Warm-Up # 7
3/12/13
• In humans, brown eyes (B) are dominant
over blue (b)*. A brown-eyed man marries a
blue-eyed woman and they have three
children, two of whom are brown-eyed and
one of whom is blue-eyed.
1) What is the man’s genotype?
2) The woman’s genotype?
3) Draw the Punnett square that illustrates this
marriage.
4) What are the genotypes of the children?
Man’s genotype: Bb
(Heterozygous Dominant)
Woman’s genotype: bb
(Homozygous recessive)
Kid’s genotypes:
2-Bb (Heterozygous
Dominant)
1-bb (Homozygous
recessive)
B
b
b
b
Bb
bb
Bb
bb
Independent Assortment, Dihybrid
Crosses, and Beyond Dominant/
Recessive
Ch 11-3
What’s This???
• Essential Question
– How are independent
assortment and
dihybrids related?
• Objectives
– What is the principle
of independent
assortment?
– What inheritance
patterns exist aside
from simple
dominance?
– Explain how Mendel’s
principles apply to all
organisms?
Independent Assortment
Review
• Genes for different traits separate
independently from one another when
gametes are formed.
• In order to come to the conclusion of
independent assortment, Mendel did an
experiment…
Independent Assortment & 2Factor Crosses
• Mendel followed 2 different genes.
• It is called a two-factor (Dihybrid) cross.
– Type of Punnett Square – 16 squares
– Results: 9:3:3:1 (ratio)
Dihybrid Crosses Example
• Followed two traits
• Gene for seed color and texture
– Seed color………….Y=yellow, y=green
– Seed texture……… R=round, r=wrinkled
• They were dihybrid, meaning both traits
were hybrids (heterozygous)
• Let’s try one………
Dihybrid Cross – RrYy x RrYy

F1 Gametes 
RY
Ry
rY
ry
RY
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
9 round-yellow: 3 round-green: 3 wrinkled-yellow: 1 wrinkled-green
Mendel’s Results
• Out of 556 seeds, he found
– 315 were round and yellow
– 108 were round and green
– 101 were wrinkled and yellow
– 32 were wrinkled and green
• Guess what ratio this is???
9:3:3:1
– 315: 108: 101: 32 ≈ _______
Mendel’s Conclusion
• He concluded: Genes that separate
independently DO NOT influence each
other’s inheritance. (independent
assortment)
• Helps account for the many genetic
variations
When Heredity Follows Different
Rules
• Some alleles are neither dominant nor
recessive.
• Many are controlled by multiple alleles
or multiple genes.
Incomplete Dominance
• Neither allele is completely dominant.
• The heterozygous phenotype is a blend
of the two homozygous phenotypes.
• Ex: Red flower x white flower = pink
flower.
Codominance
• Both alleles are ‘dominant’.
• Both colors show at the same time.
• Ex: black chicken x white chicken =
black AND white chicken (NOT GRAY).
Multiple Alleles
• Many genes have
more than 2
alleles.
• The organism still
gets ONLY 2
alleles for the
trait.
• Ex: rabbit color
and human blood
type.
Human Blood Types
Alleles = IA, IB, and i
Blood types (phenotypes) = A, B, AB, O
Phenotype
(blood type)
Genotype
A
IAIA or IAi
B
IBIB or IBi
AB
IA IB
O
ii
Let’s Practice!
• On the back of your notes, complete
the following crosses and list offspring
possibilities:
1) Mother has AB blood and Father has B
blood (recessive)
2) Mother has AB blood and Father has B
blood dominant.
IA
1)
IB
IA IB
IB
IB IB
MOM
Outcome:
1 child with AB blood (IAIB)
i
IA i
IB i
2 Children with B blood(IBIB, IBi)
1 Child with A blood (IAi)
DAD
2)
IB
IA
IB
IA IB
IB IB
MOM
Outcome:
2 children with AB blood (IAIB)
IB
DAD
IA
IB
IB
IB
2 children with B blood (IBIB)
Multiple Alleles in Dogs
• Labrador Retriever
Genetics
• Black is dominant to
chocolate B or b
• Yellow is recessive
epistatic (when
present, it blocks
the expression of
the black and
chocolate alleles) E
or e
BBEE
BbEE
BBEe
BbEe
bbEE
bbEe
BBee
Bbee
bbee
Albino Rabbit
cc
Full Color Rabbit
CC, Ccch, Cch, or Cc
Chinchilla Rabbit
cchc, cchcch, or cchch
Himalayan Rabbit
chc or chch
Polygenic Traits
•
•
•
•
•
Poly - many
Genic - gene
Traits controlled by 2 or more genes.
Wide variety of phenotypes
Ex: human skin color (controlled by >4
genes) and human eye color.
Environmental Influence
• Characteristics of an organism are
determined by its genes AND its
environment.
• Ex: you may have the genes to be 6’
tall, but if you don’t have proper
nutrition, you won’t be 6’!
Let’s Practice!
• On back of
notes, complete
the following
cross:
• F1 guinea pigs:
male BbSs x
female BbSs
• B=black fur,
b=brown fur
S=short fur,
s=long fur
Dihybird Cross Practice
• You will work with one parnter to
complete the Dihybrid cross worksheet
• Anything not finished today will need to
be finished for homework tonight