1. Mendel’s Dihybrid Cross
& PUNNET SQUARES

2. Punnett Squares
Shorthand way to find out the expected proportions of possible genotypes in offspring of a cross.
Need the genotypes of the parents to predict the possible genotypes of their offspring

3. Punnett Square- Monohybrid Cross

4. How to make a Punnett Squares
The types of gametes produced by each F1 parent are shown along the top and left sides of the square.
The possible gene combinations for the F2 offspring appear in the four boxes that make up the square.
The letters in the Punnett square represent alleles.
Example, T represents the dominant allele for tallness and t represents the recessive allele for shortness

6. In this cross we see:
Genotype:
One fourth (1/4) of the F2 plants have two alleles for tallness (TT)
2/4, or 1/2, of the F2 plants have one allele for tallness and one for shortness (Tt)
One fourth (1/4) of the F2 plants have two alleles for shortness (tt)

7. Phenotype:
Because the allele for tallness is dominant over the allele for shortness we see:
3/4 of the F2 plants tall
¼ of the F2 plants are short.
Thus, the ratio of tall plants to short plants is 3 : 1.

8. Other monohybrid crosses
Recall: Mendel had tested 7 other traits on the pea plant:
Round seeds crossed with wrinkled produced round seeds in F1, indicating round as the dominant allele and wrinkled as recessive
Yellow seeds crossed with green seeds produced yellow seeds in F1, indicating yellow as the dominant allele and green colour as recessive

9. Dihybrid Cross
A cross involving TWO different traits is called a dihybrid cross
Di- means two
Mendel watched to see if two traits stay together in the next generation OR will they be inherited independently of each other
Mendel performed another set of corsses in which he used peas that differed from each other in two traits, instead of one

10. F1: First Generation
True breeding pea plants: round yellow seeds crossed with true breeding pea plants wrinkled green seeds
Genotype for round yellow seeds: RRYY
Genotype for wrinkled green seeds: rryy
Dihybrid expect dominant trait to prevail for both traits in F1
F1 of dihybrid cross were ALL Round and Yellow

12. F2: Second Generation
Using F1 seeds to pollinate, he found a variety of phenotypes:
Round and yellow seeds (like P1)
Wrinkled green seeds (P1)
Round green seeds (NEW Phenotype combination)
Wrinkled yellow seeds! (NEW Phenotype combination)
Ratio of 9: 3: 3: 1

14. Law of Independent assortment
Genes for different traits are inherited independently of each other
Ex. Different traits in pea plant are observed in dihybrid cross such as seed shape and seed colour.
When pea plants that have a HETEROZYGOUS genotype, RrYy, they will produces gametes:
where the alleles R and r will separate from each other
and alleles from the Y and y will separate from each other
This means that alleles will recombine in 4 different ways

15. * IF alleles are inherited TOGETHER, we would see only TWO kinds of seeds, round yellow and wrinkled green (the same phenotypes as P1)

16. Punnett Square: Dihybrid Cross
Mendel Found that traits for seed shape and seed colour would be inherited independantly of each other.
P1 gametes: ALL RY or ry
F1 gametes: RY, Ry, rY, ry
Use a punnett square to show the possible genotypes of the offspring (F2 generation)

18. Results: Phenotype/Genotype:
(9) Round and Yellow: RRYY, RRYy, RrYY, RrYy
(3) Round and Green: RRyy, Rryy
(3) Wrinkled and Yellow: rrYY, rrYy
(1) Wrinkled and Green: rryy
Ratio of Phenotype:
9: 3: 3: 1

19. Word List: Punnett Square Monohybrid Cross Dihybrid Cross
Law of Independent Assortment