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

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Blending Inheritance: Offspring phenotypes, height, width, and color Observations are intermediate between the parental phenotypes.

Parent 1 Parent 2AverageOffspring

V(0)

Variation

Generations, t

0

V(0)/2

1 2 3

V(0)/4V(0)/8

Exponential Decay of Variation in a Population with Blending Inheritance

The Effect of Blending on Variation in a Population

4

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.

There is a Third Unknown Force ADDing new Variation at each generation and The ADDED variation

exceeds replacement of that lost by blending inheritance.

Remove Natural Selection and Variation increases

Darwin Infers

Ancestor: Not Variable Descendant: Highly Variable

Remove Natural Selection by feeding

and protecting

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.

Gregor Mendel (1823-1884)

• 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.

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

x x x

Pure-breeding Lines: The Raw Materials of Mendel’s Experiments

x x x

Line 1

Line 2

Mendel’s Crosses: Started by crossing different pure breeding lines.

1. Cross of Two Pure-breeding

Parentsx

F1 “First filial generation”

F2 “Second filial generation”

x

Mendel observed this pattern of P, F1, and F2 for every phenotype:seed shape, plant height, seed color, flower position, flower color, etc..

P or “Parental generation

2. Cross of Two F1 Offspring

from P

3. Offspring from F1 cross

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-breedingParents

x

F1

Because the F1 resembles the yellow parent, we say

Yellow is DOMINANT to Green x

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-breedingParents x

F1 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-breedingParents

x

F1

F2

The F2 SEGREGATES Yellow AND Green offspring.

x

The Law of Segregation

Pure-breedingParents

x

F1

F2

The F2 SEGREGATES Smooth AND Wrinkled offspring.

x

Note: Dominant Parent and the F1 have IDENTICAL Phenotypes.

But since they have DIFFERENT OFFSPRING, they must haveDIFFERENT Hereditary states.

Inheritance of Two Phenotypes appears to be Independent!

Pure-breedingParents with 2 traits

x

F1

F2

x

Yellow is still Dominant Smooth is still Dominant

Yellow and Green still Segregate 3:1 Smooth and Wrinkled still Segregate 3:1

Novel Combinations

and

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-breedingParents with 2 traits

x

F1

F2

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.

x x

Pure breedingParentals F1 Cross

YY YY

YYYYYYYY YY

Yy 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.