Gregor Mendel

“Give peas a chance!”

How a monk learned about inheritance using

a tasty vegetable

Gregor Mendel Born in 1822.

Gregor Mendel was a

monk who taught high

school and worked in the

monastery gardens.

In fact, he loved plants so

much that he spent most

of his adult life studying

how certain traits were

passed from “parent”

plant to “baby” plant.

Gregor Mendel

Considered the “Father

of Genetics” due to his

work studying

inheritance.

Genetics: the

scientific study of

heredity.

Mendel and his peas Gregor Mendel is most famous for his scientific

study of pea plants.

He chose pea plants because they reproduce

quickly and produce large amounts of offspring.

Pea plants have seven observable characteristics.

Each characteristic has two possible traits.

Characteristic Possible Traits

1. Plant height -----------------------------Long or short stems

2. Flower position along stem--------------Axial or terminal

3. Pod color-----------------------------------Green or yellow

4. Pod appearance------------------------Inflated or constricted

5. Seed texture-----------------------------Smooth or wrinkled

6. Seed color----------------------------------Yellow or green

7. Flower Color--------------------------------Purple or white

Purebred

By controlling plant pollination

(fertilization), Mendel was able to create

pea plants that were purebred for each

trait.

This is also known as true breeding.

When they self-fertilize, true breeding

parent plants always produce offspring

with the same trait.

Purebred Examples of True Breeding:

A purebred Labrador bred with a purebred Labrador

will always produce fully Labrador offspring. No

Chihuahuas???

A self-pollinating plant purebred for purple flowers

will always produce offspring with purple flowers.

A self-pollinating plant

purebred for white flowers

will always produce offspring

with white flowers.

What happens if you cross two

purebred plants?

purple flowers x white flowers

or

wrinkled seeds x smooth seeds

or

tall plants x short plants

or

Labrador x Poodle

Let’s take a look at flower color….

Mendel called the purebred parents

the P generation.

For each cross (plant “parents”),

Mendel cross-pollinated plants who

were true breeding with opposite traits.

Example: the P generation consisted of

a purebred purple flower plant and a

purebred white flower plant

Flower color cross

Mendel called the offspring plants the F1

generation (1 for 1st , F for filial- son or daughter)

Plants in the F1 generation are called hybrids

because their parents have different traits.

What do you think Mendel expected to see when

he crossed a purebred purple flower plant with a

purebred white flower plant?

All of the hybrid, or F1 Generation,

offspring had purple flowers!

Crossing two purebred plants

In every case of crossing two purebred plants,

one trait “won out” in the F1 generation.

Examples:

Purple flower color “won out” over white flower

colors

Smooth seed texture “won out” over wrinkled seed

texture.

Why do you think this is???

Dominant and Recessive

Traits can be dominant or recessive!

Mendel called the trait that appeared in the F1

generation (purple flowers in this case) the

dominant trait.

Mendel called the trait that did not appear in

the F1 generation (white flowers) recessive

trait.

What do you think happened when

Mendel let plants from the F1

generation self-pollinate?

Did he end up with purebred purple

flowers?

No!!!

Dominant and Recessive Traits

About 25% of the flowers in the F2 generation

were white!

Even though the F1 generation looked like

purebred purple plants, they carried the trait

for white flowers somewhere inside.

Where was this white flower trait “hidden”??

The traits are hidden in the genes!

A gene is a segment of DNA that codes for a particular protein. The protein results in the trait.

So….. Each characteristic = One gene

For example:

There’s a gene for plant height

There’s a gene for seed color

There’s a gene for flower color

etc...

Alleles

Each alternative (different) form of a gene is

called an allele.

Examples:

The plant height gene has two alleles, tall and

short.

The flower color gene has two alleles, purple and

white

The seed texture gene has two alleles, wrinkled

and smooth.

In other words… Inheritance is determined

by factors (genes) that are

passed from one generation

to the next.

These genes can come in

different forms called

alleles.

Law of Dominance: Some

alleles are dominant and

others are recessive

For every gene, how many alleles do you get from each parent?

This is a result of what process??

From his research, Mendel came to

two conclusions:

1. The Law of Segregation: Two factors (alleles)

control each specific characteristic (gene). These

factors (alleles) are separated during the formation

of gametes (meiosis).

2. The Law of Independent

Assortment: Factors (alleles)

for different characteristics (genes)

are distributed to gametes

independently. This means that the

allele for seed texture isn’t dependent on the allele

for plant height, etc.

As humans, our alleles are what

create the diversity among us!

Terminology

Alleles are represented with letters

Dominant – The allele/trait that is expressed (T)

Recessive – The allele/trait that is hidden (t)

Genotype – The allele makeup of a gene (TT,

Tt, tt)

Phenotype – The physical appearance of a trait

(Tall plant, purple flowers, etc)

Genotypes and Phenotypes

There are three possible genotypes:

Homozygous Dominant (BB, TT, PP)

Homozygous Recessive (bb, tt, pp)

Heterozygous (Bb, Tt, Pp)

Homozygous: Has two of the same allele.

Heterozygous: Has one of each allele.

Mendelian Genetics

Punnett Squares

Mendel’s work has allowed

us to be able to predict

possible offspring

outcomes between two

known genotypes.

This prediction is illustrated

through a punnett square.

Monohybrid Crosses

Two heterozygous

tall plants are

allowed to cross-

pollinate.

Use a punnett square

to predict the

genotype and

phenotype

probabilities.

Monohybrid Crosses

Genotype Probabilities:

TT: 25%

Tt: 50%

Tt: 25%

Phenotype Probabilities:

Tall: 75%

Short: 25%