Simple Punnett Square Warm-up
1. In cats, long hair is recessive to short hair. A
true-breeding (homozygous) short-haired
male is mated to a long-haired female. What
is the genotypic and phenotypic ratios for
their offspring?
Simple Punnett Square Warm-Up
2. Mr. and Mrs. Miller both have widow’s peaks
(dominant). Their first child also has a
widow’s peak, but their second child doesn’t.
Mr. Miller accuses Mrs. Miller of being
unfaithful to him. Is he necessarily justified?
Why or why not? Use a Punnett square to
defend your response.
Independent Assortment
Two-Factor Crosses: F1
• Mendel crossed true-breeding plants that
produced only round yellow peas (genotype
RRYY) with plants that produced wrinkled
green peas (genotype rryy)
• All of the F1 offspring produced round yellow
peas. What does this show?
• This shows that the alleles for yellow and
round peas are dominant over the alleles for
green and wrinkled peas.
Two Factor Crosses: F2
• Mendel knew the genotypes of the F1 generation
were RrYy. All heterozygous for seed shape and
seed color genes.
• When he crossed these F1 plants he found that in
the F2 generation, there were plants that had
combinations of phenotypes (combos of alleles)
not found in either parent.
• Therefore, the alleles had segregated
independently of each other.
• We call these tests two-factor or dihybrid crosses.
Independent Assortment
Independent Assortment
• The Principle of Independent Assortment states that
genes for different traits can segregate
independently during the formation of gametes.
• Independent assortment helps account for the many
genetic variations observed in plants, animals, etc.
• Example: Blue eyes and brown hair; one dominant
and one recessive trait are visible in phenotype. The
dominance of brown hair had no bearing on the
expression of blue eyes.
Setting Up a Dihybrid Cross
• In cats long hair is dominant to short hair and
short tail is dominant to long tail. A long-tailed
male cat that is heterozygous for hair length is
mated with a female cat who is heterozygous
for both traits. What are the genotypic and
phenotypic ratios for their kittens?
• First step: Write out genotypes of both parents.
Setting Up a Dihybrid Cross
• Parents genotypes – (H for hair-length; T for tail-length)
– Male: Hhtt
– Female: HhTt
• Second Step: Figure out all possible
combinations (Independent Assortment). Use
FOIL method to determine combinations.
– FOIL (First, Outside, Inside, Last)
Setting Up a Dihybrid Cross
• Possible combinations
(should be four for
each)
• Third Step: Take the
four combinations for
each parent and put
them on the outside of
your dihybrid Punnett
square.
Female Gametes
– Male: Ht, Ht, ht, ht
– Female: HT, Ht, hT, ht
Male Gametes
Fourth Step: Fill in Punnett square
Long-hair, short-tail
Setting Up a Dihybrid Cross
• Fifth step: Tally up all the different genotypes
and phenotypes from the cross.
– Determine the probability of each
genotype/phenotype and right it down as a ratio
(ex. 9:3:3:1).
Setting Up a Dihybrid Cross
• Genotypes
–
–
–
–
–
–
HHTt: 2
HHtt: 2
HhTt: 4
Hhtt: 4
hhTt: 2
hhtt: 2
• Phenotypes
–
–
–
–
Long hair, Short tail: 6
Long hair, Long tail: 6
Short hair, Short tail: 2
Short hair, Long tail: 2
 Phenotypic Ratio:
6:6:2:2 or 3:3:1:1