What is the difference between a genotype and
a phenotype? Provide an example of each.
• Genotype – allele combination for a trait (e.g.
RR, Rr, rr)
• Phenotype - the physical feature resulting
from a genotype (e.g. red, white)
• In leghorn chickens, colored feathers are due
to a dominant allele, white feathers are
recessive. Cross a homozygous white chicken
with a heterozygous chicken.
• Draw a Punnett square and provide the ratio
of possible genotypes & phenotypes.
Mendel’s Laws
Law of Dominance
In a cross of parents that are
pure for contrasting traits, only
one form of the trait will appear in
the next generation.
All the offspring will be
heterozygous and express only the
dominant trait.
RR x rr yields all Rr (round seeds)
copyright cmassengale
6
Law of Dominance
copyright cmassengale
7
Law of Segregation
• During the formation of gametes (eggs or
sperm), the two alleles responsible for a
trait separate from each other.
• Alleles for a trait are then "recombined"
at fertilization, producing the genotype for
the traits of the offspring.
copyright cmassengale
8
Applying the Law of Segregation
copyright cmassengale
9
Law of Independent Assortment
• Alleles for different traits are
distributed to sex cells (& offspring)
independently of one another.
• This law can be illustrated using
dihybrid crosses.
copyright cmassengale
10
Dihybrid Cross
• A breeding experiment that tracks
the inheritance of two traits.
• Mendel’s “Law of Independent
Assortment”
• a. Each pair of alleles segregates
independently during gamete formation
• b. Formula: 2n (n = # of heterozygotes)
copyright cmassengale
11
Summary of Mendel’s laws
LAW
DOMINANCE
SEGREGATION
INDEPENDENT
ASSORTMENT
PARENT
CROSS
OFFSPRING
TT x tt
tall x short
100% Tt
tall
Tt x Tt
tall x tall
75% tall
25% short
RrGg x RrGg
round & green
x
round & green
9/16
pods
3/16
pods
3/16
pods
1/16
pods
copyright cmassengale
round seeds & green
round seeds & yellow
wrinkled seeds & green
wrinkled seeds & yellow
12
Dihybrid Cross
• Traits: Seed shape & Seed color
• Alleles: R round
r wrinkled
Y yellow
y green
•
RrYy
RY Ry rY ry
x
RrYy
RY Ry rY ry
All possible gamete combinations
copyright cmassengale
13
Dihybrid Cross
RY
Ry
rY
ry
RY
Ry
rY
ry
copyright cmassengale
14
Dihybrid Cross
RY
Ry
rY
ry
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
RY
copyright cmassengale
Round/Yellow:
Round/green:
wrinkled/Yellow:
wrinkled/green:
_:_:_:_ phenotypic
ratio
Phenotypes
9 round, yellow - __%
3 round, green - __%
3 wrinkled, yellow – __%
1 wrinkled, green – __%
15
Dihybrid Cross
RY
Ry
rY
ry
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
RY
copyright cmassengale
Round/Yellow:
9
Round/green:
3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1 phenotypic
ratio
Phenotypes
9 round, yellow - 56%
3 round, green - 19%
3 wrinkled, yellow – 19%
1 wrinkled, green – 6%
16
Dihybrid Cross
Round/Yellow: 9
Round/green:
3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1
copyright cmassengale
17
Dihybrid Cross Practice
Traits: Plant Height x Plant Color
Alleles: T = Tall
t = short
G = Green
g = yellow
TtGg x ttgg
What are the possible gamete combinations?
(genotypes of offspring?)
Dihybrid Cross Practice
Traits: Plant Height x Plant Color
Alleles: T = Tall
t = short
G = Green
g = yellow
TtGg x ttgg
TG Tg tG tg x tg tg tg tg
What are the possible gamete combinations?
(genotypes of offspring?)
Dihybrid Cross
copyright cmassengale
20
Dihybrid Cross
TG
Tg
tG
tg
tg
tg
Tall/Green:__
Tall/yellow: __
Short/Green: __
Short/Yellow: __
_:_:_:_ phenotypic
ratio
tg
Phenotypes
Tall, green - __%
Tall, yellow - __%
Short, Green – __%
Short, yellow – __%
tg
copyright cmassengale
21
Dihybrid Cross
TG
Tg
tG
tg
TtGg
Ttgg
ttGg
ttgg
TtGg
Ttgg
ttGg
ttgg
tg
TtGg
Ttgg
ttGg
ttgg
tg
TtGg
Ttgg
ttGg
ttgg
tg
tg
copyright cmassengale
Tall/Green:__
Tall/yellow: __
Short/Green: __
Short/Yellow: __
_:_:_:_ phenotypic
ratio
Phenotypes
Tall, green - __%
Tall, yellow - __%
Short, Green – __%
Short, yellow – __%
22
Dihybrid Cross
TG
Tg
TtGg
Ttgg
ttGg
ttgg
TtGg
Ttgg
ttGg
ttgg
tg
TtGg
Ttgg
ttGg
ttgg
tg
TtGg
Ttgg
ttGg
ttgg
tg
tg
tG
tg
copyright cmassengale
Tall/Green: 4
Tall/yellow: 4
Short/Green: 4
Short/Yellow: 4
4:4:4:4 phenotypic
ratio
Phenotypes
Tall, green – 25 %
Tall, yellow – 25 %
Short, Green – 25 %
Short, yellow – 25 %
23
Dominance
Practice
A woman who is heterozygous type B marries a
man who is type O. What are the possible
blood types that their children could be?
Practice
The gene for brown eye color is dominant to the
allele for blue eyes. A man with blue eyes
marries a brown eyed woman whose mother
had blue eyes. What are the genotypes &
phenotypes of their children? What are the
genotypic and phenotypic ratios?
Henry Anonymous, a film star, was involved in a
paternity case. The woman bringing suit had
two children, on whose blood type was A and
the other whose blood type was B.
Her blood type was O, the same as Henry’s!
The judge in the case awarded damages to the
woman, saying that Henry had to be the father
of at least one of the children. Obviously, the
judge should be sentenced to Biology. For
Henry to have been the father of both children,
his blood type would have had to be what?