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2. Identification

Cytology - Depending on the size of the structural change, duplications and deficiencies may be visible as an unpaired segment at pachytene or by pairing patterns in a segment present in triplicate.

Transmission Genetics: In plants, the frequency of transmission is variable depending on the viability of the megaspores containing the duplication or deficiency. (Remember that megaspores and pollen are haploid, so a deficiency means that some genes aren't present at all in gametes carrying the deletion.) The frequency of transmission through the pollen is very low. This is either due to the nonviability of the pollen containing the duplication or deficiency or the failure to compete with the normal pollen in fertilization. 

This is another example of the value of the haploid gametophyte generation in flowering plants. In haploids, deletion of critical genes can greatly reduce viability of gametes. In animals this is of little consequence because little gene expression occurs in gametes, particularly sperm.

In maize, Dp-Df kernels borne on Dp-Df plants often make up less than 1/3 of the kernels as the Dp-Df gametes are transmitted almost exclusively through the female- egg cell.

Many chromosomal changes can be detected using FISH.

As discussed previously, interphase chromosomes tend to remain in discrete "chromosome territories", as opposed to being dispersed throughtout the entire nucleus. Use of two or more locus-specific probes can detect deletions, duplications, inversion or translocations.

Lengauer C, Kinzler KW and Vogelstein B (1998)  Nature 396: 643-649.
c) Loss of chromosomes 3 (red arrows) and chrom. 12 (yellow arrows) in colorectal cancer cells.  Interphase chromosomes were hybridized with centromeric probes for chrom. 3 (red spots) and chrom. 12 (yellow spots). Normal diploid nuclei contain two red and two yellow spots each.

d) Chromosome translocation. Metaphase plate from neuroblastoma cells was hybridized with chromosome-painting probes specific for chromosome 1 (red) and chromosome 17 (yellow)  revealing a t(1;17) translocation. (Note that chromosome 17 is small, compared to chrom. 1)

e) Gene amplification. N-myc oncogene probe (yellow); chromosome 1 (red). Figure shows N-myc amplification (arrow) within chromosome 1.

Nature396.643.Fig1.gif


C.  Mechanisms Behind Changes in Gross Chromosomal Structure


1. Unequal crossing over

If unequal crossing over occurs between two homologous chromosomes, one chromosome will gain a duplication and the other chromosome will contain a deletion.



One example is the Bar locus in Drosophila a duplication of 16A segment from region 16A1 to 16A6 of chromosome X which contains 5 bands.
 

Origin of Bar-double by unequal crossing over in the Bar-locus of the salivary gland X chromosome of Drosophila melanogaster (
Redrawn from Morgan et al., 1935. Figure 12. Cold Spring Harbor Press, New York.)

If this region is duplicated (Bar mutation), the facets in the eye are reduced in number (homozygous Bar average number of facets is 68) which narrows the normal round eye to a bar-shaped eye. If unequal crossing over occurs between two chromosomes with the Bar duplication, a Bar double is produced which in heterozygous condition reduces the eye facets to 45. Gene expression is stronger when the duplicated genes are in tandem than when they are on separate chromosomes (position effect).



 


Fig. 12.6. Illustration of the different sizes of compound eyes of the female Drosophila melanogaster
as caused by the varying numbers of facets. The size of the eye is influenced by the position effect. (From Kin, 1965. Redrawn from Oxford University Press, New York).

2. Alien chromosome additions

The presence of alien chromosomes in a genome increases the incidence of chromosome structural changes. This has been observed in wheat (Triticum) lines in which chromosomes from Aegilops were maintained (monosomic). The mechanism appears to be increased incidence of chromosome breakage. There is support for this in the higher number of deletions occurring in the chromosomes with the higher amount of heterochromatin. Endo (1990. Jpn. J. Genet. 65:135-152) produced numerous deletions by introducing Aegilops cylindrica chromosomes into hexaploid wheat cv. Chinese Spring. These deletions have been used for mapping genes on the wheat chromosomes. The number of deletions is greatest in the B-genome and these chromosomes are more heterochromatic than either the A or D genome chromosomes. A series of deficiencies were detected using Giemsa C-banding technique in chromosome 5B (Figure 6.2). Observations of deletions and translocations were made in almost half the progeny of a wheat line in which a  single species have been added through crossing and selection.

Chromosomal Aberrations--Structural Chromosome Changes

Figure 6.2 A series of deficiencies in chromosome 5B (normal chromosome 5B extreme left) detected by Giemsa C banding technique. The horizontal line represents the kinetochore. (From Endo, T.R. 1990. Jpn. J. Genet. 65:135-152.)

The ability of wheat to tolerate such drastic chromosomal deletions is probably due to the fact that it is hexaploid. Most genes are therefore present on three homeologous chromosome pairs.

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