Variation in the Number of Chromosome Segments Help
Loss of a chromosomal segment may be so small that it includes only a single gene or part of a gene. In this case, the phenotypic effects may resemble those of amutant allele at that locus. For example, the "notch" phenotype of Drosophila is a sex-linked deletion that acts like a dominant mutation; a deletion at another sex-linked locus behaves as a recessive mutation, producing yellow body color when homozygous. Deletions never back mutate to the normal condition, because a lost piece of chromosome cannot be replaced. In this way, as well as others to be explained in subsequent chapters, a deletion can be distinguished from a point mutation. A loss of any considerable portion of a chromosome is usually lethal to a diploid organism because of genetic unbalance. When an organism heterozygous for a pair of alleles, A and a, loses a small portion of the chromosome bearing the dominant allele, the recessive allele on the other chromosome will become expressed phenotypically. This is called pseudodominance, but it is a misnomer because the condition is hemizygous rather than dizygous at this locus.
A deletion heterozygote may be detected cytologically during meiotic prophase when the forces of pairing cause the normal chromosome segment to bulge away from the region in which the deletion occurs (Fig. 7.4).
Overlapping deletions have been extensively used to locate the physical position of genes in the chromosome (cytological mapping).
EXAMPLE 7.13 A laboratory stock of Drosophila females is heterozygous in coupling phase for two linked genes at the tip of the X chromosome, ac (achaete) and sc (scute). A deletion in one chromosome shows pseudodominance for both achaete and scute. In other individuals, another deletion displays pseudodominance only for achaete. Obviously, these two deletions overlap. In the giant chromosomes of Drosophila, the absence of these segments of chromosome is easily seen. The actual location of the scute gene resides in the band or bands that differentiate the two overlapping deletions.
Extra segments in a chromosome may arise in a variety of ways. Generally speaking, their presence is not as deleterious to the organism as a deficiency. It is assumed that some duplications are useful in the evolution of new genetic material. Because the old genes can continue to provide for the present requirements of the organism, the superfluous genes may be free to mutate to new forms without a loss in immediate adaptability. Genetic redundancy of this type may protect the organism from the effects of a deleterious recessive gene or from an otherwise lethal deletion. During meiotic pairing, the chromosome bearing the duplicated segment bulges away from its normal homologue to maximize the juxtaposition of homologous regions. In some cases, extra genetic material is known to cause a distinct phenotypic effect. Relocation of chromosomal material without altering its quantity may result in an altered phenotype (position effect).
EXAMPLE 7.14A reduced eye size in Drosophila, called "bar eye," is known to be associated with a duplicated region on the X chromosome. Genetically, the duplication behaves as a dominant factor. Wild-type flies arise in homozygous bar-eye cultures with a frequency of about 1 in 1600. With approximately the same frequency, a very small eye called "double-bar" is also produced. These unusual phenotypes apparently arise in a pure bar culture by improper synapsis and unequal crossing over as shown below, where the region (a-b-c-d) is a duplication.
Practice problems for these concepts can be found at:
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