The study of heredity can now be divided into three branches. Oldest among
these is the branch dealing with changes from generation to generation; these
folks are the classical geneticists who study transmission genetics. They
analyze the transmission of traits within a family pedigree or in cross-breeding
experiments on other organisms. Other researchers, who now are studying
structures and functions at the molecular level practice molecular genetics.
This is the area that has been shifted into high gear by the completion of the
Human Genome Project and spawned the new industry of genomics. The third group
deals with the distribution of genes in populations, or population genetics.
Although animal populations have been studied considerably, most of the efforts
have focused on searching the origins of human populations and tracing human
migration throughout the world. Such studies depend considerably on the sciences
of geography, ecology, archaeology, physical anthropology and linguistics.
As we noted previously, plants and animals had been bred and crossbred many
centuries before Mendel, but he selected a plant that was easy to breed and
crossbreed, planned his experiments well and kept excellent records. By keeping
his studies simple, he was able to demonstrate clearly hereditary traits which
have become known as "Mendel's laws." Since his time, the experimental plants
and animals have changed many times, being selected for their usefulness in
studying scientific problems which were of current interest. After the
importance of DNA became recognized, organisms used most commonly were those
having nuclei in each cell where the genetic material DNA is located. These
organisms are known as eukaryotes, meaning they have a true nuclei bordered by
Six organisms have been most popular research subjects (we'll not try to
impress you with all their Latin names): yeast, fruit flies, roundworms, a
mustard-like weed, mice, and humans. This by no means completes the list, as
many studies have also used orange bread mold, various protozoans, algae, corn
and chickens. And what do we have in common with these strange bedfellows? Quite
a lot! It's true, we don't fit in little bottles and we tire of a diet of
bananas, but we do have similar traits in common with our diminutive cousins
here on Planet Earth. But first please note that we refer to the genetic
composition of an organism as a "genotype" and the observable traits as
"phenotypes." And even though Mendel never used these terms, credit his work for
inspiring the creation of a new lexicon for future scientists!
Mendel's LawsMendel's first law is the principle of segregation.
In practice this means that the two members of a gene pair (known as alleles)
segregate from each other when they form a mature reproductive cell (a gamete)
which is ready and waiting to fuse with another similar cell of the opposite
sex. Result: a diploid cell called a zygote. The alleles carry alternative forms
of a gene -- in the pea seed, one would be S, which is the smooth form and the
other is the s allele, or the wrinkled form.
His second law is the principle of independent assortment. Members of
different allele pairs are transmitted independently of one another during
production of gametes.
While it is quite simple to interbreed other organisms - in fact they seem to
participate in the studies quite enthusiastically -- we dare not even think of
interbreeding humans, lest we be banished from NEHGS and cyberspace and acquire
an X-rating for their web site! The penalty for controlled crosses is
crucifixion. But Genealogists to the Rescue!
Human geneticists, in contrast to inhuman geneticists, analyze genetic traits
by pedigree analysis, documenting recurrences of a trait in family trees. Using
charts called genograms, the patterns of heredity become apparent. They are also
useful in understanding complex family relationships and environmental effects.
Later we'll examine them in more detail.
Pink-Eyed Fruit FliesWhile progress in human genetics took a turn
for the worse in the era of eugenics, Thomas Hunt Morgan and his associates were
doing some fascinating studies with Drosophila fruit flies. Breeding these
little creatures with variable eye colors, wing features and other mutations,
they were able to confirm the laws of Mendel. But they went further than Mendel,
describing small structures they called "chroma (color) soma (body)" or
chromosomes in the nuclei of their cells. The chromosomes appeared in pairs,
derived from each of the parents, and were found to be the location of the
factors of inheritance, or genes. The locations of these genes could vary, i.e.,
"crossovers" could occur. Eventually over 3,000 genes were described in
Drosophila. Science had progressed from picking peas to swatting flies. Morgan's
group made enormous contributions to the science of genetics and fruit flies are
still swarming around mashed bananas in many laboratories.