Gregory Johann Mendel

• 1822- 1884• Austrian monk• Experimented with

pea plants• He thought that

‘heritable factors’ (genes) retained their individuality generation after generation

Terms to Know and Use

• Gene - a heritable feature: i.e. flower color coded for on a chromosome

• Trait - variant for a gene: i.e. a purple flower, determined by alleles

• Dominant trait - expressed over recessive trait when both are present

• Recessive trait - not expressed when the dominant trait is present

• Allele - a variation of a gene responsible for different traits, often expressed as A or a

• Locus - location of a gene,or allele, on a chromosome

• Chromosome - strand of DNA that codes for genes

• Haploid - one copy of a chromosome

• Diploid - two copies of a chromosome

• Gamete - a spermatozoa or oocyte (egg) cell, they are haploid

• Zygote – diploid, fertilized egg (2N)

• Genotype - the type of alleles on a chromosome: genetic makeup

• Phenotype - the physical result of a genotype ex. blue eyes

• Hybrid – an organism with two different alleles

Garden Pea Experiments 1856-64

• Mendel disagreed with the “Blending Theory” of inheritance.

• Started with34 kinds peas

Pea Characteristics

Trait on the left is dominant. Trait on the right is recessive.

Dominant Recessive

Mendel Determined that Alleles Separate During Meiosis

Parents:

Assume Round is S and the other form is s.

Punnett Square used to predict

the Genotype of Offspring

S S

S Ss

Smooth

Ss

Smooth

s Ss

Smooth

ss

Smooth

So, cross purebred smooth peas with a plant having the recessive wrinkled form of the peas and see the result

No Blend as

expected!

Mendel’s Hypotheses1. There are alternate forms

of ‘genes’ = alleles

2. For each trait, organisms have 2 genes, one from mom & one from dad

3. Sperm and egg each carry 1 allele/trait because alleles separate

4. Dominant alleles mask the effects or recessive alleles

A A

A

A

AA

AA

AA

AA

Fat

her

cont

r ibu

tes:

Mother contributes:

oror

Pure Breds Crosses – have all dominant traits

a a

A

A

Aa

Aa

Aa

Aa

Fat

her

cont

r ibu

tes:

Mother contributes:

oror

Pure Bred Dominant Father breeds with Pure Bred Recessive

Mother

Purebred Dominant

+ Purebred Recessive

a Hybrid ----> Aa

Describing Alleles Properly

• Aa means the critter is HETEROZYGOUS

It has two different alleles: A & a

The critter is not a purebred but a hybrid

• AA or aa means the critter has the same two alleles, so it is HOMOZYGOUS

Test Cross

Mendel's Laws of Inheritance

1. Genes produce traits (characteristics)

2. Law of Segregation alleles are separated during meiosis into different eggs/sperm

3. Law of Independent Assortment suggested that other alleles do not influence how other alleles come together (how they assort) (demonstrated with a “dihybrid cross”)