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)
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 Bb = Brown All of the offspring will
have brown eyes,
but they will be
heterozygous
individuals. b

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

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

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
B b Bb bb b

14. Ratios Genotype Ratio – 0 BB to 2 Bb to 2 bb (reduce)
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%____