I.B. 31, Spring, 2008
Feb. 11, 2008
Basic Mendelian Genetics Lecture
This is a “background” lecture, so if you remember your basic genetics, sit back
and relax – it will be over soon!
Through much of human history, the idea has been prevalent that behavioral traits,
like physical ones, are inherited.
Certainly the domestication of dogs 14,000 years
ago depended on this idea whether or not it was overtly expressed.
Even today, we
use terms like "a chip off of the old block", "like father, like son", and "it runs in
Tell story of Bartolommeo de Vinci, the 45 year younger half sib of
Leonardo was the son of their father, Piero, a notary, and Caterina, a
peasant girl from Vinci.
Bartolommeo, also a notary, went back to Vinci, found a
peasant girl who matched the description of Caterina, and married her.
also named Piero, was a skilled artist, but died young.
Darwin included in his Theory of Natural Selection the premise
that phenotypic traits varied and that variation had a heritable component.
recognized two kinds of variation:
Continuous Variation - small differences from individual to individual.
Sports - occasional major variants (albinos, crumpled wings, etc.)
Both were known to be inherited (from domestication studies), but Darwin
thought that "sports" were too rare to play a major role in evolution.
He accepted the prevalent view of "Blending Inheritance": traits of offspring
tended to be the average of the traits of the parents.
He recognized that
interbreeding would eliminate variation.
This is one reason that he stressed
geographical isolation in speciation.
Isolation prevented interbreeding
throughout the entire species and allowed natural selection to accumulate
small differences in local populations.
He could not explain the source of heritable variation.
"Pangenesis" was the
best he could come up with.
This was the Larmarkian idea that somatic cells
contained gemmules that were influenced by the environment and by organ
use and disuse (eyeless cave species of fish).
Gemmules then moved to sex