Punnett Squares:Genetic Crosses

Making Genetic Predictions

• Mendel found that you could predict thetraits of a percentage of the offspring.

• He invented and used the tool of a Punnettsquare to illustrate these percentages.

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)

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

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.

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

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

Punnett Squares

B B

b

b

Bb = BrownAll of theoffspring willhave brown eyes,but they will beheterozygousindividuals.

Punnett Squares

• Eye Color– Father has Brown eyes and is

heterozygous (Bb).– Mother has Brown eyes and is

heterozygous (Bb).

Punnett Squares

BB Bb

Bb bb

B

b

B bBB = BrownBb = Brownbb = Blue

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

Punnett Squares

• One parent is heterozygous for the trait the other parent is homozygous

• Eye Color– Father (Bb) Brown– Mother (bb) blue

Punnett Squares

Bb bb

Bb bb

b

b

B b

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%

Mendel’s Work

• Pea Color:• Yellow: Y• Green: y• P Generation• YY x yy

Mendel’s Work

• Crossed two of the F1 tosee what the F2 wouldlook like.

Multiple Characteristics

• Punnett Squares work with multiple characteristics.

• Crosses can be done with an assortment of genes.

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

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)

BE Be bE be

BE

Be

bE

be

Phenotype Ratios

• Phenotypes– Brown Hair Brown Eyes _________– Brown Hair Blue Eyes _________– Blonde Hair Brown Eyes _________– Blonde Hair Blue Eyes _________

Phenotype Ratios

• Phenotypes– Brown Hair Brown Eyes ____9____– Brown Hair Blue Eyes ____3____– Blonde Hair Brown Eyes ____3____– Blonde Hair Blue Eyes ____1____

Phenotype Percentages

• Phenotypes– Brown Hair Brown Eyes ____56%___– Brown Hair Blue Eyes ____19%___–Blonde Hair Brown Eyes ____19%___– Blonde Hair Blue Eyes ____6%____