Download Mendelian Genetics

Mendelian
Genetics
Review
• What is cell theory?
– All living things are made of cells
– Cells are the basic unit of life
– Cells come from other cells
• DNA  RNA  protein (The Central Dogma)
Mendelian Genetics
What do you inherit from your parents?
1) money/property
2) your genes
Which is more important???
Much of what we understand about heredity
is the result of experiments conducted by
one person: an Austrian monk named
Gregor Mendel, who lived and worked in a
monastery in the mid-1800s
What does that say about the relationship
between science and religion?
Gregor Mendel
• born in 1822
• Lived in what is
now the Czech
Republic
• Tended the garden
of a monastery while
living there as a
monk
• Taught high school
science!!
Mendel’s Plants
• Mendel worked with pea plants in
his garden
• Mendel cultivated several stocks
of pure-breeding pea plants
• These plants had a number of
traits that occurred in one of two
forms:
Green or yellow seed color
Round or wrinkled seeds
Tall or short plant height
And a variety of others...
Mendel’s Crosses
• After establishing his purebred peas,
Mendel started his experiments
• Important vocabulary for
understanding Mendel:
–P = parental generation
–F1 = first generation of offspring;
“first filial” Filius is latin for “son”
–F2 = second generation of offspring
Mendel’s Crosses
• When he crossed tall plants with
other tall plants, what type of
offspring do you think he got?
• You guessed it…
tall offspring!!!
•Tall + tall (P) = tall (F1)
Mendel’s Crosses
• When Mendel crossed short
plants with other short plants,
what do you think he got?
• You are a genius!!!
Short offspring
• Short + short (P) = short (F1)
Mendel’s Crosses
• When he crossed tall plants with
short plants, what do you think
he got?
• Tall + short (P) = ??
• He got ALL TALL plants in the F1
generation!!!
This was quite a
surprise to him.
Why was Mendel surprised?
• Mendel was surprised because he
had expected the traits to blend:
–Tall + short = medium
• Instead, the tallness of one parent
had somehow masked the shortness
of another parent in the offspring
• Tall + short = Tall????
Mendel’s Conclusions
1) Mendel realized that biological
inheritance is controlled by factors
(genes) from each parent
Mendel’s Conclusions
2) These genes came in two different forms.
These forms are called alleles.
For example: the gene that controls the height of a plant
has a tall form and a short form. The gene for flower
color has a purple form and a white form.
Mendel’s Conclusions
3) Some alleles are
dominant while
others are recessive.
When both types of
alleles are present,
dominant alleles
mask (cover up)
recessive alleles.
Dominant allele
Recessive allele
Mendel’s Conclusions
• So each plant must have two alleles
for each trait
• Short plants must have two alleles for
short plant height (tt)
• Tall plants may have either two alleles
for tall plant height (TT) or one allele
for tall and one allele for short (Tt)
Tall/Tall (TT) or
Tall/Short (Tt)
Short/Short (tt)
P Generation
The plants are
homozygous-they have two
copies of the
same allele for
plant height:
TALL/TALL or
SHORT/SHORT
F1 Generation
These plants are
heterozygous-they have two
different alleles
for plant height:
TALL/SHORT
But they are all
tall…why?
Genotype vs. Phenotype
• An organism’s genetic make-up =
genotype
• An organism’s physical appearance =
phenotype
• Possible Genotypes = TT, Tt, tt
• Possible Phenotypes = Tall, short
Genotype vs. Phenotype
• Do you know the genotype of a short
plant?
–Genotype = tt
• What about the genotype of a tall
plant?
–Genotype = TT or Tt (you can’t tell
by its appearance)
Mendel’s Crosses
•What did Mendel do next?
•He crossed the tall F1 plants from the
previous cross with each other to see
what would happen…
• When he did this, he got more
interesting results--what do you think
happened??
Mendel’s Crosses
• The results of the F1 cross were:
–75% tall plants
–25% short plants
• Again Mendel was surprised
because the short, recessive allele
had returned--why did it reappear
in the F2 when it wasn’t observed
in the F1?
Mendel’s Conclusions
• Mendel knew that for there to be
short plants in the F2 generation,
those plants must have received
two short alleles (tt) from their
parents
Mendel’s Conclusions
• He figured out that each parent
contributes one allele (for a certain
trait) to the offspring
• Why does one parent only
contribute one allele to its
offspring--instead of two???
Mendel’s Conclusions
Two Scenarios:
1) If this plant is
homozygous tall (TT) it
can only pass on a tall
allele (T) to its offspring
2) If this plant is
heterozygous tall (Tt) it
can pass on either a tall
allele (T) or a short allele
(t) to its offspring
Mendel’s Conclusions
Because this
plant is short (tt)
it can only pass
on a short allele
(t) to its
offspring!
Parent
Possible Gametes
T
Tall Plant
TT
T
Tall Plant
T
Tt
t
Parent
Possible Gametes
Short
Plant
t
tt
t
Mendel’s Conclusions
• Mendel called this Segregation:
–The two alleles from each
parent are segregated from
each other so that each gamete
only carries a single copy of
each gene
We can use Mendel’s Law of
Segregation to figure out the
genotypes and phenotypes that
result from crosses between
pea plants or any other
organism…using a tool called
Punnett Squares!
Male Gametes
Female
Gametes