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
PF1
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