Gregor Mendel: Musings of a Czech Monk

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Gregor Mendel Musings of a Czech Monk
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Darwins Dilemma Remember that Darwins theory
of natural selection claimed that there is a
struggle for existence and that in that struggle,
the fittest survive Natural selection therefore
implies that an organism that is best-equipped to
survive the rigors of its environment is most
likely to bear offspring and that these offspring
will pass on advantageous traits to their
offspring Implications wonderous monsters
with new, advantageous traits will eventually
replace less well-adapted individuals and give
rise to new species
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Problem How do new characters survive through
several generations ? Remember that at this
time, people believed that traits were passed on
through the blood (so why dont new traits just
get diluted out ?) Darwin never figured out this
problem !
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The ultimate irony
Among several books on Darwins bookshelf were
two books, one by H. Hoffmann, and another by
W.O. Focke that referred to the work of Gregor
Mendel, It was Mendels experiments on pea
plants that provided the answers to Darwins
dilemma Apparently, Darwin never had a chance to
read Mendels work before his death (Doh !)
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Gregor Mendel (1822-1884)
Born to peasant parents in Heizendorf, Austria
(now Hyncice in Czechoslovakia) Entered
monastery Brunn in Moravis, (now Brno in
Czechoslovakia). Attended University of Vienna
to get a teaching diploma University examiner
failed him with the comments, " he lacks insight
and the requisite clarity of knowledge".
For many years took care of a monastry
garden (apparently had lots of time on his hands)
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• From 1858 to 1866, Mendel bred garden peas in his
  monastery garden and analyzed the offspring of
  these matings.
• The garden pea was good choice of experimental
  organism because
• many varieties were available that bred true for
  clear-cut, traits like
• seed texture (round vs wrinkled)
• seed color (green vs yellow)
• flower color (white vs purple)
• tall vs dwarf growth habit

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Anatomy of a flowering plant

• Pea plants have both male and female reproductive
  organs. 
• As a result, they can either self-pollinate
  themselves or cross-pollinate with another
  plant. 
• In his experiments, Mendel was able to
  selectively cross-pollinate purebred plants
  with particular traits and observe the outcome
  over many generations. 

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One of his simple experiments crossing purebred
round and wrinkled-seeded plants
The first cross
Mendel crossed a pure-breeding round-seeded
variety with a pure-breeding wrinkled-seeded
one.
Result
All the peas produced in the second or hybrid
generation were round.
Soround wrinkled round
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Second cross
Mendel then allowed round hybrid peas to self
pollinate
Result
The wrinkled trait (which had disappeared in his
hybrid generation) reappeared in 25 of the new
crop of peas.
Soround round 75 round 25 wrinkled
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Third cross
Mendel then allowed some of each offspring from
the second-cross generation to self-pollinate
Results
1. Crossing of wrinkled seeds from second cross
produced only wrinkled seeds 2. Round seeds
produced 2/3 round seeds and 1/3 wrinkled seeds
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Explanation

• In the organism there is a pair of factors that
  controls the appearance of a given
  characteristic. (We call them genes.)
• The organism inherits these factors from its
  parents, one from each.
• Each is transmitted from generation to generation
  as a discrete, unchanging unit.
• When the gametes are formed, the factors separate
  and are distributed as units to each gamete (sex
  cell). This statement is often called Mendel's
  rule of segregation.
• If an organism has two unlike factors in a factor
  pair (we call members of such pair alleles) for a
  certain characteristic, one may be expressed to
  the total exclusion of the other (dominant vs.
  recessive).

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First Cross
R round allele (dominant always expressed) R
wrinkled allele (recessive hidden when combined
with dominant)
R (round) R (round)
r (wrinkled) Rr (round) Rr (round)
r (wrinkled) Rr (round) Rr (round)
So we have a genotype (organisms characteristics
in genetic code) and a phenotype (physical
expression of genotype)
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Second Cross
R (round) r (wrinkled)
R (wrinkled) RR (round) Rr (round)
r (wrinkled) Rr (round) rr (wrinkled)
Recessive allele is hidden when combined with
dominant allele Recessive allele only shows up
when combined with another recessive allele
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Third Cross
r r R r R r R R
r rr rr Rr rr Rr rr Rr Rr
r rr rr Rr rr Rr rr Rr Rr
R Rr Rr RR Rr RR Rr RR RR
r rr rr Rr rr Rr rr Rr Rr
R Rr Rr RR Rr RR Rr RR RR
r rr rr Rr rr Rr rr Rr Rr
R Rr Rr RR Rr RR Rr RR RR
R Rr Rr RR Rr RR Rr RR RR
Genotype 16 RR 32 Rr 16 rr Phenotype 48
round 16 wrinkled
So crossing all types, get ratio of 121 for
RRRrrr But ratio of 13 for round wrinkled
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Mendels Particulate Inheritence Significance
to Evolution
Indicated that in sexual organisms, each parent
has two factors for each trait (these split when
sex cells are made, then recombine in
offspring) Also indicated that traits are passed
on more like particles than as a fluid (i.e.
factors are discrete entities that are not
diluted). Natural selection can, however, act on
populations on organisms according to the
fitness of the phenotypes
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But
Some characteristics such as hair and eye colour
may not significantly affect the survival of
organisms such as humans (at least at the present
time- but who knows for the future) What might
the future hold for the apparently neutral
characteristics humans hold ?
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End of Lecture