CYTOGENETICS; CHROMOSOMAL ABERRATIONS
PART I: Changes in Chromosome Number
Changes in the number of sets of chromosomes per cell -EUPLOIDYEach genome has one complete set of chromosomes
Humans have 23 chromosomes per set (genome) N = 23
Corn plants have 10 chromosomes per set N= 10
Flies have 4 chromosomes per set N= 4
'Ploidy ' levels are defined by the number of sets of chromosomes per cell
Most higher organisms have 2 sets of chr0mosomes in each somatic cell and thus are said
to be "diploid". They are said to be 2N.
Gametes (eggs or sperm) have one set of chromosomes so are haploid (or monoploid) (1N)
Some cells in liver and bronchial tissues have 4 sets (4N) so are tetraploid cells. However
it is very rare to find "polyploid" animals capable of survival and reproduction.
(The term polyploid is often used any time more than 2 sets of chromosomes are found.)
Dr. Bickham in our WFS department discovered a "triploid" species of turtles in South
America that reproduces by parthenogenesis (unreduced, unfertilized eggs develop into
adults.
Human triploids occur rarely. While not grossly malformed, the vast majority are aborted
as fetuses or are stillborn. Most are thought to result from simultaneous fertilization of an
egg by two sperm. (How many chromosomes would be present in the cells of these
individuals?)
By contrast, polyploidy is very common in plants.
Many ornamental flowers are tetraploids or hexaploids; they produce gametes with 2 and 3
sets of chromosomes respectively. As long as every chromosome has a partner for meiosis,
functional gametes can form. Gametes without exact whole sets of genomes are almost
always non-functional and non-viable.
The cotton we grow in Texas (Gossypium hirsurtum) is almost all "tetraploid". It has 2
complete sets of chromosomes from an "old world" diploid cotton species and 2 sets of
chromsomes from a "new world" diploid species. Thus, each chromosme has a natural
partner, and cotton is said to be an "allotetraploid" or "amphidiploid" meaning that
although it has 4 complete sets of chromosomes it behaves like a diploid. Although the
hybridization and doubling of chromosomes originally occurred in nature, it was
reproduced by Beasley here at TAMU. (The Beasley Lab on campus is named for him.)
A chemical called colchicine prevents spindle fibers from forming so the chromosomes
do not separate in mitosis or meiosis. It is used to trap chromosomes at metaphase for
preparing human karyotypes and for doubling the chromosome composition of plants.
Common 'bread wheat' is a hexaploid. It has 6 sets with 7 chromosomes in each set Like
cotton, it is actually a 'hybrid' of 3 species that have genomes called A, B, and D. Thus
wheat is AA, BB, DD in genome composition, with 7 chromosomes in each set.
'Macaroni' wheat is AA BB.
Bananas are a very interesting example of "applied genetics" involving ploidy. The
commercial varieties -the ones we eat- are triploids created by crossing tetraploids by
diploids. The resulting plants are triploid. When meiosis occurs in a triploid, very few
gametes have only euploid (true sets) of chromosomes since some of the "odd"
chromosomes that do not have a partner migrate by chance to either pole. Since
'unbalanced' gametes are nonfunctional, the result is one of the neat features of bananas they have no seeds. The plants are reproduced asexually from cuttings, so many banana
trees can be started from a single plant.
A not so desirable result of banana breeding is that almost all of the bananas being grown at
any one time are genetically identical. Monoculture in turn means that when a new virulent
pathogen arises or is introduced, it can be devastating to fruit production.
Seedless watermelons are made the same way, but do not reproduce asexually, so triploids
must be remade every year by crossing 2Ns by 4Ns.
Seedless oranges and Thompson (seedless grapes) are not 3N; they develop from
unfertilized ovary tissues.
ANEUPLOIDY; extra or missing chromosomes; incomplete sets
The effects of extra and missing sex chromosomes have already been covered, so here we
will consider only the cases of somatic chromosomes. Just as is the case with sexchromosomes, gametes with extra or missing chromosomes arise as a consequence of nondisjunction during meiosis. In most cases, except for the smallest chromosomes, the
presence of extra or missing chromosomes in gametes and/or zygotes incompatible with
life.
Monosomy: A monosomic by definition is missing one chromosome (2N-1). In humans
the standard definition of the karyotype would be 45 XX (or XY) - 21 if one copy of
chromosome 21 is missing, etc. In humans, the only monosomics that can survive to birth
are monosomy 21 and monosomy 22, the smallest chromosomes. Both of these
monosomies lead to severe developmental and mental retardation.
Plants that are tetraploids or hexaploids can often survive when one chromosome is
missing, but there is usually an associated phenotypic abnormality. Otherwise, a missing
chromosome is almost always lethal.
Trisomy: A trisomic individual has one extra chromosome. The most common trisomy in
humans is trisomy-21, (47 XX or XY + 21) which is also called Down's syndrome for the
doctor who described the condition in 1866. (The chromosomal defect was not discovered
until 1959.)
Trisomy 21 patients have broad faces, thick tongues, stubby fingers, respiratory problems,
heart defects and a high risk of leukemia. As they age, the tangled brain amyloids
associated with Alzheimers are also prevalent. The best known characteristic is mental
retardation. Unless the individuals are mosaic (having some cells with 46 chromosomes
and others with 47) the final IQ is between 25 and 50, meaning there is little chance for self
care.
For more information see:
http://www.ncbi.nlm.nih.gov:80/entrez/dispomim.cgi?id=190685
The extra chromosome 21 can come from either the sperm of egg, but most often it is
derived from the egg. This is especially true in "older" mothers - there is a large increase in
the frequency of trisomy-21 babies associated with the age of the mother, and a relatively
low increase with paternal age. Mothers at age 25 have less than 1 trisomy-21 infant in
1200, at 35 it is 1 in 365 and at 4o, the risk is almost 1%.
Trisomy 13 (Patau syndrome) infants are very rare. They are extremely retarded, have
numerous defects including cleft palate and heart defects that limit life expectency
severely.
Trisomy 18 (Edwards syndrome) humans have a life expectancy of about 2 months and
again show extreme mental retardation.
The majority of conceptions where any type of trisomy occurs are aborted spontaneously.