- We use heredity to mean different things:
- Like for like inheritance (as when we point out which traits of a
child resemble which parent or relative)
- Running in the family (alcoholism; making money; love for travel;
piety; etc.) which is often not inherited but cultural
- Congenital (born with a trait; sometimes inherited; but also can be
damage caused by a virus or other agent during pregnancy and definitely
not inherited; and it can be cultural like inheriting royal titles or
inheriting wealth).
- Transmitted by genes (bingo! That's what we want to study)
- What did people do before Mendelism was introduced in 1900?
- They believed it was an unsolvable mystery
- They believed it was something shaped and constructed by the
environment This is called lamarckism or the theory of inheritance of
acquired characteristics. Are the children of athletes and construction
workers more muscular than those of professors and musicians? Probably
not. Training can build anyone's muscles. What might be inherited is a
better coordination or more efficient metabolism utilizing glucose and
converting it into energy. This is not a well studied trait so it's easy
to mouth off one's own biases.
- Why is it called lamarckism?
- It was developed by Jean Baptiste Lamarck, a French biologist (he
named the field "biology") who was active about 1790 to 1810. He
believed, as did almost all his contemporaries, that the environment
altered heredity and for a century or more people sought good environments
to produce healthy offspring and blamed bad environments for degenerate
offspring and blamed just about everything on bad environments.
- What discredited lamarckism?
- The work of August Weismann, a German biologist, whose publications
followed a century later (the 1880's to 1900). He proved that mutilations
did not cause hereditary change (e.g., circumcisions among Jewish males
has not shortened the foreskin at birth although Jews have done this for
more than 2500 years). Weismann cut tails off mice for six generations
and showed no change in tail length at birth.
- Weismann proposed instead a theory of the germplasm. He argued that
reproductive (germ) cells are set aside to be passed on to the next
generation and body cells (soma) die with the individual and make no
contribution to the heredity of the next generation. Thus changes in body
tissues (e.g., muscles) would not alter the germ cells of an athletic
person.
- What's in the germplasm that leads to heredity?
- Hereditary units were proposed in the 1860s. This followed the idea
of units of matter (the atomic theory of 1810) and units of life (called
cells) in the late 1830s. Of the three people who proposed such units,
Herbert Spencer, Charles Darwin, and Gregor Mendel, only Mendel's theory
of hereditary units was put to experimental test. Mendel worked out two
laws of heredity in the 1860s but his work was largely ignored for the
next 35 years as irrelevant, mystical, Pythagorean, or discredited.
- How did hereditary units win out over other theories?
- Two fields came together in 1903. One was the study of cells and
how they divide (a field called cytology). The other was a field called
breeding analysis which was invented by Mendel. A student, Walter Sutton
working at Columbia University suggested locating hereditary units (later,
in 1909, called genes) in the chromosomes that were observed in the nuclei
of cells. He showed that Mendel's laws were predicted by the events
(called meiosis) leading to sperm and egg formation. In 1906 a name was
given to this new field, William Bateson called it genetics.
- How much of our being is inherited?
- Our genes provide us with the construction of our bodies including
all organs and tissues. The enzymes, structural proteins, regulators, and
other major components of metabolism and embryology are all controlled by
genes. What is not controlled by genes are the modifications of our
organs by training or disease; and most important to us, much of the
behavior that shapes our lives (we learn a language, we are not born with
a language). We can learn a lot or a little; we can develop our
personalities or we can be largely unreflective persons. We might be able
to work out the sequence of nucleotides in Shakespeare's genes but we will
not find a single sonnet in those genes. We inherit potentials and ranges
and limits for our physical traits and our behavioral traits.