1. The Monohybrid Cross …

2. Mendel’s Work, applied
These problems will focus on one trait (like plant height for example)‏
We will use Punnett’s Squares to determine what happens to this trait over the course of a few generations
This method can be used on almost all genetics problems

3. An Example…
We will replicate one of Mendel’s examples.
Cross a pure bred tall plant with a pure bred dwarf plant. Use Punnett’s squares to show the F1 and F2 generations

4. Pheno geno P1 X P2 X Tall Short TT tt t T
Possible
Gametes
Tall
Short TT tt t T
In this case, each parent can only give one possible gamete

5. Now to make the square…
The number of possible gametes per parent decides the dimensions of the square.
Our example will use a 1 x 1 square… T
Parent’s Gametes t
T t
Offspring’s genotype
Parent’s Gametes

6. Now the analsis… F1 generation
T t
This means that all of the offspring in the F1 generation will have the genotype T t. The Genotypic ratio is all Tt
Knowing that T is the dominant allele, all of the offspring will have the phenotype “Tall”. The Phenotypic Ratio is All Tall.

7. All of the F1 pea plants are Tall
Our Statement must state what we see!
All of the F1 pea plants are Tall
Next, we will cross two plants from the F1 generation.

8. Pheno geno F1 X F1 X Tall Tall Tt Tt T or t T or t
Possible
Gametes
Tall
Tall Tt Tt
T or t
T or t
Each parent can give either one of their gametes

9. Now, to make the square: One parent’s possible gametes T t
The other’s possible gametes T t
Offspring’s Genotypes TT Tt Tt tt

10. Now, more analysis… F2 generationTT Tt tt T t
1 of the 4 has the genotype TT,
2 have the genotype Tt, and
1 has the genotype tt.
The genotypic ratio is 1 TT : 2 Tt : 1 tt
3 of the 4 offspring will be Tall (either Tt or TT), and 1 will be short (tt). The Phenotypic ratio is going to be 3 Tall : 1 Short

11. Remember! The Statement!
State what we SEE
The F2 phenotypic ratio for pea plants is
3:1 Tall:Short.

12. The Guinea Pig problem…
Suppose that Guinea Pig colour is determined by one gene. B (brown coat) is dominant to b (grey coat)‏
Show the F1 generation for a cross of B b x B b using Punnet’s squares
Give the Genotypic and phenotypic ratios for each generation

13. The Guinea Pig Problem:
Pheno
geno
P X P2 X
Possible
Gametes
Brown
Brown Bb Bb
B or b
B or b
Both Parents can give either gamete

14. The Guinea Pig Problem B b Genotypic Ratio: 1BB : 2Bb : 1 bb
Phenotypic Ratio:
3 Brown : 1 Grey BB Bb B b Bb bb
Statement: The F1 phenotypic ratio for Guinea Pig’s coat colour is 3 Brown : 1 Grey

15. The Guinea Pig problem… Tips for monohybrids
Do each generation one step at a time
What gametes can each parent contribute?
What are the dimensions of each square?
Count how many of each type of offspring you will have in terms of BOTH genotype and phenotype.
REMEMBER YOUR STATEMENTS

16. The Test Cross
What if you had a female brown Guinea Pig at home, and wanted to know her GENOTYPE?
How would you be able to find out? We’d use a Test Cross
A Test Cross is crossing the unknown with a homozygous recessive.
In this case, you would need a male homozygous recessive… he would have to be grey

17. Baby Guinea Pig…
Lets say we do this cross…and get this result? x =

18. This was the cross: BB x bb
We know the grey Guinea Pigs is bb. But the Brown Guinea Pig could be BB or Bb.
All Brown offspring means that our brown Guinea Pig was genotype BB, because 100% of the offspring had the dominant trait.

19. Baby Guinea Pigs…
How about this result? x =

20. This was the cross: Bb x bb
Half brown and half grey offspring means that our brown Guinea Pig was genotype Bb, because Half got the ‘B’ gamete and half got the ‘b’ gamete.
Half express the recessive trait, and half express the dominant trait.