Transcript hybrid generation

Gregor Mendel: Musings of a Czech Monk
Darwin’s Dilemma: Remember that Darwin’s 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 welladapted” individuals and give rise to new species
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 don’t new traits
just get diluted out ?)
Darwin never figured out this problem !
The ultimate irony
Among several books on
Darwin’s bookshelf were two
books, one by H. Hoffmann,
and another by W.O. Focke
that referred to the work of
Gregor Mendel,
It was Mendel’s experiments
on pea plants that provided
the answers to Darwin’s
dilemma
Apparently, Darwin never had
a chance to read Mendel’s
work before his death (D’oh !)
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)
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
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.
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.
So…round + wrinkled = round
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.
So…round + round = 75 % round + 25 % wrinkled
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
Explanation
1.In the organism there is a pair of factors that controls the appearance
of a given characteristic. (We call them genes.)
2.The organism inherits these factors from its parents, one from each.
3.Each is transmitted from generation to generation as a discrete,
unchanging unit.
4.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.
1.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).
First Cross
R = round allele (dominant – always expressed)
R= wrinkled allele (recessive – hidden when combined with dominant)
r
(wrinkled)
r
(wrinkled)
R
(round)
Rr
(round)
Rr
(round)
R
(round)
Rr
(round)
Rr
(round)
So we have a genotype (organism’s characteristics in genetic code)
and a phenotype (physical expression of genotype)
Second Cross
R
(wrinkled)
r
(wrinkled)
R
(round)
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
Third Cross
r
r
R
r
R
r
R
R
r
r
R
r
R
r
R
R
rr
rr
Rr
rr
Rr
rr
Rr
Rr
rr
rr
Rr
rr
Rr
rr
Rr
Rr
Rr
Rr
RR
Rr
RR
Rr
RR
RR
rr
rr
Rr
rr
Rr
rr
Rr
Rr
Rr
Rr
RR
Rr
RR
Rr
RR
RR
rr
rr
Rr
rr
Rr
rr
Rr
Rr
Rr
Rr
RR
Rr
RR
Rr
RR
RR
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 1:2:1 for RR:Rr:rr
But ratio of 1:3 for round: wrinkled
Mendel’s “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
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 ?
End of Lecture