Mendelian Genetics

Biology B/Genetics

Gregor Mendel Gregor Mendel was a

priest and high school science teacher who lived in the 17th century.

Mendel was also in charge of the garden at the monastery where he lived.

One of the many plants that Mendel worked with in his garden were pea plants

Pea Plants Like many plants, peas

use parts of their flowers to reproduce: The male part of the

flower produces pollen, which contains male sex cells

The female part of the flower produces eggs, which are the female sex cells.

Pea Plants There are two ways for

pea plants to reproduce: True breeding is when

the flower self pollinates (asexual reproduction)

Cross breeding is when the flower is pollinated by another flower (sexual reproduction)

Mendel’s Experiment Mendel observed that

some of his pea plants were tall, while others were short.

Height is an example of a trait, which is a specific characteristic that varies from one individual to another.

Mendel decided to study how this trait got passed from parent plant to daughter plant when his pea plants cross pollinated.

Make A Prediction:

Mendel cross pollinated a tall and a short pea plant.

He called these plants the P Generation (P=Parent)

What do you think the offspring looked like?

The Results:

Much to Mendel’s surprise, the resulting offspring were two tall pea plants.

He called these plants the F1 Generation (F= Filial, Latin for son)

What happened to the gene for a short pea plant?

Alleles Mendel correctly came

to two conclusions:

1. Inheritance is determined by genes, which are factors that are passed from one generation to the next. Different forms of the gene are called alleles.

Example: The are two alleles for plant height: tall and short

The Principle of Dominance

2. An organism with a dominant allele for a particular form of a trait will always have that form. An organism with a recessive allele for a particular trait will have that form ONLY when the dominant allele for the trait is NOT present.

Example: Tall height is dominant and short height is recessive.

Punnett Squares If we use a diagram called

a Punnet Square, we can understand how Mendel’s conclusions explained what happened with the pea plants:

Both parents were hybrids, which means that they carried both the dominant tall allele AND the recessive short allele.

Phenotype and Genotype Phenotype describes the

physical characteristics of an organism. Example: If the pea

plant is tall or short.

Genotype describes the make-up of an organism’s genes. Example: If the pea

plant has two tall alleles, two short alleles or one tall and one short allele.

Another Experiment…

Mendel did one more experiment to test his theory.

This time he cross pollinated two tall pea plants from the F1 Generation.

Make a prediction for the F2 Generation…

The Results Just as Mendel expected,

this third generation contained one short and one tall plant.

Why did the short plant “reappear”?

…Because the recessive allele had been “hidden” by the dominant trait, but was still present in both parents’ genotypes.

“Hidden” Recessive Alleles

…Because the recessive allele had been “hidden” by the dominant trait, but was still present in both parents’ genotypes.

More Punnett Squares