1. Mendel’s Second Experiment
Inheritance of Two Traits
Dihybrid Cross

2. Monohybrid Review
How many traits were being looked at during the Mendel’s monohybrid cross?
The cross was between one trait only
What was being crossed in Mendel’s monohybrid cross?
Two purebred organisms for the first cross
Two F1 Heterozygote organisms in the second

3. 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 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)

4. P X P When the two parents were crossed, BB X bb, what resulted?
The Results were four heterozygous offspring.
These are the F1 generation

5. F1 X F1
When the two F1 generation were crossed, BB X bb, 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

6. 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

7. 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

8. 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 bred 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)

9. 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

10. P X P Dihybrid Cross RY ry
RrYy

11. Then What? Just like last time he crossed the two F1 Progeny
What do you think he got?
He got 9 Yellow and round
He got 3 Green and round
He got 3 Yellow and Wrinkled
He got 1 Green and Wrinkled

12. What’s The Ratio?
The ratio that he got was?
9:3:3:1

14. How to determine the Gametes
Think Back to Grade 9 math (or before maybe)
What was foil?
(x + n) (y + z) = xy + xz + ny + nz

15. Same rule applies
When looking at the genotypes of the parents to identify the possible gametes perform the same type of foil as we do in math
RrYy = RY + Ry + rY + ry

16. 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

17. The Law of Independent Assortment
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

18. The Law of Independent Assortment
The second law of inheritance states that the inheritance of alleles for one trait does not affect the inheritance of alleles for another trait
Different pairs of alleles are passed to the offspring independently of each other

19. The Law of Independent Assortment
This means that offspring may have new combinations of alleles that are not present in either parent

20. Class Work
Read Pages 136 – 140 (including the Sample problem on page 140)
Complete questions 1-9 on page 141
Complete questions 1-3 on page 29 (course pack)