Transcript on Mendel`s principles of heredity

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
or
Wrinkly
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 Experiments
The results showed that all the offspring of the
first generation (F1) had only smooth peas.
X
Parents
F1
What had happened to the wrinkly trait?
Mendel’s Experiments
Mendel 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 Experiments
It did!
X
Parents
F1
X
F2
The
wrinkly
trait
reappeared
in the F2
generation
about one
out of four
times!
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.
Ss
Parents
Gene with
2 alleles
ss
s
Parents’
sex cells
s
Ss
Alleles united
in the offspring
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.
As the symbols indicate, this
pea expresses the trait for
smoothness (S) but also
carries the allele for wrinkliness (s)
Ss
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
chromosomes,
spread out and
stained, as seen
through a
microscope.
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