NonMendelian Inheritance Patterns
There are other patterns of inheritance that
do not follow Mendel’s Principles.
Be sure you recognize what is different or
what Law is violated by these different
patterns.
1. Co-dominance
Co-dominance=when two alleles are both
expressed (neither masks the other)
• Ex = human blood type
• 3 different alleles (= versions of genes
coding for one trait) include IA, IB, and i
that code for A, B, and “o” blood types.
NOTE: Positive or Negative is another trait
and it follows Mendel’s Law of Dominance
Blood Types (continued)
•
•
•
•
Genotype 
Phenotype
IAi or IAIA  Type A blood
IBi or IBIB  Type B blood
ii  Type O Blood (the most common)
IAIB  Type AB blood (Co-dominant)
Why are there different blood
types anyway?
• Because there is a connection between
blood type and susceptibility to certain
diseases.
--AB nearly completely resistant to
Vibrio cholera (a diarrheal) disease
(European descent)
--O slightly more resistant to malaria
(African descent)
2. Incomplete Dominance
• Also known as “blending” b/c neither allele
in a pair is fully expressed
• Example seen in Shorthorn Cattle
C = color gene with
alleles possible = CR (red), CW (white)
• Cross a red bull with a white cow (Punnett
Square)
Shorthorn Cattle
CRCW  Roan Cattle

3. Linked genes = found on the
same chromosome
• Ex: the genes for petal shape & color are
linked (C=curved or c=straight / W=white
or w=blue); C & w are linked, c & W are
linked.
• Cross two heterozygotes (Punnett Square)
• What Mendelian law do these results
violate?
Crossing over switches linked genes
Read pp. 276-7 in text
4. Sex-linked Traits
• Also known as “X-linked”
• Mendel’s laws indicate that the parents’
gender should not matter. If you cross a
purple flowered plant with a white
flowered, it should not matter which is from
the male pollen and which is from the
female ovule.
• Example 1 = eye color in fruit flies
• Example 2 = Color blindness in humans /
normal color seeing is dominant to color
blindness but the gene that codes for this
trait is linked (found on) to the X
chromosome
• X-C = normal color vision
• X-c = color blindness
• y (doesn’t code for color-seeing)
What letter or number do you see?
Light-sensitive opsin
proteins made in the eye
& needed for color vision
are encoded by a cluster
of genes on the X
chromosome. Mutations
in these genes can lead
to an insensitivity to
certain colors (like red
and green) when seen
together (“color vision
deficiency”)
• Cross a
normal-seeing
male with a
carrier female.
X-C y
x
X-C X-c
• How does a colorblind male
demonstrate NON-Mendelian
inheritance? Hint: How many
recessive alleles must an
organism normally inherit to
express a recessive trait?
5. Multiple Alleles (vs. just two)
• Sometimes a trait is coded for by more
than just two alleles
• Example = human blood type has 3 alleles
A, B, or O
@ 9q34 =
locus of where the
gene is found
6. Polygenic Traits
• More than one set of genes coding for a trait
(NOT the same as multiple alleles)
• Eye color is influenced by many genes coding
for different kinds of pigment as well as where in
the iris those pigments are found (some have
been located on chromosomes 15 & 19)
• Think of having 3+ colors from a crayon box and
how much and in what order you mix them on a
white page dictates the final color.
• http://www.thetech.org/genetics/ask.php?id=203
7. Sex-influenced Traits
• Aka, “Gender-influenced”
• Usually influenced by sex hormones like
estrogen, testosterone
• Examples include baldness in humans,
plumage in birds, horns on cattle
8. Environmentally-influenced
• Color of the Hydrangea flower determined
by the pH of the soil
• Acidic soilblue flower
• Basic soilpink flower
• How about “intelligence” in humans?
9. Transposons (Jumping Genes)
& Their Effect On The Kernel
Color Of Indian Corn
Grains of Indian corn come in
different colors like purple or
yellow. But what happens
when they appear
mottled/streaked?
• This mottling effect defies Mendelian
principles of genetics b/c individual grains
may be multicolored rather than just one
single color or another (purple vs. yellow).
• The explanation? Involves "jumping genes"
or transposons, and earned Dr. Barbara
McClintock the Nobel Prize in Medicine
(1983).
10. Genomic Imprinting
• In recent years it has become apparent that the
parental origin of genetic material does have an
impact on gene expression and this effect has
become known as genomic imprinting.
• See http://epigenome.eu/en/2,5,126 on how
ligers created by male lions mating with female
tigers are much larger than tigons created by a
male tiger with a female lion.
Who’s your Daddy?
• Imprinted genes are either expressed
only from the allele inherited from the
mother or in other instances from the
allele inherited from the father.
• Any examples of this in humans?
Genomic Imprint of
Chromosome #15
• Genetic deletion on the q arm of
Chromosome #15 depends on
whose chromosome you got it from.
• If you got the deletion from Mom
Angelman Syndrome
• If you got it from Dad Prader-Willi
Syndrome.
• Angelman  Absence
of speech, mild to
moderate mental
retardation, small
hands/feet, laugh a lot,
dancing gait so called
“Happy Puppet
Syndrome.”
• Prader-Willi 
Severe obesity,
hyperactivity &
severe mental
retardation
Review
Get organized!
Subtopic of Genetics = Patterns of Inheritance
-- “How Do Characteristics Vary Within a
Species?” Lab
-- Notes of Mendelian Genetics incl. CH 11
-- Chapter 11 Homework Packet (4 pages)
-- Notes on NonMendelian Genetics
-- Passing on My Genes Simulation
-- Testing Concepts + Spot Handout
• Can you identify different described traits
as following Mendelian or Non Mendelian
inheritance?
• Do you understand Mendel’s 3 Principles
of Inheritance?
• How do NonMendelian Patterns “violate”
Mendelian genetics?
• Can you do a monohybrid, dihybrid, or
sex-linked cross? 1. Show the genotype of
parents; 2. The resulting Punnett square; &
3. The Phenotypic Ratios