Punnett Squares

Punnett Squares for IGCSE Biology

1. Punnett Squares: Genetic Crosses

2. Making Genetic Predictions Mendel found that you could predict the traits of a percentage of the offspring. He invented and used the tool of a Punnett square to illustrate these percentages.

3. Punnett Squares The Law of Dominance Traits are shown with a Capital letter for dominant and a lower case for recessive – Brown Eyes (B) Blue Eyes (b)

4. Punnett Squares Eye Color – Brown eyes B – Blue eyes b Purebred individual with brown eyes (BB) – Homozygous Purebred individual with blue eyes (bb) – Homozygous Hybrid individual with brown eyes (Bb) – Heterozygous

5. Punnett Squares Eye Color – Father is (BB) – Mother is (bb) Their Genes Are Crossed – BB x bb – Since we are looking at only 1 gene, it is called a monohybrid cross.

6. Law Of Segregation Law of Segregation – Alleles for the same trait must be in separate sex cells. – Father is (BB) as his diploid genotype – Mother is (bb) as her diploid genotype Somatic Cells BB bb Sex cells B B b b

7. Monohybrid Crosses Monohybrid cross deals with one gene – We are working with only eye color Setting up a punnet square The sex cells go on the outside of the square

8. Punnett Squares B B b b Bb = Brown All of the offspring will have brown eyes, but they will be heterozygous individuals.

9. Punnett Squares Eye Color – Father has Brown eyes and is heterozygous (Bb ). – Mother has Brown eyes and is heterozygous (Bb ).

10. Punnett Squares BB Bb Bb bb B b B b BB = Brown Bb = Brown bb = Blue

11. Ratios Genotype Ratio – 1 BB to 2 Bb to 1 bb Phenotype Ratio – 3 Brown eyes to 1 blue Eyes What is the chance of this couple having a child with blue eyes? – 1/ 4 or a 25 % chance

12. Punnett Squares One parent is heterozygous for the trait the other parent is homozygous Eye Color – Father (Bb) Brown – Mother (bb) blue

13. Punnett Squares Bb bb Bb bb b b B b

14. Ratios Genotype Ratio – 0 BB to 2 Bb to 2 bb (reduce) – 0 BB to 1 Bb to 1 bb Phenotype Ratio – 2 Brown eyes to 2 blue eyes (reduce) – 1 Brown eye to 1 blue eye What is the chance of this couple having a child with blue eyes? – ½ or 50%

15. Mendel’s Work Pea Color: Yellow: Y Green: y P Generation YY x yy

16. Mendel’s Work Crossed two of the F1 to see what the F2 would look like.

17. Multiple Characteristics Punnett Squares work with multiple characteristics . Crosses can be done with an assortment of genes .

18. Dihybrid Crosses The Parents are heterozygous for both hair and eye color ( BbEe : Brown hair and Brown Eyes). – B: Brown hair - E: Brown eyes – b: Blonde hair - e : Blue eyes Cross = BbEe x BbEe

19. Mendel’s Laws The law of segregation – You can’t have the same letters in a Sex cell The law of independent assortment – The two genes will have their alleles placed into the same sex cells randomly. Work with one parent at a time – BbEe (B b) (E e) – Possible sex cells – (BE) (Be) ( bE ) (be)

20. BE Be bE be BE Be bE be

21. Phenotype Ratios Phenotypes – Brown Hair Brown Eyes _________ – Brown Hair Blue Eyes _________ – Blonde Hair Brown Eyes _________ – Blonde Hair Blue Eyes _________

22. Phenotype Ratios Phenotypes – Brown Hair Brown Eyes ____ 9____ – Brown Hair Blue Eyes ____3____ – Blonde Hair Brown Eyes ____ 3____ – Blonde Hair Blue Eyes ____ 1____

23. Phenotype Percentages Phenotypes – Brown Hair Brown Eyes ____56%___ – Brown Hair Blue Eyes ____19%___ –Blonde Hair Brown Eyes ____ 19%___ – Blonde Hair Blue Eyes ____ 6%____