Non-Mendelian Genetics
Inheritance Concepts Mendel
Never Imagined!
Mendelian Genetics
(Complete Dominance)

Only two possible Phenotypes: either
dominant or recessive

Tall plant X Short plant = Tall plant
(TT)
(tt)
(Tt)
Non-Mendelian Genetics

But, not all inheritance is based on the
rules of Complete Dominance!!
Introduction
Mendelian Genetics describes inheritance
patterns based on Complete Dominance
or Recessiveness.
 There are other types of inheritance that
Mendel never considered:






Incomplete Inheritance
Codominance
Multiple Alleles
Polygenic Traits
Sex-linked
Incomplete Inheritance
The hybrid (heterozygous) offspring
displays a THIRD Phenotype!! Neither trait
is completely dominant, as a result, there
appears to be a blending phenotype.
Red Flower X White Flower = Pink
(RR)
(WW)
(RW)

Incomplete Inheritance Problem

What is the probability of pink flowers if
pink flowers are bred with red flowers?
50%
chance
of Pink
Flowers
Incomplete Inheritance Problem

What is the probability of white flowers if
pink flowers are bred with pink flowers?
CoDominance

Both traits are dominant, and show up in
the phenotype together. Co means
“together”

Black Cow X White Cow = Spotted Cow
(BB)
(WW)
(BW)
CoDominance Problem

What are all the possible phenotypes
when two spotted cows are bred?
Possible
phenotypes
are a black
cow, 2
spotted
cows, and a
white cow
Below are imaginary organisms. Note horn shape, leg
length, fur color and tail shape. Now determine the type of
inheritance of the four traits by examining the genotypes of
the parents and offspring.
Blood type displays both co-dominance
and complete dominance

Red Blood cells can either have a carbohydrate
on their surface or not.


The presence of a carbohydrate (I) is dominant to the
absence of a carb (i).
Additionally, there are two types of carbs that
may exist on the surface of RBCs called A (IA)
and B (IB).

Cell surface carbs A and B are codominant, which
means they could also show up at the same time
on an RBC.
Blood Type Inheritance Rules:
1.
2.
3.
4.
A person with the IA allele
will have A carbohydrates.
A person with the IB allele
will have B carbohydrates.
A person with the recessive
i allele will have no
carbohydrates.
Or a person with both IA and
IB alleles will have both A
and B carbohydrates on
their cells.
Cells
Genotypes
Blood types
Type A
A
A
A
I i, I I
blood
Type B
B
B
B
I i, I I
blood
IAIB
Type AB
blood
ii
Type O
blood
Blood Type Problems
If a woman with AB blood
has children with a man
who has type O, what will
be the possible genotypes
of their children? What will
be their blood types?
A
I
i
i
B
I
I A i IB i
I A i IB i
Blood Type Problems
Blood Type Problem 2: A
woman with type B blood
has a child with type O
blood. How is this possible
if her husband has type A
blood?
Multiple Alleles

When more than 2 varieties exist in a trait.
Many animals have a variety of coat
colors.
Polygenic Traits
Require more than one gene (allele) to
determine trait.
Skin tone is determined by 4-6 genes—that
means that there may be six different
chromosomes involved!
Sex-linked Inheritance

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Genes for some traits are found on the sex
chromosomes (X or y)
Most of these traits are recessive the normal
gene is dominant
Heterozygous Females (XXc) are carriers. They
do not show the trait, but carry a gene for the
trait.
Homozygous Females (XcXc) have the trait
Males with the gene (XcY) have the trait.—They
do not have another X to counterbalance the
affected gene
Color blindness is a sex-linked
recessive trait. The gene for this trait is
inherited through the X chromosome.
If a woman with normal vision
has children with a man
who is colorblind,
what are the chances
that their children will be
colorblind?
Will any children be carriers
of the trait?
Xc
Y
X
c
XX
XY
X
XXc XY
Non-Mendelian Genetics
What are the similarities and differences
between each of these forms of inheritance?
How will you remember? What are
examples of each type?
Incomplete inheritance
 CoDominance
 Multiple Alleles
 Polygenic Traits
 Sex-linked inheritance
