Mendel’s Genetics
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Transcript Mendel’s Genetics
Mendel’s Genetics
Monohybrid Cross
Gregor Mendel
As discussed last
class, he was an
Austrian monk
He worked at St.
Thomas Monastery
He studied both
mathematics and
botany
Why did he actually
Succeed?
He chose the appropriate organism to
study
He designed and performed his
experiments correctly
Analysis of the data was done properly
The Patient
The common pea plant also
known as Pisum sativium
It was chosen for 4
reasons
It was commercially
available
Easy to grow and mature
The sex organs are entirely
enclosed in the flower
Different varieties had
different traits
Purebred
What is a purebred organism?
It is an organism that has descended from
ancestors of a distinct type or breed
Why was this important to Mendel and
his pea plants?
This is because he used purebred plants
to conduct all of his experiments
How did he make sure?
How did Mendel make sure that his
organisms (pea plant) were pure bred?
He bred them as everyone else bred plants, and
he looked at one trait at a time
He selected from the progeny, only the plants
that had the trait that he was looking for
He then bred them together (F1 x F1) and then
he continued this for several generations until no
more of the other trait showed up
What happened Next?
He repeated this for all the other
traits until he had pure bred plants
with each trait.
The first of the Crosses
So what did he end up doing next?
He started to cross breed
He designated the parents as the “p”
generations and crossed a truebreeding tall pea plant, with a truebreeding short pea plant
The offspring were of course the F1
generation and he also denoted them as
Hybrids
That F1 Generation
What do you think he saw?
What was the phenotype of the
plants?
All of the plants were tall
What is this type of crossing is called?
Cross?
Well what is the Greek word for a
single unit?
Well, think back to the first unit, what
did we call a single sugar
MONOSACCARIDE
So now what is the Greek word for a
single unit?
MONO (Greek word for single or alone)
Creatively
Creatively the name given to this type
of cross is Monohybrid
It is a single cross between two
purebred plants giving us a hybrid
species
What did his experiment look
like?
That’s the Phenotype
We have now seen the phenotype
All of the F1 generations plants were tall
What do you think that the genotype must
be of the offspring?
First we have to learn some more terms
Terms
Homozygous Dominant (TT) – Two alleles for
a trait that are the same as a result of pure
breeding
Heterozygous (Tt) – Having two alleles for a
trait that are different
Homozygous Recessive (tt) - Two alleles for
a trait that are the same as a result of pure
breeding
The First Cross
Tall
Short
PF1
Lets back track
Yesterday we looked at the different
combinations of alleles that an
individual can have
We also talked about dominance and
recessiveness
What did we say the dominant allele was?
What about the recessive allele
What were the parents?
The parents that was tall
had a double dominant allele
The parent that was short
had a double recessive allele
Both of these parents were
homozygous
When he Crossed
When the parents were crossed, the
F1 generation was completely
heterozygous
They all expressed the dominant trait,
but why?
This was because the dominant allele
trumps the recessive allele
So the First cross is done,
what about the second?
Second Cross Genotype
Mendelian Ratio
As Mendel completed the experiment,
he found that 75% of the offspring of
the F1 cross were the dominant (tall)
and 25% were the recessive (short)
The Ratio of 3:1 is known as the
Mendelian ratio
Based on observations, Why
did this happen?
Each parent in his F1 Generation starts
with two hereditary “factors.” One
factor is dominant and the other
recessive
The factors separate out in the and
only one of the two factors contribute
to the phenotype of the offspring
Why did this happen?
The offspring inherit one factor from mom
and one from dad. If the dominant factor is
present, it will be expressed, even if the
recessive one is present
The recessive factors will be express only if
the recessive factors are present
The first Law of heredity
His results gave rise to his first law
The law of Segregation
The law of segregation
His law of segregation states that inherited
traits are determined by pairs of “factors”
These factors segregate in the gametes
(after meiosis)
Mendel did not know that his factors were
actually genes, we know this today
What do we call this?
Punnett Square’s
What do they do for us?
Well, they are used by geneticists so that
they can calculate the expected ratio
(probability) and to suggest possible
combinations of alleles in the offspring
They also tell us something about the
phenotype (the appearance of a trait in an
organism)
So we saw TT x tt
And for that cross we ended up having
a ratio of?
All were Heterozygous (100% - 0%)
What about when we cross the F1 x F1
generations?
F1 X F1
What’s the ratio when we are talking
about phenotype?
3:1 as well, three of the offspring will
present the dominant gene while the
fourth will present the recessive gene
What about the genotype ratio?
1:2:1 (one homozygous dominant, two
heterozygous, and one homozygous
recessive)
Example
Class Work
Read Pages 123 – 129
Complete Questions P135, 1-6, 8-16