Outline the experiments carried out by Gregor Mendel

IntroductionGregor Mendel (1860s), an Austrian Monk,

formulated the principles of genetics by careful and methodical experimentation with garden peas.

IntroductionHe wrote that ‘factors’ (genes) were passed

on from parent to child that could produce the same physical characteristics.

It wasn’t until the 1900’sthat Mendel’s findings wereformally recognised.

Mendel and GeneticsThe appearance of an organism is the result of

two factors:1. the gene being expressed2. the environment

Mendel stated that each individual has two factors for each trait, one from each parent.

The two factors may or may not contain the same information.

The alternative forms of a factor are called alleles.

Homozygous and Heterozygous traitsIf the two factors are identical the

individual is called homozygous for the trait (pure bred)

If the two factors have different information, the individual is called heterozygous. (hybrid)

In heterozygous individuals the only

allele that in expressed is the dominant.

The recessive allele is present but its expression is hidden.

Mendel’s experimentMendel chose garden peas as he was also

able to strictly control the breeding patterns of his peas for the following reasons; They were easy to grow, produced new generations quickly, and had easily distinguishable characteristics.

What did Mendel look at?Mendel examined the following seven

characteristics found in peas: Flower colour (purple or white) Flower position (axial or terminal) Seed colour (yellow or green) Seed shape (round or wrinkled) Pod shape (inflated or constricted) Pod colour (green or yellow)Stem height (tall or short)

Why was Mendel successful?Accuracy, reliability, validity…He cross-fertilised plants by hand with a small

brushHe prevented pollination by other plants by

covering the flowers with small sacksHe crossed pure breeding parents with easily

identifiable characteristicsEach characteristic studied was determined by 1

pair of genesGenes for each characteristic are found on

separate chromosomesHe did multiple crosses and repeated it

thousands of times (large sample sizes and repetition)

Why was Mendel successful?Mendel ensured pure bred plants by

allowing them to self-fertilise over many generations while removing any plant with a contrasting characteristic.

He eventually got a different set of pure-breeding plants with each of his seven characteristics.

Why was Mendel successful?Once Mendel was sure he had 2

populations of plants with contrasting features (e.g. Purple flowers and white flowers), he cross bred them and grew the seeds that resulted. This was called the first generation (or F1).

He noticed that this produces all purple flowers

Why was Mendel successful?He then self fertilised these purple flowered

plants (F1 x F1) and grew these offspring (F2). Instead of all purple, this produced 75% purple and 25% white flowered plants (3:1 ratio).

He noticed this ratio for each of the seven characteristics he studied.

Dominant and recessive factorsEach of the seven traits that Mendel studied

had a dominant and a recessive factor.

When two true-breeding plants were crossed, only the dominant factor appeared in the first generation of offspring.

The recessive factor appeared in the second generation in a 3:1 (dominant : recessive) relationship.

Lets do some Monohybrid crosses (punnett squares) to show this…