HeredityMendel and His Peas

Biology

First, Let’s Review!

Sexual ReproductionProcess of a sperm fertilizing an egg

GametesSex cells (eggs, sperm)

Meiosis ReviewMeiosis

Cell division that results in haploid sex cells

Number of chromosomes in somatic (body) cells= 46

These are diploid cells!

Number of chromosomes in gametes (sex cells)= 23

These are haploid cells!

What is Heredity?

Heredity is the passing of traits from parents to offspring.

Hair color, eye color, height, etc.

But sometimes both parents will have brown eyes, but their child has blue eyes.

Who was Gregor Mendel?

Mendel was born in 1822 in Austria.

He grew up on a farm and knew a lot about cultivating flowers and fruit.

He became a monk and started to work in the monastery garden.

He noticed that certain traits were passed from parent plants to offspring plants.

A Mystery

Mendel knew that sometimes the patterns of inheritance seemed simple and sometimes they were not.

Sometimes a trait that appeared in one generation did not show up in any of the offspring in the next generation, but suddenly the trait would appear again in the third generation.

Mendel’s Peas

Mendel chose peas to conduct his experiments with.

Peas grow fast, can self-pollinate, and they come in many different varieties.

A self-pollinating plant contains both male and female reproductive structures.

More Peas

Mendel chose to study only one characteristic at a time, such as color, height, or shape.

Mendel chose plants that had two forms for each of the characteristics he wanted to study. Ex: yellow and green peas.

True-Breeding Plants

When a true-breeding plant self-pollinates, it will always produce offspring with the same trait the parent plant has.

For example: Green pea plants will always produce green pea offspring.

Cross-Pollination

Mendel wanted to find out what would happen if he bred two plants with different characteristics.

In cross-pollination, the anthers of one plant are removed so the plant cannot self-pollinate. Then pollen from another plant is used to fertilize the plant without anthers.

Mendel’s First Experiment

Mendel crossed plants that had round peas with plants that had wrinkled seeds.

The offspring from this first cross are known as the first generation.

What do you think the offspring looked like?

Dominant Vs. Recessive

One trait always appeared (round) and one trait always disappeared (wrinkled).

Mendel called the trait that always appeared the dominant trait.

The trait that disappeared is called the recessive trait.

Mendel’s Second Experiment

Mendel then let the first generation plants self-pollinate.

The offspring of the first generation are called the second generation.

What do you think happened?

The wrinkled seeds appeared again!

Mendel’s Results

Mendel decided to count the number of plants with each trait that turned up in the second generation.

Mendel noticed that there were patterns and ratios to how the traits showed up.

Genes and Alleles

Mendel realized that each plant had two sets of instructions for each characteristic.

Each parent donates one set of instructions, called genes, to the offspring.

The two genes that determine a characteristic are known as alleles.

The Punnet Square

A Punnet square is used to see all the possible combinations of alleles from the parents.

Dominant alleles are symbolized with capital letters.

Recessive alleles are symbolized with lowercase letters.

Genotype

The inherited combination of alleles is known as the offspring’s genotype.

Phenotype

An organism’s actual appearance is known as phenotype.

Probability

The mathematical chance that an event will occur is known as probability.

Usually expressed as a fraction, ratio or percentage.

We use this mathematical method to calculate genotype probability.

Let’s Try One!

Blue eyes