I. Gregor Mendel and his Experiments

A. Background B. Testing “Blending Inheritance” C. Mendel’s Model

Background

Austrian monk

First to determine basic rules of inheritance in eukaryotes

Published in 1860s, little attention (work rediscovered in the 1900s)

Laid foundation of genetics

Experimental Organism: Garden Pea

Advantages:

Inexpensives, easy to obtain

Identifiable traits

Easy to grow, short generation time

Easy to control pollination

Many varieties available- Mendel collected 34 strains

Developed True Breeding Lines

True Breeding: always express same phenotype after self fertilization, no exceptions

(2 years of true breeding before starting experiments)

Blending Inheritance Hypothesis:

Gametes contain sampling of fluids from parents

Fuse during reproduction

Fluids blend, offspring will have intermediate phenotype

Prevailing idea in mid- 1800s

Experimental Crosses

Mating 2 organisms to see offspring’s phenotype

P generation: Parental Generation

F1 generation: 1st Filial generation (filius, latin for son)

F2 Generation: 2nd Filial generation

Crossed true breeding plants (P generation) with contrasting traits

Blending Inheritance Hypothesis Predictions

If blending is accurate F1 phenotype should be intermediate between P phenotypes (ex. Red and white flower will make pink, next generations will not be red or white)

Mendel’s Experiments

Start with true breeding P generation

Mate P with opposite phenotypes

Cross pollinates violet and white flowers to get all violet in F1

Then cross pollinated F1 to get 3:1 ratio of violet to white in F2 generation

Observations of Mendel’s Experiments

F1 always resembled just 1 parent, not intermediate

Other phenotype absent

F2 progeny mix of both P phenotypes

Traits absent in F1 reappear in F2 generation

3:1 ratio consistent

Conclusions of Mendel’s Experiments

No intermediate phenotypes appeared

Lost phenotypes reappear

Blending of fluids cannot explain either observation

Blending hypothesis, rejected

Mendel’s particulate inheritance hypotheses

Characters determined by “heritable factors”

Each character controlled to 2 factors, 1 from each parent

Now we know: heritable factors = genes

4 components of Mendel’s Model

1. Alleles

2. 2 Factors for each characters

3. Dominance

4. 2 Principles of Heredity

Alleles

alternative versions of a gene

A A  a a

I I I I

B B b b

A= black hair

a= red hair

2 Factors for each characters

Diploid individuals inherent 2 copies o each gene

1 from each parent

May be identical (as in true breeding lines), may be different

Found on homologous chromosomes (Mendel did not know that)

Dominance

If 2 alleles differ

Dominant allele: determines phenotype

Recessive allele: has no noticeable effect on phenotype

Example: flower color- Purple (P) dominant over white (p)

Purple flower -> PP/ Pp or white flower -> pp (homozygous recessive)

2 Principles of Heredity

• Laws of Segregation

• Law of Independent Assortment

Laws of Segregation

The two alleles for a character segregate during gamete formation, each gamete only gets 1 (anaphase I)

Example: seed color

Dominant allele: yellow Y

Recessive allele: green y

Possible Genotype: YY Yy yy

Homozygous: 2 of the same allele at a locus

Heterozygous: 2 different allele at the locus

Possible gametes are y or Y

Law of Independent Assortment

Genes on different chromosomes assort independently during gamete formation

Due to random orientation of tetrads during metaphase I

Importance of Independent Assortment

• Results in genetic recombination- new combination of alleles in offspring

• This is 2nd mechanism for increasing genetic variation in sexual reproduction

• Mendel did not know mechanism behind any of this

• Independent assortment + crossing over = lots of new variation