Transcript on Mendel`s principles of heredity

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 Mendel
Heredity 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.
In one
experiment he
examined how
the trait of pea
shape was
transmitted in
one type of pea
plant.
Mendel’s Experiments
Mendel’s Experiments
• This pea plant always produced peas of two
basic shapes:
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 (first
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!
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.
Ss
Mama
pea
Parents
Gene with
2 alleles
ss
Papa
pea
S
Each
parent’s sex
cells
provide
only half
(i.e., one
allele)
s
Ss
One Allele
from each
parent united
in the offspring
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.
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
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.
1.
2.
Ss
Questions
If “S” = smooth trait and “s” = wrinkly trait, is this pea heterozygous or homozygous?
Is smoothness or wrinkliness dominant in this plant?
Mendelian inheritance in humans
a) Mid-digital hair
b) Tongue
rolling
c) Widow's peak
d) Earlobe attachment
e) Hitchhiker's
thumb
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
f) Relative
finger length
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
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