Mendel’s Second
Experiment
Inheritance of Two Traits
Dihybrid Cross
Monohybrid Review

How many traits were being looked at
during the Mendel’s monohybrid cross?
– __________________________________
What was being crossed in Mendel’s
monohybrid cross?
 _________________________________
 _________________________________

Back Track for a Second

What exactly was a purebred species in
terms of genotype?
– It was a species that had the same alleles in
its genetic makeup (either BB or bb)

What about a hybrid species in terms of
genotype?
– It was a species that had different alleles in
its genetic makeup (like Bb)
PXP

When the two parents
were crossed, TT X tt,
what resulted?

The Results were four
heterozygous
offspring.

These are the F1
generation
F1 X F1

When the two F1 generation
were crossed, TT X tt, what
resulted?

The Results were three
dominant and one recessive in
a 3:1 phenotypic ratio but in a
1:2:1 genotypic ratio

These are the F2 generation
The Law of Segregation

What does the Law of Segregation state?
– It states that inherited traits are determined
by pairs of “factors”
– These factors segregate (separate) in the
gametes
What happened next?
Mendel completed the monohybrid cross
and started to ask and formulate new
questions
 What do you think he was thinking of
next?

– He wanted to know if the inheritance of one
characteristic influenced the inheritance of a
different characteristic
What did he try next?
He started all over again
 He produced purebred plants just like last
time with the traits he wanted to examine
 He bread pea plants that were round and
yellow pure plants (dominant for both)
and then he did the same for wrinkled and
green (recessive for both)

What did he try next?
He then crossed them again
 The P X P ended up yielding what?

– The F1 ended up being all yellow and all
round, just like last time
– There seems to be some similarity
P X P Dihybrid Cross
RY
RY
RY
RY
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
Then What?
Just like last time he crossed the two F1
Progeny
 What do you think he got?

– He
– He
– He
– He
got
got
got
got
9
3
3
1
Yellow and round
Green and round
Yellow and Wrinkled
Green and Wrinkled
What’s The Ratio?

The ratio that he got was?
– 9:3:3:1
How Many?

How many plants do you think that
Mendel used for this experiment
– 551 pea plants
– And of that 551 this was the ratio that he got
320 round and yellow
 104 round and green
 101 wrinkled and yellow
 26 wrinkled and green

What did this tell him?

The ratio of 9:3:3:1 could be explained if
the alleles from one trait were inherited
independently of the alleles for another
trait
The Law of Independent
Assortment
The Law of Independent
Assortment

The second law of inheritance
states:_____________________________
__________________________________
__________________________________

Different pairs of alleles are passed to the
offspring independently of each other
The Law of Independent
Assortment

This means that offspring may have new
combinations of alleles that are not
present in either parent