Evolution and Genetic Engineering - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 701

Zygotes produced from gametes involving duplications are often viable and may or may not
have any serious problems. For example various sorts of duplications are related to color
vision conditions, many of which are quite subtle in their effects, as are certain anemias
involving abnormal hemoglobin called the thalassemias.
Chromosomal translocations involve a relocation of a segment of chromosome. Intra-
chromosomal translocations move a segment within the same chromosome. Inter-chromo-
somal translocations move a segment of a chromosome to a non-homologous chromosome.
Chromosomal translocations happen during early phases of mitosis or meiosis. The involved
chromosomes come in close contact in a process called “crossing-over”; DNA strands of two
chromosomes break at a certain point and then relegate, causing a transfer of a DNA segment
from one chromosome to another. This process can very adversely affect the gene expression
of the transferred genes. Several genetically inheritable disorders result from these transloca-
tions. Certain types of Down syndrome involve translocations between chromosome 14 and
chromosome 21.
Chromosomal inversion describes a segment of DNA chopped from the parent molecule
and inserted after being flipped over by end to end (180 ). The effect of DNA inversions is
unpredictable and can vary from disastrous effects to no effect. These sorts of inversions
are important in reshuffling genes on a chromosome.
Transposable elements are stretches of DNA which can insert themselves into new
regions of a chromosome. Because of this ability they are often called jumping genes. The
simplest transposable element, called an insertion element, consists of a gene for an enzyme
called transposase which is required for the insertion process. This gene is flanked by special
sequences called inverted repeats which the enzyme must recognize for insertion to take
place.
More complex transposable elements have other genes and carry these genes along with
them. Such transposable elements are called transposons.
Transposons can be of benefit to the organism by providing a mechanism for insertion of
beneficial genes. For instance, the genes that confer resistance to antibiotics in bacteria are
carried by transposable elements to other nonresistant bacteria.
Transposable elements may also insert themselves into the middle of genes in which case
the gene sequence is disrupted. For instance, the allele for wrinkled seeds in peas studied by
Gregor Mendel is actually the gene for round peas into which a transposon has inserted itself.
14.1.3.2. Changes in Chromosome Number
Aneuploidy is a condition that a normally diploid individual ends up either with extra
copies of homologous chromosomes or fewer than the normal diploid number. This happens
when homologous chromosomes fail to segregate properly during meiosis. This failure of
homologous chromosomes to segregate is called nondisjunction.
There are a number of different types of aneuploidy. The most common types are mono-
somy in which the diploid individual has only one member of a certain homologous
chromosome.
The other common type of aneuploidy is called trisomy because the individual has three
copies of the chromosome. Aneuploidy leads to a number of syndromes in humans. For
example, trisomy 21 leads to Down syndrome, characterized by mental retardation and other
abnormalities.