1. Using Punnett Squares
Dominant & Recessive

2. Punnett Squares
It is a special chart, or grid system, named after its inventor Reginald Crundall Punnett. (devised in the early 1900’s)

3. Punnett Squares Geneticists often
use Punnett squares to depict genetic crosses and to determine the possible gamete combinations of the offspring.

4. Punnett Squares
Punnett square: a diagram used to visualize genetic crosses (a chart that shows all the possible combinations of alleles that can result from a genetic cross)

5. Punnett Squares
How to work Dominant and Recessive Genetic Problems with Punnett Squares

6. Show a cross between a tall and short pea plant
Step One
Write down the set of contrasting traits.
Assign the capitol letter to the dominant trait and the lower case letter to the recessive trait.
Tall
Short T t

7. Show a cross between a tall and short pea plant
Step Two
Write down possible genotypes and phenotypes. TT Tt tt
Tall
Short

8. Show a cross between a tall and short pea plant
Step Three
Fill in the genotypes of the parents on the punnett square.

9. Punnett Squares t t
On the top- the possible gametes of the female are listed.
Down the left side- the possible gametes of the male are listed T T

10. Show a cross between a tall and short pea plant
Step Four t
Fill in the genetic possibilities of the F1 T T

11. Show a cross between a tall and short pea plant
Step Four t t
Take first allele from the male and first allele from the female and place in the first square. T t T T

12. Show a cross between a tall and short pea plant
Step Four t t
Take first allel from the male and second allele from the female and place in the second square. T t T T

13. Show a cross between a tall and short pea plant
Step Four t t
Take second allele from the male and first allele from the female and place in the third square. T T T t

14. Show a cross between a tall and short pea plant
Step Four t t
Take second allele from the male and second allele from the female and place in the fourth square. T T T t

15. Show a cross between a tall and short pea plant
Step Four t
Fill in the genetic possibilities of the F1 T T t T t T T t T t

16. Show a cross between a tall and short pea plant
Step Five
Write down the genotypic ratio and the phenotypic ratio.
Purebred dominant : Hybrid : Purebred recessive
Tall : Short

17. Show a cross between a tall and short pea plant
Step Five
Write down the genotypic ratio and the phenotypic ratio. T t T T
0:4:0
4:0

18. Punnett Squares F f
The gametes are then combined in each of the boxes within the square to give the possible gamete combinations of the offspring. F1
1:2:1
3:1

19. Punnett Squares F f
Besides showing possible gene pairings, a Punnett square gives the probability of each pairing.
It shows how often, on the average, a given pairing will occur. F1
1:2:1
3:1

20. Punnett Squares
Punnett squares cannot tell which traits a specific offspring will exhibit but rather the chance, or odds, of having certain traits
1:2:1
3:1

21. Punnett Squares
The number of boxes in a Punnett square does not stand for the number of offspring an organism will produce. The boxes represent the genetic possibilities of the offspring.

22. Punnett Squares
A small # of offspring- may not have the exact ratio of gene pairings.
1:2:1
3:1