Transcript Punnett Squares: Genetic Crosses

Punnett Squares:
Genetic Crosses
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.
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 = Brown
All of the
offspring will
have brown eyes,
but they will be
heterozygous
individuals.
Punnett Squares
• Eye Color
– Father has Brown eyes and is
heterozygous (Bb).
– Mother has Brown eyes and is
heterozygous (Bb).
Punnett Squares
B
BB
b
Bb
Bb
bb
B
b
BB = Brown
Bb = Brown
bb = 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
B
b
b
b
Bb
bb
Bb
bb
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 to
see what the F2 would
look 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
– Brown Hair Blue Eyes
– Blonde Hair Brown Eyes
– Blonde Hair Blue Eyes
____9____
____3____
____3____
____1____
Phenotype Percentages
• Phenotypes
– Brown Hair Brown Eyes
– Brown Hair Blue Eyes
–Blonde Hair Brown Eyes
– Blonde Hair Blue Eyes
____56%___
____19%___
____19%___
____6%____