The Work of Gregor Mendel

Learning Objectives Describe Mendel’s studies and conclusions about

inheritance. Describe what happens during segregation.

Mendel’s Experiments

• Peas are a “model system.”

• Started with “true breeding” plants

• Trait: specific characteristic (e.g., seed color, plant height) of an individual

• Hybrid: created from cross of true-breeding individuals

Pollination and Fertilization in Plants

Male part

Female part

Cross-Pollination

Pollen

Pea Characteristics

Dominant and Recessive Traits

Yellow seed color is to green seed color.

Green seed color is to yellow seed color.

dominant

recessive

The F1 Generation

P parent

F1 offspring

Mendel’s F1 CrossesWhen Mendel crossed plants with contrasting traits, the hybrid offspring showed traits of only one parent.

The F2 Generation

What proportion of F2 offspring were short?

What proportion of F2

offspring were tall?

1/4

3/4

Genes and Alleles

• Genes: passed from one generation to the next; determine an individual’s characteristics

• Alleles: the different forms of a gene

Characteristics such as pod color

are determined by .

Yellow and green pod colors are

determined by .

The allele for green pods is to the allele for yellow.

genes

alleles

dominant

Principle of Dominance

• Some alleles are dominant, some recessive.

• An organism with at least one dominant allele will exhibit that trait.

• An organism with a recessive allele will exhibit the trait only in the absence of a dominant allele.

dominantrecessivedominant

Segregation

Segregation: separation of alleles during gamete formation

The Formation of Gametes

Eggs and sperm

Fertilization