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Complex
Inheritance
Polygenic Inheritance
• Many genes working together to
control a single phenotype
• Just break down the word:
– “poly” means many
– “genic” means genes
• Height and skin color are traits that
are controlled by more than one
gene
Polygenic Inheritance
• Here’s how it works. Let’s say 3
genes control height
– A, B, & C are “tall” alleles each
contributing a unit of tallness
– a, b, & c are “short” alleles each
contributing a unit of shortness
Polygenic Inheritance
• THINK! – What is the phenotype for
each of the genotypes below?
• AABBCC = Extremely Tall
• aabbcc =
Extremely Short
• AaBbCc =
Average Height
THINK! - Can you think of other genetic combinations that would still turn out to be
average height? Let’s List them.
Extend: Can you figure out the Genotype
(Letters) for each person?
1. Slightly Below Average?
AaBbcc
3. tad shorter than
extremely tall?
aABBCC
2. Slightly Above Average?
AABBcc
What will the children look like? There are 4
possibilities. Fill in the chart.
AABBCC
x
Tall Father
AaBbcc
Shorter Mother
Child’s allelle (letter)
combination
What is the child’s height?
AABBCc
Very tall
AaBBCc
AABbCc
AaBbCc
Above average
Above average
Average
Polygenic Inheritance
• THINK! – Can you think of more
examples for polygenic inheritance?
– HINT: Think of traits in which the
phenotype has a broad range of
variation.
• Weight
• Hair Color
• Eye Color
What are the Human Blood
Types?
BLOOD TYPE A
BLOOD TYPE AB
BLOOD TYPE B
BLOOD TYPE O
Multiple Alleles
• Example: Blood Type in Humans
– There are 3 alleles floating in the
population: IA, IB, i; but a person will
only get 2 of them.
– The IA & IB alleles are said to be
codominant.
– Codominant means both alleles are
expressed. Thus, the person has type
AB blood.
Blood Type in Humans
Blood Type Problems
• THINK! – Amy is
heterozygous for
type A blood. Jim
is homozygous for
type B blood.
What is the chance
they will have a
child with type B
blood?
IB
IB
IA
i
IA IB
IB i
IA IB
IB i
Blood Type Problems
• THINK! – Jill is
blood type O. She
has two older
brothers. One has
type A blood and
one has type B
blood. What are
the genotypes of
the parents?
IB
i
IA
IA IB
IAi
i
IB i
ii
Sex-Linked Inheritance
• Sex is determined by chromosomes
X
X
X
XX
XX
Y
XY
XY
• THINK! – Who determines the sex of the
baby, mom or dad? Dad
Sex-Linked Inheritance
• Any gene located on the X
chromosome is considered to be sexlinked.
• Sex-linked inheritance was
discovered by Thomas Hunt Morgan
in fruit flies.
Sex-Linked Problems
• THINK! – Morgan
took a pure redeyed female and
crossed it with a
pure white-eyed
male. What did
the Punnett Square
look like?
• THINK! – What did
the F1 generation
look like?
– All red eyed flies
XR
XR
Xr
XR Xr
XR Xr
Y
XR Y
XR Y
Sex-Linked Problems
• THINK! – Next, Morgan
took a red-eyed female
from the F1 generation
and a red-eyed male
from the F1 generation
and mated them. What
did the Punnett Square
look like?
• THINK! - What did the
F2 generation look like?
– Only the males had
the recessive trait of
white eyes
XR
Xr
XR
XR XR
XR Xr
Y
XR Y
Xr Y
Sex-Linked Traits
• In sex linked traits:
– Females need 2 copies of a recessive allele to show the trait
– Males only need 1 copy of a recessive allele to show the trait
– Mothers pass sex-linked traits to their sons via the X
chromosome
• Other sex-linked traits include:
– Colorblindness
– Male pattern baldness
– Hemophilia (blood clotting disorder)
Examples of Vision of the Color Blind
http://www.vischeck.com/examples/
Color Blind Test
http://www.toledobend.com/colorblind/Ishihara.asp