Mendel’s Principles of Heredity

By

Gabriel Tordjman

For

Darwin’s Tea Party

Mendel’s Principles of Heredity

Gregor Mendel (1822-1884)An Austrian monk born in Heinzendorf and who died in Brno (both now in modern day Czech Republic).

Gregor Mendel (1822-84)Mendel’s Principles of Heredity

•As a substitute teacher at a technical school, Mendel conducted experiments on thousands of plants between 1856-1863.

•These lead to discovery of the basic principles of heredity, also called Mendel’s laws of heredity.

Gregor Mendel (1822-84)

• These principles of heredity confirmed that traits are passed down from one generation to another by distinct units, later called genes.

• They also established the idea of dominant and recessive traits.

Mendel’s Experiments

• In one experiment he examined how the trait of pea shape was transmitted in one type of pea plant.

• This pea plant always produced peas of two basic shapes and no others:

Smooth Wrinkly or

Mendel’s Experiments

Mendel took plants he knew always produced only smooth peas and crossed them with plants he knew always produced wrinkly peas and examined the results.

X

Mendel’s ExperimentsThe results showed that all the offspring of the first generation (F1) had only smooth peas.

X

F1

Parents

What had happened to the wrinkly trait?

Mendel’s ExperimentsMendel decided to take the F1 generation and cross them with each other to see if the wrinkly trait would reappear in the next generation (F2).

Mendel’s ExperimentsIt did!

X

F1

Parents

F2

The wrinkly trait reappeared in the F2 generation about one out of four times!

X

Mendel’s Experiments

• Mendel tried the same experiment to examine the transmission of other traits, such as flower colour in certain plants (white or purple flowers).

• The same pattern and results reappeared!

Analysis of Mendel’s Experiments: The Genetic Hypothesis

All these results could be explained with 5 basic ideas:

1. There were separate, distinct units (later called genes) responsible for hereditary traits.

gene

trait

Analysis of Mendel’s Experiments: Alleles - 2 alternative

forms of a trait 2. Each unit (gene) had two alternative forms (later called

alleles), one derived from the male and one derived from the female parent, that come together in the offspring.

Parents

ss

Ss s

s

Ss

Alleles united in the

offspring

Gene with2 alleles

Parents’ sex cells

Analysis of Mendel’s Experiments:Dominant and Recessive Traits

3. Though two alternative forms of a gene (alleles) can exist in the offspring, only one is visible or expressed, while the other is covered up or masked. – The expressed trait is called dominant. – The covered up trait is called recessive.

Ss

As the symbols indicate, thispea expresses the trait forsmoothness (S) but also carries the allele for wrinkliness (s)

Analysis of Mendel’s Experiments:Independence of traits

4. One trait - e.g., pea shape, does not influence another shape - e.g., flower colour.

Analysis of Mendel’s Experiments:Statistical Prediction of Traits

5. One could calculate the statistical probability of certain traits appearing or not appearing in generations of large populations.– Dominant traits appeared in a ratio of 4:1 or 3:1– Recessive traits appeared in a ratio of 1:3 of 1:4

From Mendel to DNA• Mendel had not actually seen genes - this was an

inference or deduction from his experiments.

• He had only observed outward visible traits that seemed to confirm unknown inner entities later called genes.

gene

trait?

From Mendel to DNA:Cell Biology (Cytology)

Later investigators, using the microscope, began to search for physical evidence of genes in the cells of micro-organisms, plants, and animals.

From Mendel to DNA:Cell Biology (Cytology)

• They noticed that just before cells divide, certain structures in the nucleus of the cell make duplicates of themselves.

• These were called chromosomes.

From Mendel to DNA:Cell Biology (Cytology)

Chromosomes were thus once believed to be the genes responsible for all hereditary traits

Human Human chromosomes, chromosomes, spread out and spread out and stained, as seen stained, as seen

through a through a microscopemicroscope.

From Mendel to DNA:Cell Biology (Cytology)

• But later researchers discovered that the chromosomes were only a kind of package that contained the genes.

• Genes were insides the chromosomes and the hunt was on for discovering their exact chemical structure and function.

Stay tuned for the further adventures of the science of genetics in the 20th Century!

The End