Gregor Mendel

The Father of Genetics

Gregor Mendel

• Born in 1822 in what is now called the Czech Republic

• Studied Math and Physics and became a monk and a teacher

• Studied Pea plants because of their distinct varieties

• His experiments showed that inheritance follows patterns

• His conclusions became the foundation for modern genetics – unfortunately,

his ideas were still not well known or accepted when he died at the age of 61.

Why Pea Plants?

• Mendel decided to study pea plants because:

• They have clearly opposite traits (tall versus short; purple versus white, etc.)

• They grow quickly

• They are able to self-pollinate and cross-pollinate

When a plant with purple flowers pollinates itself, it will produce all

purple flowers. This is called True Breeding.

Which Traits did Mendel Study?

The First Experiments

Mendel first crossed

purple flowered pea plants

with white flowered pea

plants.

ALL of the offspring had

purple flowers –NO white

flowers!

The Second Experiment

Next, Mendel allowed one of the

offspring from the first experiment

to self-pollinate.

This cross resulted in a purple to

white ratio of 3:1 – EVERY TIME!

Dominant and Recessive Traits

Dominant Trait

• The trait that appears

• The dark brown hair

is dominant

Recessive Trait

• The trait that does not appear

• The light brown hair is recessive.

Mendel’s Conclusions

• Mendel discovered that there were two sets of instructions for each trait.

• For example, when he crossed the purple and white flowered plants, he knew the

instructions for white flowers were still there because the white flowers showed up in

the 2nd generation.

• Scientists call theses instructions for traits GENES.

• The different forms of a gene are called ALLELES.

• In Mendel’s experiments, the purple flower color was a dominant allele and the white

flower color was a recessive allele.

Mendel’s Conclusions Continued

• PHENOTYPE is how the trait appears.

• GENOTYPE is both inherited alleles for a trait.

• For example, Mendel’s first set of offspring were all purple. The phenotype for all 4

plants was purple.

• However, we know that these purple flowers were a result of a cross between purple

and white flowers. So the genotype carried the dominant trait P=Purple and the

recessive trait p=white.

Punnett Squares

• Since purple is the dominant allele, anytime there is a dominant allele (P), the offspring will be purple.

• The possible genotypes for purple flowers are:

PP or Pp

• The possible genotypes for white flowers are:

pp (Both recessive alleles must be present in order for the recessive trait to show up)

Practice a Punnett Square

• Create a Punnett Square to show a cross between a black haired puppy (BB)

and a brown haired puppy (bb). Black hair = B; Brown hair = b

What are the genotypes and phenotypes of the 4 puppies? What is the ratio of black to brown?

Practice a Punnett Square - Solved

• Create a Punnett Square to show a cross between a black haired puppy (BB)

and a brown haired puppy (bb). Black hair = B; Brown hair = b

Bb Bb

Bb Bb

The genotypes are all Bb.

The phenotypes of the 4 puppies: All 4 puppies have black hair.

The ratio of black to brown is 4:0 or 100% black

B Bb

b

Homozygous / Heterozygous

• Genotypes such as PP or pp are known as HOMOZYGOUS.

• (Remember, ‘homo’ means “same.”)

• Genotypes such as Pp are known as HETEROZYGOUS.

• (Remember, ‘hetero’ means “different.”)

Practice a Punnett Square

• Create a Punnett Square to show a cross between a homozygous dominant black

haired puppy and a heterozygous brown haired puppy. B=Black; b=brown

What are the genotypes and phenotypes of the 4 puppies? What is the ratio of black to brown?

Practice a Punnett Square

• Create a Punnett Square to show a cross between a homozygous dominant black

haired puppy and a heterozygous brown haired puppy. B=Black; b=brown

BB

Bb Bb

The genotypes are BB, BB, Bb, Bb

The phenotypes of the 4 puppies are 100% Black

What is the ratio of black to brown? 4:0 or 100% black

B BB

b

BB

Review

• What is the phenotype of a puppy with the genotype bb?

• The puppy is brown

• What are the possible genotypes for a flower that is purple?

• PP or Pp

• Is the genotype Pp homozygous or heterozygous?

• Heterozygous (the alleles are different)

Review

• What are the possible genotypes for a white flower?

• pp only

• What is the ratio of black to brown dogs with a cross of Bb x bb?

• 1:1 or 50% black to 50% brown

• Which trait shows up in the phenotype, the dominant or the recessive gene?

• The dominant trait is the one that shows up.

• What is the only genotype that will allow the recessive genes to show up in the phenotype?

• Two recessive alleles, such as bb or pp