NOTES – Complex
Inheritance Patterns
Most human traits have more than 2 or 3 forms.
Types of inheritance that are more complex than
simple dominance, incomplete dominance or
codominance control these traits.
Multiple Allelic Inheritance (Multiple Alleles)

Traits are controlled by 1 gene with more than 2
alleles

Ex. The ABO Blood Type System in Humans

The gene that controls ABO blood type has 3 alleles:

A = Type A blood B = Type B blood o = Type O Blood

Alleles A and B are codominant and allele o is
recessive
There are 6 possible genotypes and 4
possible phenotypes:

A A or A o
=
Type A Blood

B B or B o
=
Type B Blood

AB
=
Type AB Blood

oo
=
Type O Blood
Blood Cells





There are four basic Blood
types:
Type A with A antigens on the
red cells and anti B antibodies
in the plasma.
Type B with B antigens on the
red cells and anti A antibodies
in the plasma.
Type AB with both A and B
antigens on the red cells and no
blood type antibodies in the
plasma.
Type O with no antigens on the
red cells and both anti A and
anti B antibodies in the plasma
Polygenic Inheritance



Traits are controlled by more than 1 gene
There are many possible phenotypes
Ex. Skin color is controlled by at least 3
genes



A = dark skin
B = dark skin
C = dark skin
a = light skin
b = light skin
c = light skin




Differences in skin color are largely due to
differences in the amount of melanin, a
dark pigment produced by skin cells.
Each “dark skin” allele (A, B, C) codes for
high melanin production
Each “light skin” allele (a, b, c) codes for
low melanin production
Each dark skin allele (A, B, C) in the
genotype adds a small but equal amount
of pigment to the skin
Some possible genotypes and phenotypes:



aabbcc =
AABBCC=
AaBbCc =
very light skin
very dark skin
medium skin
What would a Punnett Square for
skin color look like?

Cross two medium skin tone parents

AaBbCc X AaBbCc

Each parent could produce 8 different
possible combination to pass on: ABC,
ABc, AbC, Abc, aBC, aBc, abC, or abc
Blood Type Is Polygenic

Another level of specificity is added to blood
type by examining the presence or absence of
the Rh protein



Each blood type is either positive "+" (has the Rh
protein) or negative "-" (no Rh protein)
For example, a person whose blood type is "A
positive" (A +), has both type A and Rh proteins on
the surface of their red blood cells
The Rh protein gene has two alleles:

R = protein present (+)
r = protein absent (-)
Polygenic Blood Type

What is the phenotype of a person who
has the blood type genotype ABRr?


What is the phenotype of a person who
has the blood type genotype oorr?


Type AB +
Type O –
What would be the genotype of a person
who is A +

AARR or AoRR or AARr or AoRr