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PHENOTYPIC EFFECTS OF ABNORMAL CHROMOSOME NUMBER, CONTINUED
First, a look at
SYNTENY (from the Greek syn, "same" and tene,
Simply defined, SYNTENY is the presence in two different species of
segments of DNA with the same gene sequences.
Synteny provides evidence that multiple translocations and
chromosomal rearrangements of originally similar chromosomes has been an
important feature of evolutionary change.
Example: human vs. mouse genome:
MUTATIONS/CHANGES AT THE LEVEL OF THE ENTIRE CHROMOSOME SET
One chromosome set: HAPLOIDY
Two chromosome sets: DIPLOIDY
Multiple chromosome sets: POLYPLOIDY (You can specify how many sets!)
In most animals, polyploidy is lethal (it is known
in a few reptiles, fish and invertebrates, but it's always lethal
in mammals and birds, as far as we know). Haploidy is normal in some
species, such as honeybees (drones), but it is not common.
In plants, polyploidy is an important mechanism
- ALLOPATRIC speciation can occur when a formerly
contiguous population becomes divided by a geographic or other
impassable barrier. Over time, genetic drift, mutation, natural
selection and other factors can result in the reproductive isolation
of the separated parts of the original population, creating two
- SYMPATRIC speciation occurs when a subset of a population becomes
reproductively isolated from the larger population, WITHOUT geographic separation.
In some cases, a genetic event can cause the instantaneous reproductive
isolation of part of a population. This type of sympatric speciation is
sometimes called STASIPATRIC speciation. In plants, this can occur due to polyploidy (the new polyploids
can't back cross with the original, parental population).
Three general ways this can happen:
1. Autopolyploidy (all chromosome sets from the
2. Allopolyploidy(chromosome sets from different
species) - two forms of this type.
Note: usually, a polyploid plant is much bigger,
more robust and healthy than its parental diploid stock!
Fun with Polyploidy
Some species are sufficiently closely related that their genes, when combined
in a hybrid individual, provide the necessary information for a viable
organism--but not for that organism to undergo normal meiosis.
Homo sapiens 2n - 46
Pan troglodytes 2n - 44
Hybrid between the two would be 2n = 45.
(BUT THIS HAS NOT HAPPENED, DESPITE WHAT THE WEEKLY WORLD NEWS WILL TELL
In some cases, closely related species *can* hybridize, and if their
chromosome sets differ in number, each chromosome in the hybrid is univalent
(i.e., it has no homologous "mate"), and meiosis cannot proceed normally (no synapsis; no crossing over; random
segregation of c'somes into forming gametes; inviable gametes)
Viable hybrids have been produced in the laboratory, and it's possible that
they could occur in similar fashion in natural situations. Here's an
example of an artificially produced "new species." (Well, sort of.)
Brassica oleracea (cabbage) x Raphanus sativa (radish)
Animals can also produce hybrids (e.g. horse x donkey yields a mule;
lion x tiger yields a liger or tigon), but these are almost invariably
animals have chromosomally determined sex, and polyploidy interferes with
animals have multiple biological isolating mechanisms (geographic, temporal,
behavioral etc.) which tend to prevent natural interbreeding between species.
plants retain meristematic (embryonic) tissue throughout their lives and are
self-fertile: these conditions are favorable for successful allopolyploidy.
ANEUPLOIDY VS. POLYPLOIDY: Gene Balance
Note that in almost all cases, aneuploidy is much worse for the
organism that has it than polyploidy.
Also, monosomies (haplo-abnormal) are much worse than trisomies
This is probably due to GENE BALANCE, which means that proper
development depends not only on the absolute quantity of transcripts of a
particular gene, but also on the RATIO of that gene product to the other
gene products produced in a normal cell.
Presumably, the amount of transcript from a given gene is directly
proportional to the number of copies of that gene in the cell. This
In a polyploid individual, gene products should still be present in
the same ratios as found in a normal, diploid cell.
In an aneuploid individual, however, the PROPORTIONS of gene products
will be different from those found in a normal, non-aneuploid individual.
The delicate balance and interaction between gene products, when
changed, results in mild to severe abnormalities in aneuploid
This will probably make more sense when we discuss DEVELOPMENTAL
GENETICS, since it is here that we see how relative proportions of gene
products has a profound effect on determining the direction of
Note that X-chromosome inactivation (dosage compensation) is related
to this phenomenon. The Y chromosome is believed to be a degenerate X
chromosome (far back in evolutionary history...). In a sense, it can
said that the normal condition for the X chromosome is MONOSOMIC, except
that there are about 18 genes that are operational even on the Barr Body.