Download Mol Bio CH1 Sept 13

Gregor Mendel
-Presented work on heredity in 1865
-Attempted to demonstrate the usefulness of laws of heredity for
evolution, was ignored
Mendel’s first experiment on Heredity
- Do Peas “Breed True”
Yes. Traits are inherited from parents to offspring
- Why was this necessary?
Mendel’s second experiment:
-Bread varieties with different
traits
-Carried out breeding for multiple
generations (F1, F2…)
-Found that one trait was lost
during F1 generation, but returned
in lower numbers in F2 generation
3:1 (75%:25%)
-Concluded that each trait has 2
“factors”
-Concluded that each “factor”
segregates independently
during formation of sex cells (1st
law of heredity)
QuickTime™ and a
TIFF (U ncompressed) decompressor
are needed to see this pi cture.
Dominance is not required
- Independent segregation still
explains heredity when traits
“blend”
QuickTime™ and a
TIFF (U ncompressed) decompressor
are needed to see this picture.
Mendel’s third experiment:
-Bread varieties with multiple
alternative traits
-Observed 9:3:3:1 ratio, including
new combinations of traits
QuickTime™ and a
TIFF (U ncompressed) decompressor
are needed to see this pi cture.
-Concluded that traits are
independently assorted. 2nd law of
heredity.
-(This law isn’t always correct, and
probably shouldn’t be called a law)
Mendel was ignored because the mechanism of heredity was
unknown
-Darwin’s black box was Mendel’s black box too
-Mendel’s results were replicated by 3 others in 1900, geneticists started
taking Mendel seriously
-Walter Sutton published “The chromosomes in heredity” in 1903
“Gene Linkage” occurs when two genes reside on the
same chromosome - violates the 2nd law of heredity
- Gene linkage can be broken
by “crossing over” during
meiosis
QuickTime™ and a
TIFF (U ncompressed) decompressor
are needed to see this picture.
Physical proof of crossing over
-Even before chromosomes
were firmly established as
molecules of heredity,
crossing over had been
demonstrated (1931)
QuickTime™ and a
TIFF (U ncompressed) decompressor
are needed to see this picture.
-Diagram shows the results
of no crossing over and
crossing over when
individual chromosomes
can be identified
Chromosome mapping
-Morgan and undergrads looked at “mutations” in fruit flies
QuickTime™ and a
TIFF (U ncompressed) decompressor
are needed to see this picture.
-Could use the frequency of “recombinants” (broken linkage groups) to
estimate the relative positions of genes on a chromosome
QuickTime™ and a
TIFF (U ncompressed) decompressor
are needed to see this picture.
The larger the distance, the more likely a cross over even will
occur
QuickTime™ and a
TIFF (U ncompressed) decompressor
are needed to see this picture.
-Morgan et al. helped establish that genes reside on
chromosomes.
Could map the relative distance of genes on a chromosome
Could explain all of Mendel’s results with genetics (Morgan) and
cytology (Sutton)
-Morgan was not taken seriously by classic biologists.
Where do mutations come from?
Are “fruit fly” mutations significant enough for large scale
evolution?
What are genes?
- DNA or Protein? (Chromosomes are made of both)
- Genes must be able to self replicate to allow heredity
Various observations suggested that genes are correlated to with
enzymes……..
……..Do genes control enzymes, are enzymes synthesized by
genes?
By the mid 1900’s biologist could say”
-Genes segregate independently in sex cells
-Genes reside on chromosomes
-Give a relative position to genes on chromosomes
-Could explain much of Darwin’s black box by genes on chrom., inherited
by offspring with crossing over, mutation