Monohybrid &
Dihybrid Crosses
Probability and Genetics!
Why did Mendel use Pea Plants?
- easy to control pollination
Pea Flower
Pistil
(female)
produces
ovules
Stamens
(male)
produce
pollen
- seven distinct, inherited traits
Seed shape
Seed color
Pod shape
Pod color
Flower color
Flower location
Plant size
P1
F1
F1
F2
Cross Fertilization of Parents
Pollen
Cross-Fertilize
Pollen
P
Purebred
Purple-flowered
Parent
P
F1
All Purple-flowered
Offspring
Purebred
White-flowered
Parent
Self-fertilization of F1
F1
Self-Fertilize
F2
75% Purple
25% White
F2
F2
F2
Punnett squares
•The probability the next generation will
exhibit a certain trait can be shown with a
Punnett square.
• The genotype of the mother is
shown on the top of the square
(TT)
• The genotype of the father is
shown on the side of the square
(tt)
• The various combinations of
genes (genotypes) are shown in
the squares. All of these offspring
will be (T) tall and carry the
recessive (t) short allele.
T
T
t
Tt
Tt
t
Tt
Tt
Monohybrid Cross
(crosses that examine the inheritance of only
ONE specific trait)
•What happens if you cross a tall (heterozygous)
plant (Tt) with a short plant (tt)?
T
t
t
Tt
tt
t
Tt
tt
Genotypic Ratio:
TT : Tt : tt
0 : 2 : 2
Phenotypic Ratio:
Tall : Short
2 : 2
Punnett squares
•If deafness in dogs are recessive (dd),
show what offspring could result from two
dogs that are heterozygous (Dd)
3 possible genotypes: DD, Dd, dd
Genotypic Ratio: DD : Dd : dd
1 : 2 : 1
2 possible phenotypes:
Not deaf or Deaf
Phenotypic Ratio: Not Deaf : Deaf
3
: 1
D
d
D
DD
Dd
d
Dd
dd
Test Cross
•Farmer Dan breeds guinea pigs.
•There is a gene in guinea pigs for
teeth length. Teeth can either be
normal (T) or bucked (t).
•Farmer Dan buys what he thinks is a
normal TT guinea pig. How can he be
sure it is “pure” (TT) and not a
heterozygote (Tt)?
Test Cross:
Breed the unknown genotype with a homozygous recessive.
A homozygous dominant
guinea pig will result in
offspring that all have
normal teeth.
T
T
t
Tt
Tt
t
Tt
Tt
A heterozygous guinea
pig will result in a 2:2
or 1:1 ratio of normal to
buck-teethed offspring.
T
t
t
Tt
tt
t
Tt
tt
Dihybrid Cross
(crosses that examine the inheritance of
only TWO different traits)
• Mendel also studied dihybrid crosses.
– He crossed a plant with yellow round peas with a
plant with green wrinkled peas.
Mendel and Dihybrid Crosses
• Mendel began his experiments with purebred
(homozygous) parent plants. RRYY x rryy
– Remember each trait has 2 genes due to getting
half your genes from mom and half from dad.
• How many gamete options do you have for RRYY??
RY
• How many gamete options do you have for rryy?
ry
• These are the gametes from the “P” (parent)
generation.
Mendel and Dihybrid Crosses
“P” generation: RRYY x rryy
RRYY
rryy
ry
ry
ry
ry
RY
RrYy
RrYy
RrYy
RrYy
RY
RrYy
RrYy
RrYy
RrYy
RY
RrYy
RrYy
RrYy
RrYy
RY
RrYy
RrYy
RrYy
RrYy
Mendel and Dihybrid Crosses
What is the genotypic ratio?
16 RrYy:0 anything else
Phenotypic ratio?
16 Round Yellow:
RY
0 anything else
ry
ry
ry
ry
RrYy
RrYy
RrYy
RrYy
RY
RrYy
RrYy
RrYy
RrYy
RY
RrYy
RrYy
RrYy
RrYy
RY
RrYy
RrYy
RrYy
RrYy
Mendel and Dihybrid Crosses
He then crossed two “F1” organisms:
RrYy x RrYy
RrYy
RY
RrYy
RY
Ry
rY
ry
Ry
rY
ry
RRYY RRYy RrYY RrYy
RRYy RRyy RrYy Rryy
RrYY RrYy rrYY rrYy
RrYy Rryy rrYy
rryy
Mendel and Dihybrid Crosses
RY Ry rY ry
RY
Ry
rY
ry
RRYY RRYy RrYY RrYy
RRYy RRyy RrYy Rryy
RrYY RrYy rrYY rrYy
RrYy Rryy rrYy
rryy
What is the genotypic ratio for the F2 generation?
1 RRYY:2 RRYy: 1 RRyy: 2 RrYY: 4 RrYy: 1 rrYY: 2
Rryy: 2 rrYy: 1rryy
Mendel and Dihybrid Crosses
Phenotypic ratio for the F2 generation?
RY
RY
Ry
rY
ry
Ry
rY
ry
RRYY RRYy RrYY RrYy
RRYy RRyy RrYy Rryy
RrYY RrYy rrYY rrYy
RrYy Rryy rrYy
rryy
9:16 Round Yellow,
3:16 Round Green,
3:16 Wrinkled Yellow,
1:16 Wrinkled Green
Mendel and Dihybrid Crosses
9:16 yellow and round,
3:16 green and round,
3:16 wrinkled and yellow,
1:16 wrinkled and green.
Mendel and Dihybrid Crosses
• Mendel continued to find this approximately
9:3:3:1 phenotypic ratio in the F2 generation,
regardless of what 2 different traits he chose.
Mendel and Dihybrid Crosses
–This is due to his Law of
Independent Assortment.
• Each allele pair separate
independently from other allele pairs
during meiosis (gamete formation)…
–different traits are inherited
separately.
Dihybrid Cross Practice
• Suppose you are studying the color and
texture of kernels on a corn cob.
– Kernels can be either purple (R) or yellow (r).
– They can also be smooth (T) or wrinkled (t).
– Predict the phenotypic and genotypic outcome of
crossing a dihybrid (heterozygous) purple smooth
with a yellow wrinkled.
• Don’t forget to answer the questions on the
bottom of your note template.
Answer:
phenotypic and genotypic outcome of crossing
a dihybrid (heterozygous for both) purple smooth with a yellow wrinkled
Cross: RrYy x rryy
RrYy
rryy
ry
ry
ry
ry
RY
RrYy
RrYy
RrYy
RrYy
rY
rrYy
rrYy
rrYy
rrYy
4:16 Purple Smooth,
4:16 Purple Wrinkled,
4:16 Yellow Smooth,
4:16 Yellow Wrinkled
Ry
Rryy
Rryy
Rryy
Rryy
ry
rryy
rryy
rryy
rryy
• Parent generation (AABB x
aabb) are purebred for both
traits because they are
homozygous for both traits.
• Possible gametes
AaBb -> AB, Ab, aB, ab
• Genotype: AAbb ->
Phenotype: Purple, short