History of Mendel’s

work

Watch Video from 2:45 – 11:45 to introduce the thoughts of the ancient Greeks to Darwin…

Watch Video from 11:45 - 17:47 to review Mendel’s life and work…

Mendel was a monk in the 1800’s

He grew garden peas to see how characteristics were inherited.

He looked at 7 traits• Flower color• pea shape• pod shape• pea color• pod color• plant height• position of the flowers

Mendel started by self-fertilizing the plants (how they naturally reproduce).

• This means that he had them mate with themselves.

He called these plants pure or true because when he did this they always resulted in identical offspring

Sperm carrying pollen grains released from stamens land on egg containing carpel

Next he mated a PURE purple with a PURE white flower. • When you mate 2

different plants it is called cross fertilization.

The offspring of these crosses were HYBRIDS. They were no longer pure because they were mixtures of two different plants.

Finally, Mendel self-fertilized the offspring and looked at the grand-offspring.

Mendel described how the plants’ appeared, or their physical traits, with the term phenotype and their genetic appearance, or genotypic traits, with the term genotype.

Watch Video from 17:47 – 23:00 to review Mendel’s findings…

Based on his experiments we found:•There are alternative forms of genes he

called factors (today we call these alleles)

GENE ALLELES

Height Tall (T) or Short (t)

Flower Color Purple (P) or White (p)

Seed Shape Round (R) or Wrinkled (r)

Based on his experiments we found:• For each characteristic an organism has 2 factors – one

from each parent If these 2 factors are the same we call it homozygous

If these 2 factors are different we call it heterozygous

GENEALLELE

from MOM

ALLELEFrom DAD

Genotype?

Height Tall (T) Tall (T) (TT) Homozygous

Flower Color White (p) Purple (P)(Pp)

Heterozygous

Seed Shape Round (R) Round (R) (RR) Homozygous

Based on his experiments we found:• When two factors are different, the one that is expressed

is dominant, while the one that is hidden is recessive.

GENEALLELE

from MOMALLELE

From DADPhenotype?

Height Tall (T) Short (t) Tall

Flower Color White (p) Purple (P) Purple

Seed Shape Round (R) Round (R) Round

Based on his experiments we found:• The sperm and egg are able to carry only 1 factor for

each gene because the factors segregate during gamete production. This is called the law of segregation.

Watch Video from 23:02 to the end to explore Mendel’s Second Set of Experiments…

Crossed 2 traits instead of just 1

He followed the same experimental protocol – •Two different pure parents were cross –

fertilized

•Offspring were self – fertilized

He discovered that traits are inherited independently of one another (you can have black fur and red or black eyes)

He called this his Law of Independent Assortment

It turns out that Mendel’s hypotheses, and his two laws, hold true for garden peas and many human traits. However, that is not the end of the story. Today we know…

Damn straight…

That while all genes have alternative forms, some genes have more than two… For instance while peas can only be tall (T) or short (t), many human traits have more variety – like with blood we can have A (IA), B (IB) or O (i) alleles.

That while a mixture of tall (T) and short (t) alleles in pea plants results in a tall phenotype, in humans it does not.

Tt in humans results in a medium height. Outside of the world of peas – not all traits have a clear cut dominant and recessive.

That while segregation should always occur during the process of meiosis, in rare cases – like Down Syndrome – it does not.

That while independent assortment does occur for many traits - it is not always true.

For instance, in humans if two genes are right next to each other on a chromosome chances are they will NOT sort independently.

We say they are linked. In these instances independent assortment does not occur.