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In keeping with the concept that people of earlier
ages were not stupid, it was long understood that most
reproduction produced offspring that somehow were a combination of
the traits of the parents. If you bred individuals
with some common trait together, you increased the chance that such a
trait would be prominent in the offspring, but it didn't always
work that way. You could make plant cuttings and pretty much
get a duplicate plant, traitwise, but pollination produced the
same sort of mixtures that animal reproduction gave. How did
it work? Were there hidden rules that could be discovered and
applied to make results more predictable?
Gregor
Mendel was fascinated with science and Nature, but in
order to continue his education beyond a certain point, he had to enter a monastery in what was
then Austria (now Brno of the Czech Republic). In a classic
example of using the materials at hand, Mendel took advantage of
the monastery's production of peas: for years he bred peas
in special separate plots, focusing on a few one-or-the-other features
(such as Tall / Short, or Wrinkly Pea / Smooth Pea) that he could
isolate in a "pure" form. After hundreds of tests,
with new plots of hybrid plants and plots of hybrid-hybrid
crosses, Mendel developed the first basic rules of what would come
to be called genetics.
Mendel decided that traits were determined by
some sort of internal codes which he called genes.
For any particular trait, there might be code variations
that changed the nature, like leaf color, or degree of the trait, such as
one
variation in the Height Gene for Tall and one for Short.
These variations he called alleles, which are
carried in each individual in pairs (although there may be way
more than two allele variations in a population, each individual
just gets two), of which only one is passed on to offspring from each
parent.
For each trait that is determined by a single gene, an individual's
particular version of that trait is a product
of their two alleles. Mendel thought
that every single allele pair was sorted separately for
reproduction, which turned out later to not be true, but many of
them do sort separately.
As a side effect of Mendel's choice of pea
traits, he also discovered a genetic feature called dominance,
where the presence of a dominant allele can completely hide
the presence of a recessive allele. This either-or
condition was necessary to his working out genetic rules, but it
has since been found (and he probably realized) that many traits
don't follow such an extreme pattern. Many allele effects are not
that strong or weak, but rather blend to produce the
trait. (In human eye color, very dark alleles can be
dominant over very light alleles - brown can dominate blue - but
middle-strength alleles blend, such as when green and light brown
produce hazel). It also turns out that many traits, in fact
most traits, are a product of at least two genes working together,
two alleles per gene; these multiple-gene traits
do follow rules based upon Mendel's discoveries, but the complex
mixing of many alleles makes predictions of offspring traits a
matter of probabilities. With all that we know of genetics,
breeding racehorses is still a matter of mating parents with
desired traits and hoping everything mixes properly in the
offspring, which is what they were doing before Mendel was born.
Mendel wrote and published several papers on his discoveries, but no one
in the scientific community noticed. When he died, as far as he knew
his research and findings would make no difference to the world.
However, in the very early 1900s, three scientists investigating the same
sorts of processes discovered Mendel's papers, and his rules became the foundation of a
basic building block of biology today.
We know quite a bit more about genes today. The
researchers who followed up on Mendel's work through the 20th
century discovered some additional details:
Groups of genes are linked together.
It turns out that our cells do not carry thousands of separate
codes floating about; genes are bound together along
the lengths of chromosomes. This is called linkage:
for example, genes that are found on sex chromosomes exhibit sex
linkage (not because they have anything to do with sex, but because
they are on "sex-determining" chromosomes). One feature of any
species is its characteristic chromosome number, usually an
even number in sexually-reproducing species because of the
pairs (humans have a chromosome number of 46, 23
pairs). There are advantages and disadvantages to both low
and high chromosome numbers: with low numbers, the
distribution of copies that must happen whenever new cells are
made is easier - there are fewer chromosomes and chromosome copies
to control; but with higher numbers, the sorting of various
allele combinations goes up, increasing potential variability in
offspring, which increases evolutionary flexibility. The numbers
vary widely, including some plants
with amazingly high numbers.
One weakness of Darwin's original theory is that
it could not explain where "new" (really, old but
dramatically changed) features came from - natural variations
couldn't really explain how snake venom evolved from saliva, for
instance. But many years later, mutations were
discovered: spontaneous changes in genetic material,
producing new alleles that usually have no altering effect on the coded
trait, or a bad effect if an effect exists, but
occasionally might produce a useful change. And a
particular type of mutation, called genetic redundancy
(which is actually one of two types of genetic redundancy), could duplicate
genes, allowing the mutational change of an existing trait (say,
snake saliva to venom through changes of the "extra"
saliva gene) while preserving the original action (not losing the
original saliva). This might also explain why more
"advanced" organisms seemed to have more genes than more
"primitive" ones.
Later, it was discovered what exactly a gene
is: a stretch of the molecule DNA that
contains a sequence that can be used to produce particular proteins.
Cells then use those proteins to create the "traits"
that Mendel connected to genes. His height gene, for
instance, codes for a type of plant growth hormone. Alleles
produce proteins that have molecular
differences, sometimes slight, sometimes major, often changing how things
work, and how the trait looks.
Since Mendel, it has been discovered that in
some circumstances having two different alleles can be better than
a pair of either variant, a condition called hybrid vigor.
This often is the reason that human genetic diseases have spread
through a population. For example, the gene for the oxygen
carrier hemoglobin has a allele that makes a nonround version of
the protein; if you get a pair of these alleles, it gives
you sickle-cell anemia, a dangerous disease that impairs
circulation. In areas where malaria, a disease caused by a
blood parasite, is common, people carrying one regular hemoglobin
allele and one sickle-cell allele have more resistance to malaria
than those with two regular alleles, giving them an advantage that
passes sickle-cell alleles to their offspring. Other genetic
diseases may have similar effects: cystic fibrosis alleles
might make single-carriers resistant to diarrhetic diseases (which
kill huge numbers across the world), Tay-Sachs seems to produce a
tuberculosis resistance, and schizophrenia (which is a
multiple-gene trait and harder to sort out) may in some
combinations increase creativity and the willingness to take
risks.
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