Download Mendel`s 3 Conclusions

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)
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
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
 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 3 Conclusions
1) Mendel realized that biological
inheritance is controlled by factors
(genes) from each parent
Mendel’s 3 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 3 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 3 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
F1 Generation
The plants are
homozygous--they
have two copies of
the same allele for
plant height:
These plants are
heterozygous--they
have two different
alleles for plant
height:
TALL/TALL or
SHORT/SHORT
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!
Punnett Square
Male Gametes
Female
Gametes