This document discusses Punnett squares and how they can be used to predict the traits of offspring from genetic crosses. It explains Mendel's work using Punnett squares to determine percentages of traits in offspring from monohybrid and dihybrid crosses. Specifically, it provides examples of Punnett squares showing crosses between parents with different genotypes for traits like eye color to determine the possible offspring genotypes and phenotypes in ratios or percentages.

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%____