humans have acted as agents of genetic selection, by breeding offspring
with desired traits. All our domesticated animals (dogs, horses,
and food crops (wheat, corn) are the result.
most of this
time, humans had no idea how traits were inherited. Why?
parents (or don't) in bewilderingly complex ways. That is because
individuals in nature contain many genes,
and many different versions (alleles)
of each gene. Consider these three individual orchids:
1866, Gregor Mendel discovered independent assortment of
dominant and recessive expression. Traits appear in pairs; separate
in the gametes; recombine in pairs, in offspring. (Today we know Mendel
only studied unlinked traits: on separate chromosomes, or so far apart
that crossover frequency approached 50%).
happens when you breed them? Try it out -- Orchid
cross. (To cross, click on one of the orchids.)
Mendel's work was
lost. Only in the past century did humans learn the fundamental
How and why
resemble their parents; and how the inherited information functions to
make organisms look and behave as they do.
-- Walter Sutton and Theodore Boveri, using dyes synthesized
by the German organic chemistry industry, observed that "colored
in cells behaved in ways parallel to the hypothetical agents of
proposed by Mendel. These bodies were called chromosomes.
-- Nettie Stevens observed in Tenebrio beetles that
pairs of homologous chromosomes are the same size, except for one pair
which determines sex -- X, Y.
-- Thomas H. Morgan correlates the X chromosome with
inheritance of the white eye trait in
Drosophila -- a strain of
flies discovered by an undergraduate lab assistant, cleaning out old
of flies in Morgan's lab. Morgan went on to make many important
in fly genetics and linkage analysis that apply to all diploid
-- Beadle and Tatum determined in Neurospora that each gene
encodes one product (protein). (Later, we learned that RNA can be a
not always transcribed to protein; for example, a ribosomal RNA.)
-- Oswald Avery identified DNA as the genetic material.
of DNA can transfer genes into bacteria cells, and transform them
-- Rosalind Franklin and Maurice Wilkins showed
that DNA is a double helix.
Watson and Frances Crick determined the structure of the base
which enable replication producing two identical daughter helices.
-- Jacob and Monod figured out regulation of the lac operon.
-- Barbara McClintockdiscovered transposable elements in corn;
found in bacteria and animals.
-- Temin and Balitimore discovered reverse transcriptase in
an enzyme later used to clone genes based on the RNA encoding the
-- Maxam, Gilbert, Sanger, others -- developed methods to
The first transgenic mammals were made.
-- Kary Mullis invented the polymerase chain reaction (PCR),
a thermostable enzyme from a thermophilic bacterium discovered by
Brock at a geyser in Yellowstone. Mullis sold the process to a
company, and earned very little. Brock didn't earn a cent.
The first bacterial genome sequence, Haemophilus
was completely determined.
-- Ian Wilmut
cloned the lamb Dolly
from adult mammary gland tissue.
--Completion of the human genome
-- Whole organs grown in culture?
-- Chimp/human hybrids demand human rights?
-- Self-aware computers demand human