one partner (the normal partner in this case). Chromosome pairing in an individual homozygous for the deletion will appear normal although the chromosomes are shorter. The map distance for markers flanking the deletion will be reduced. Examples: The best known examples of deletions affecting humans are " cri du chat ", a condition where a deletion in the short arm of chromosome 5 leads to extreme mental retardation (IQ < 20), microcephaly, and a mewing-like cry, even in heterozygous individuals. It occurs in 1 in 20- 50,000 births, generally as a result of new mutations. Some cases, about 12% arise from translocations, which will be covered later. Another rather common example is a deletion on chromosome 22 that leads to a high rate of
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