1. The Monohybrid Cross …

2. Monohybrid Crosses
Monohybrid crosses are used to determine the probability of certain genetics outcomes in offspring.
Mendel is famous for his discovery that there are 2 factors for each trait called alleles.
Genes can be dominant or recessive

3. Expectations for Genetics Problems
After reading the problem, determine the genotype of the parents
Indicate the different possible gametes
Show possible combinations using a punnett square and state the genotypic and phenotypic ratios
Answer the questions in sentence form

4. Rules for Crosses Capital letters represent the dominant trait
Small letters represent the recessive trait
Usually the first letter of the allele is used (ie. T (tall)

5. 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

6. 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

7. 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

8. 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

9. Now the analysis… 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.

10. 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.

11. 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

12. 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

13. Now, more analysis… F2 generationTT Tt tt T t
1 of the 4 has the genotype TT, have the genotype Tt, and 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

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

15. The Hamster problem…
Suppose that hamster 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

16. Both Parents can give either gamete
The Hamster 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

17. The Hamster 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 hamster’s coat colour is 3 Brown : 1 Grey

18. The Hamster 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

19. The Test Cross
What if you had a female brown hamster at home, and wanted to know her GENOTYPE?
How would you be able to find out? We’d use a Test Cross
Used for organisms with an unknown genotype (but known dominant phenotype)
determines if their dominant phenotype is due to a heterozygous or homozygous genotype
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

20. Baby Hamsters…
Lets say we do this cross…and get this result? x =

21. Therefore, we know this was the cross: BB x bb
We know the grey hamster is bb. But the Brown hamster could be BB or Bb.
All Brown offspring means that our brown hamster was genotype BB, because 100% of the offspring had the dominant trait.
Therefore, we know this was the cross: BB x bb

22. Baby Hamsters…
How about this result? x =

23. Therefore, we know this was the cross: Bb x bb
Half brown and half grey offspring means that our brown hamster 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.
Therefore, we know this was the cross: Bb x bb

24. Test Cross Practice
In dogs, there is an hereditary deafness caused by a recessive gene,“d.” A kennel owner has a male dog that she wants to use for breeding purposes.
The dog can hear, so the owner knows his genotype is either DD or Dd.
If the dog’s genotype is Dd, the owner does not wish to use him for breeding so that the deafness gene will not be passed on.
What genotype should the owner cross the dog with to determine his genotype?
Do the cross for both possible options (DD or Dd). State the genotypic and phenotypic ratios.

25. Practicing Monohybrid Crosses
1) a) Mendel crossed a purebred tall (TT) plant with a
 purebred dwarf (tt) plant. What are the possible genotypes for this cross?

26. Practicing Monohybrid Crosses - 1) b)
b) If two F1 plants are crossed, what is the probability of having a dwarf plant?

27. Practicing Monohybrid Crosses - 2)
2) In humans, the allele A, for pigment formation is dominant to the allele a, for inability to form pigment. aa individuals are albino. Determine the expected genotypic and phenotypic ratios expected from a cross between two individuals heterozygous for this trait.

28. Practicing Monohybrid Crosses - 3)
3) Some people are PTC tasters (T) and some are non-tasters (t). What is the probability of a grandchild being a non-taster if his paternal grandfather is a homozygous dominant taster, his paternal grandmother is a non-taster and his mother a non- taster?

29. Now work on your genetics problem solving work package.