WHY MENDEL? WHY PEAS?

genetics: the branch of biology that studies the transmission of hereditary information from parents to offspring

WHY MENDEL? WHY PEAS?

Mendel was a monk who was educated--he knew MATH! He also had a lot of free time.
As for the peas? Well, they were:
* easy to grow
* easy to pollinate (either self or cross)
* easy to study their TRAITS (characteristics) such as: height, seed pod shape, seed color, seed pod color, seed texture,flower position, and seed shape

Mendel kept very accurate records for hundreds of individual plants. He used statistical analysis (MATH) to study the traits of different generations. Through his findings, he proposed that the traits were passed on by some kind of hereditary factors (now know to us as DNA).

Here's basically what he did:

One trait he studied was plant height. Mendel pollinated all tall plants for many generations to get a pure population of tall plants. He also pollinated all short plants to get a pure population of small plants. See picture below...

X means to cross
(as in cross-pollinate)

tall plants X tall plants as well as short plants X short plants

He then cross-pollinated a member of the tall pure population with a member of the short pure population...

P1 (pure parent generation)

P1 TALL PLANT X P1 SHORT PLANT

ALL TALL PLANTS

F1 (first filial [family] generation)

Where did the short trait go? Mendel then crossed two tall members of the F1 generation.

F1 TALL PLANT X F1 TALL PLANT
THIS IS CALLED A MONOHYBRID CROSS (hybrids known for one trait)

F2 (second filial generation)

For every 3 TALL PLANTS there was 1 SHORT PLANT

So, the short trait did not disappear. It was present in the F1 generation, but IT WAS NOT EXPRESSED (this means it did not show up!) Somehow, it was expressed (visible) in the F2 generation.

I. Mendel's Principal of Dominance

* because the tall trait showed up more than short, Mendel call this trait DOMINANT

* the short trait, because it seemed 'weaker' than the tall trait, was called RECESSIVE

After this discovery...well, um...NOBODY CARED! Later on, when scientists studies meiosis in Drosophila (genus name for fruit flies), they linked together Mendel's factors with the chromosomes in gametes.

* Mendel's factors are now called GENES

* genes are segments of chromosomes (DNA) that code for a characteristic

* these characteristics can be:

a) physical traits (can be seen like eye color, hair color, height, etc.)

b) chemicals produced in the body (like for hormones or enzymes)

genes are lined up on chromosomes in a certain order, like beads on a string

* homologous (similar) chromosomes have the same order of genes...HOWEVER..

* these chromosomes might have the same forms of a gene

ex. trait=eye color can be blue, black, brown, hazel, green, etc.

* these different forms of a gene are called alleles

* in simple patterns of inheritance, there are 2 different forms of a gene (alleles), where one is dominant and one is recessive

* DOMINANT IS SHOWN BY CAPITAL LETTERS

* recessive is shown by lowercase letters

ex. in Mendel's peas T = tall t = short

* diploid (having 2 sets of chromosomes) organisms have 2 copies of genes, one on each chromosome they have

* the 2 copies can be the same--HOMOZYGOUS, like TT (homozygous dominant) or tt (homozygous recessive)

*the 2 copies can be different--HETEROZYGOUS Tt (heterozygous)

Notice that what the organism looks like and the kind of genes it has are two separate ways to classify an organism.

phenotype: what the appearance of an organism is (how it looks) ph = physical

genotype: what the genetic makeup of an organism is (what genes it has) gen = genes

genotype can be:
homozygous dominant----------> TT
heterozygous (a hybrid--a mix)---> Tt
homozygous recessive----------> tt

recessive phenotypes can only be expressed (show up) when there are 2 copies of a recessive gene present at the same time...if a dominant gene was there, then that would be expressed, hiding the recessive gene!