Mendel’s Principles of Heredity

By

Gabriel Tordjman

For

Darwin’s Tea Party

Last update: April 2012

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).

Before MendelHeredity was a mystery

“The laws governing inheritance are quite unknown; no one can say why the same peculiarity … is sometimes inherited and sometimes not so; why the child often reverts in certain characters to its grandfather or grandmother or other much more remote ancestor…”

- Charles Darwin, Origin of Species 1872

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 lad 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 physical-chemical units, later called genes.

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

Mendel’s ExperimentsIn one experiment he examined how the trait of pea shape was transmitted in one type of pea plant.

Mendel’s Experiments

• This pea plant always produced peas of two basic shapes:

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 (first 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!

Mendel tried the same experiment to examine the transmission of other traits, such as flower colour.

Analysis of Mendel’s Experiments: The Genetic Hypothesis

All these results could be explained with 5 basic ideas:

1. There are separate, distinct physical 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

One Allele from each

parent united in the

offspring

Gene with2 alleles

Each parent’s sex

cells provide

only half (i.e., one

allele)

Mama pea

Papa pea

Analysis of Mendel’s Experiments:Dominant and Recessive Traits

3. Though two alleles of a gene 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

Analysis of Mendel’s Experiments:

• When both alleles of a gene produce the same trait, the gene is said to be homozygous.

• When both alleles of a gene produce the different traits, the gene is said to be heterozygous.

Ss

Questions1.If “S” = smooth trait and “s” = wrinkly trait, is this pea heterozygous or homozygous?2.Is smoothness or wrinkliness dominant in this plant?

Mendelian inheritance in humans

d) Earlobe attachmenta) Mid-digital hair b) Tongue rolling

c) Widow's peak e) Hitchhiker's thumb

f) Relative finger length

All these traits (and more) are transmitted in Mendelian fashion in humans

Yale University Science Education Outreach Program, http://info.med.yale.edu/labmed/seop/supplementary/supp_traits.htm

Mendelian inheritance in humans: sickle cell anemia

• Sickle cell anemia is a single gene, recessive disease that causes red blood cells to “sickle” (“C” shaped) as shown here.

• The disease can be painful if one allele is inherited and even more serious if two alleles are inherited (one from each parent).

Mendelian inheritance in

humans: sickle cell anemia.

Note how the disease is passed down exactly like

pea shape or flower colour in Mendel’s

peas

http://www.patient.org.in/blood/sickle-cell-anemia.htm

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 the existence of unknown inner entities (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