|prev page||PLNT3140 Introductory Cytogenetics
Lecture 19, part 2 of 4
frequency of transmission is variable depending on the viability
the duplication or deficiency. (Remember that megaspores and
haploid, so a deficiency means that some genes aren't present at
gametes carrying the deletion.) The frequency of transmission
pollen is very low. This is either due to the nonviability of
the duplication or deficiency or the failure to compete with the
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
Dp-Df gametes are transmitted almost exclusively through the
3 (red arrows) and chrom. 12 (yellow arrows) in colorectal
cells. Interphase chromosomes were hybridized with
probes for chrom.
3 (red spots) and chrom. 12 (yellow spots). Normal diploid
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.
|One example is the Bar
locus in Drosophila a duplication of 16A segment
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).
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.
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
that it is hexaploid. Most genes are therefore present on three
homeologous chromosome pairs.
|Unless otherwise cited or referenced, all content on this page is licensed under the Creative Commons License Attribution Share-Alike 2.5 Canada|
|prev page||PLNT3140 Introductory
Lecture 19, part 2 of 4