The Most Serious Scientific Challenge to Darwin
came from Genetics
• Hereditary Mechanism was unknown. Why do
Offspring tend to resemble their Parents?
• Observation: When one parent is tall and the other
parent is short, the offspring are intermediate in
height. Offspring traits are a blend of the parental
traits.
• None of the offspring are as extreme in height as the
parents: Offspring are intermediate in height.
Height of the
Short Parent
<
Height of the
Offspring
<
Height of the
Tall Parent
Blending Inheritance: Offspring phenotypes, height, width, and color
Observations
are intermediate between the parental phenotypes.
Parent 1
Average
Offspring
Parent 2
The Effect of Blending on Variation in a Population
Exponential Decay of Variation in a Population
with Blending Inheritance
V(0)
Variation
V(0)/2
V(0)/4
V(0)/8
0
1
2
3
4
Generations, t
Vt = (1/2)tV0 like the geometric growth model but b < 1.
Darwin and Heritable Variation
• Darwin observed that domesticated Plants and
Animals were more variable than their wild
ancestors. Vdomestic descendant > Vwild ancestor
• Therefore, when protected from Natural
Selection in domestication, variation increases
within populations!
Two Forces Reduce Variation in a Population
Natural Selection reduces Variation.
Reproduction with Blending Inheritance reduces Variation.
Remove Natural Selection and Variation increases
Darwin Infers
There is a Third Unknown Force ADDing new Variation
at each generation and The ADDED variation
exceeds replacement of that lost by blending inheritance.
Ancestor: Not Variable
Descendant: Highly Variable
Remove Natural
Selection by feeding
and protecting
Gregor Mendel (1823-1884)
Experiments in Plant Hybridization
(1865)
by Gregor Mendel
• Experimental studies of hybridization were
popular among amateur horticulturists and
professional nurserymen; not among scientists.
•Augustinian monk who taught natural science to
high school students.
• Mendel was interested in theories of evolution. For
example, he once found an atypical, unusual variety
of an ornamental plant. He took it and planted it next
to the typical variety to "support or to illustrate
Lamarck's views concerning the influence of
environment upon plants."
• He found that the offspring of the atypical plant
retained the essential traits of the parents and did
not reflect the influence of the environment as
predicted by Lamarck: Evidence against Lamarck.
• This test stimulated his idea of heredity.
Pure-breeding Lines: The Raw Materials of Mendel’s Experiments
Line 1
x
x
x
Line 2
x
x
x
Mendel used Pure-breeding Parental Lines
to start all of his experiments.
Because they are pure breeding, the parental lines are
fixed for hereditary factors determining color.
Fixed = All individual are identical, i.e., not varying in heredity
Mendel’s Crosses: Started by crossing different pure
breeding lines.
1. Cross of Two
Pure-breeding
Parents
x
P or “Parental generation
2. Cross of Two
F1 Offspring
from P
x
F1 “First filial generation”
3. Offspring
from F1 cross
F2 “Second filial generation”
Mendel observed this pattern of P, F1, and F2 for every phenotype:
seed shape, plant height, seed color, flower position, flower color, etc..
The Law of Dominance: describes the relationship between the P
and F1 phenotypes: The phenotype of F1 individuals resembles
the phenotype of one but not the other of the pure breeding
parental lines. This parental phenotype is called the dominant
phenotype. The factor from the parental line with the dominant
phenotype is also said to be dominant to the factor from the other
parental line. The factor from the other parent, which is not
expressed in the F1 phenotype, is called recessive.
Pure-breeding
Parents
F1
x
x
Because the F1 resembles the
yellow parent, we say
Yellow is DOMINANT to Green
The Law of Dominance: The phenotype of F1
individuals resembles the phenotype of one but not the
other of the pure breeding parental lines.
Pure-breeding
Parents
F1
x
x
Note: Dominant Parent and the F1 have identical phenotypes.
The Law of Segregation:
• Describes the relationship between the P and F2
phenotypes.
• When two pure breeding parental lines are crossed
(P1 x P2) to produce an F1, and the F1 individuals are
freely crossed among themselves (F1 x F1) to produce
an F2 generation, the phenotypes of the two parent
lines segregate in the F2 in predictable ratios.
• The ratio of dominant to recessive phenotypes in the
F2 generation is 3: 1.
The Law of Segregation: When two pure breeding parental
lines are crossed (P1 x P2) to produce an F1, and the F1
individuals are freely crossed among themselves (F1 x F1) to
produce an F2 generation, the phenotypes of the two P lines
segregate in the F2 in predictable ratios: the ratio of dominant
to recessive phenotypes in the F2 generation is 3:1.
Pure-breeding
Parents
F1
F2
x
x
The F2 SEGREGATES Yellow
AND Green offspring.
The Law of Segregation
Pure-breeding
Parents
F1
F2
x
x
The F2 SEGREGATES Smooth
AND Wrinkled offspring.
Note: Dominant Parent and the F1 have IDENTICAL Phenotypes.
But since they have DIFFERENT OFFSPRING, they must have
DIFFERENT Hereditary states.
Inheritance of Two Phenotypes appears to be
Independent!
Pure-breeding
Parents with 2 traits
F1
x
x
Yellow is still Dominant
Smooth is still Dominant
Novel
Combinations
F2
and
Yellow and Green still Segregate 3:1
Smooth and Wrinkled still Segregate 3:1
The Law of Independent Assortment
Describes the relationship between the P and F2
phenotypes when the parent lines differ in TWO
TRAITS.
• When two pure breeding parental lines differing in
TWO traits are crossed (P1 x P2) to produce an
F1 and the F1 individuals are freely crossed
among themselves (F1 x F1) to produce an F2, the
P phenotypes appear
1. in novel combinations;
2. in predictable ratios, 9:3:3:1;
3. And the Law of Segregation still applies to
Each Phenotype considered separately.
The Law of Independent Assortment: When two pure breeding parental lines
differing in TWO traits are crossed (P1 x P2) to produce an F1 generation and the
F1 individuals are freely crossed among themselves (F1 x F1) to produce an F2
generation, the P phenotypes (1) appear in novel combinations, (2) in predictable
ratios, 9:3:3:1; and, (2) the Law of Segregation still applies to each P phenotype.
Pure-breeding
Parents with 2 traits
F1
F2
x
x
Mendel’s Hereditary Model:
1] Equal, biparental, particulate inheritance and
each parent has 2 factors and transmits one to each offspring.
2] The factors are constant and do not change by association, I.e.,
constancy of factors.
Pure breeding
Parentals
F1 Cross
YY x YY
YY
YY
YY
YY
YY
Yy
x Yy
Yy
Yy
yy
Mendel’s Theoretical Laws: Inferences and
Hypotheses from his Observational Laws
• Law of Particulate Inheritance:
Inheritance is equal, biparental, and particulate.
Each parent has 2 factors and transmits one
to each offspring.
• Law of Constancy of Factors: The hereditary factors
are constant and do not change by association or
by transmission.