CHROMOSOMES - GET YER GENES ON!
by
Geneva Coats,
TheDogPlace.org
Genetics Editor / Sept. 2010
 Specific
characteristics of living organisms are determined by their “GENES”. Genes
are “coding” segments made up of a substance called DNA. The DNA in your
genes is arranged in specific patterns. Different genes are strung together
in long rows to form a rope-like chain called a “chromosome”. Each
chromosome contains thousands of genes.
Chromosomes are instruction panels; they provide the
blueprint to make an organism what it is. Chromosomes carry all of the
information necessary to help living things grow, survive and reproduce.
Chromosomes are located inside the cells of the body in a central control
area called a “nucleus”. These chromosomes determine not only what you look
like, but also how your body functions and, to a large extent, how you act,
think and feel.
 The
DNA that makes up genes and chromosomes is like a computer code of
instructions. Chromosomes in the nucleus build an instruction copy of
themselves and send that instruction to other parts of the cell, the
ribosomes. The ribosomes in turn manufacture proteins according to
instructions provided. These proteins might be enzymes for body metabolism,
or proteins for building body tissues.
Chromosomes Double in Normal Cell Division
During normal cell division for growth or cell
replacement and repair, chromosomes double and then split apart to form two
cells from one. Now both of these cells will end up with identical
chromosomes within their nuclei. However, there is a special type of cell
division that happens to produce the special reproductive or “germinal”
cells. Instead of doubling, the germinal cells are produced by splitting up
the original chromosomes.
These reproductive germinal cells, the sperm and the eggs (ova), therefore
will contain only HALF the number of chromosomes as do the other cells of
the body. When a sperm cell combines with an ovum, VOILA! there is then a
complete set of genes with a full set of instructions to create a new living
being. This new creature will have half his genes originating from his
father’s sperm, and the other half will have been contributed by his
mother’s ovum.
This process involves something known as “random fertilization”. What does
that mean?
The chromosome combination contributed by a sire to his offspring is random,
and can vary considerably. Half his chromosomes will end up in that sperm
cell…but how many different combinations of chromosomes can there be in any
one sperm cell?
Astounding Canine
Chromosome Math
 Let’s
check it out. Humans have 46 chromosomes, arranged in 23 pairs, that divide
and split up to form germinal cells, and they assort independently. To form
a germinal cell, there are 2^23, or
8 million, possible different assortments of chromosomes that could be
inherited for each individual cell!! The ovum also has 8 million possible
different chromosome combinations. 8 million X 8 million = 64 trillion
possible unique combinations of chromosomes for every human offspring
created from any given mating! See how unique you are! Even your siblings
may have quite a different genetic makeup than you do!
A human cell has 46 chromosomes, arranged in 23 pairs. A canine cell,
however, has 78 chromosomes, arranged in 39 pairs. Each sire can produce
roughly 550 BILLION different assortments of chromosomes in their sperm
cells. Multiply that by the 550 billion possible combinations of chromosomes
in the dam’s ova, and there is a possible 300,000,000,000,000,000,000,000
DIFFERENT combinations of chromosomes that can be produced for any
individual dog created from any specific pairing.
WOW! That’s a lot of zeros. How do you read such a number? It is roughly the
same number as the estimate of the number of stars in the visible universe,
or 30 billion trillion. Each dog from any certain mating is as
unique in his genetic makeup as a star! That's a very nice comparison, I
think.
 But
wait! There is another factor that can further increase genetic variety in
offspring. This is the phenomenon known as “genetic crossover”. Crossover
commonly happens during cell division to produce sperm and ova. What does
“crossover” mean? Let’s see….remember we said that each chromosome has a
partner chromosome with similar genes on it. During cell division, part of
one chromosome may break off and swap material with its partner. This means
that sometimes the chromosome that you inherit is totally different from the
original one your parent has. The crossover process “shuffles the deck” so
to speak, to produce even more variety in offspring. It would be impossible
to estimate how much more variety this effect produces! But we would need
millions more universes filled with billions more stars to get close to the
number of unique combinations of chromosomes possible to achieve with any
specific mating.
Astronomical Genetics
This vastly inconceivable number implies a rich potential
to produce dogs that have a very unique and highly individualized genetic
makeup. This inherent variety in the dog genome is how man has been able to
create so many different breeds with characteristics as different as those
noted between a Chihuahua and an Irish Wolfhound. Compare the variety in
dogs to that of humans, who all look remarkably similar…even people of
different races. We have fewer chromosomes to resort and recombine, and less
chance of isolating and promoting different specific traits.
Now do you still think that one or two litters are enough to judge what your
dog can produce? Although, I am sure there are animal rights
“overpopulation” handwringers out there who believe that every intact dog
will produce billions of puppies in just seven years. Hmmm, I only wish I
could get more than three or four in a litter to select from!
And just think, each and every chromosome contains thousands of individual
genes! In the next issue, we’ll talk about how those genes combine and
recombine to work their magic!
http://www.thedogplace.org/Genetics/Chromosomes-Genes-10094_Coats.asp
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