Punnett Squares

1. Punnett Squares

2. Mendel’s Second Cross • Once Mendel had crossed the P generation, he discovered the traits of the F1 generation • However Mendel continued his observations and crossed the F1 generation

3. Mendel’s Second Cross • When Mendel crossed the second generation, he saw a reappearance of recessive traits • The recessive traits that had been lost reappeared in the F2 generation

4. Mendel’s Second Cross • In our flower problem this means there would be a reappearance of the white flower • The dominant purple flower was seen in 75% of the plants • The recessive white flower was seen in 25% of the traits

5. Predicting Mendel’s Results • Before we can predict Mendel’s results we have to determine the difference between a genetic outcome and a physical outcome • The genetic outcome can be different than the physical outcome

6. Predicting Mendel’s Results • A genotype is the genetic makeup of a trait • A genotype is the makeup of the alleles that makeup a trait • A phenotype is the physical result of genetic information • It is the physical outcome of the genetic information

7. Video • http://www.mtv.com/videos/true-life-im-an-albino/1654511/playlist.jhtml

8. Using Punnett Squares • A phenotype can be dominant or recessive • However a genotype can be double dominant, double recessive or mixed • A mixed genotype is called heterozygous • A pure genotype is called homozygous

9. Using Punnett Squares • Mendel saw a predictable pattern in the results he was getting • These results were predictable because of a small chart that can be easily read • The way that we can determine the outcome of any cross is a Punnett square

11. Homozygous Cross • In a homozygous cross we are going to cross with two different homozygous traits • This means we are going to cross two different pure breeding traits • This is like Mendel’s P generation

12. Homozygous Cross • The first thing to do is to fill in the parents in the Punnett Square • The two parents go along the top and along the side of the square • The dominant trait is represented by a capital letter • The recessive trait is represented by a lower case letter q q Q Q

13. Homozygous Cross • The next thing we would do is fill in the dominant traits • You would fill in the dominant traits for every row and column that is labeled with a dominant trait • You would fill it in with a capital Q q q Q Q

14. Homozygous Cross • The next thing we would do is fill in the recessive traits • You would fill in the recessive traits for every row and column that is labeled with a recessive trait • You would fill it in with a lower case q q q Q Q

15. Homozygous Cross • This would give you all of the possible outcomes if you crossed an homozygous dominant and a homozygous recessive trait • You can figure out the genotypes and phenotypes

16. Practice • Cross a tall pea plant (homozygous dominant) and a dwarf pea plant (homozygous recessive)

17. Heterozygous Cross • The same procedure can be used for a heterozygous cross • You would use the same procedure • First fill in the genotypes of the two parents • Remember to use capital and lowercase letters a A A a

18. Heterozygous Cross • Next you would fill in the dominant traits • Finally you would fill in the recessive traits • This would give you all of the possible outcomes • Are they all the same? a A A a

19. Heterozygous Cross • When you fill in this cross you may notice you get two different phenotypes and three different genotypes • You get a 75% of the dominant phenotype and 25% of the recessive phenotype

20. Practice • Fill in a Punnett square that has two heterozygous green pea pods • Recessive pea pods would be yellow • What is the outcome of genotypes and phenotypes

21. Practice • Fill in a homozygous dominant free earlobe student and a heterozygous free earlobe student • What are the genetic outcomes

22. Practice • Fill out a heterozygous hitch hikers thumb and a homozygous recessive non hitch hikers thumb • What are the genotypes and phenotypes?