NON MENDELIAN
INHERITANCE
EXCEPTIONS TO THE RULES
DOMINANCE: IT’S NOT JUST A GOOD
IDEA… IT’S THE LAW!
“Complete Dominance” =
• Dominant alleles cannot be “hidden”
• AA or Aa will both show the “A” trait
• Recessive alleles can be masked, however…
• aa the only genotype to show the recessive trait…
CONFOUNDING OBSERVATION?
Sometimes the heterozygous offspring have a trait
that isn’t exactly the trait of either purebred parent
Translation?
 Gregor Mendel was either twice the genius people give him
credit for being…
 OR…
 He was lucky to have chosen the pea plant!
INCOMPLETE DOMINANCE VS. CODOMINANCE
“Incomplete Dominance”
Codominance
Hybrid (heterozygote) is
a “blend” of two
parent phenotypes
Dominant allele isn’t
completely so…
• Again, no “recessive”
allele
• Both "dominant" traits
appear together in the
phenotype of hybrid
organisms.
AA = Dominant 1
BB = Dominant 2
AB = Dominant 1 &
Dominant 2
A
A
a
Aa
Aa
a
Aa
Aa
IS INCOMPLETE DOMINANCE REAL?
• On the macroscopic level, snapdragons are pink…
• In the microscopic level, they are red and white
(just like codominance)
ANOTHER EXAMPLE OF
CODOMINANCE:
ROAN COWS
AND HORSES
Red x White  Red
and White hairs
MULTIPLE ALLELES
• Phenotype affected by more than 2
alleles
• not blended phenotype
• human ABO blood groups
• 3 alleles
• IA, IB, i
• IA & IB alleles are co-dominant
• i allele recessive to both
EXAMPLE OF MULTIPLE ALLELES:
HUMAN BLOOD TYPE
Alleles:
IA=A carbohydrate
IB=B carbohydrate
i=O (no carbohydrate)
TYPES OF BLOOD
A
Anything strange
about this one?
B
AB
O
PLEIOTROPY
• Most genes are pleiotropic
• one gene affects more than one phenotypic
character
• 1 gene affects more than 1 trait
• dwarfism (achondroplasia)
• gigantism (acromegaly)
ACROMEGALY: ANDRÉ THE GIANT
INHERITANCE PATTERN OF
ACHONDROPLASIA
Aa
x aa
Aa
x Aa
dominant
inheritance
A
a
a
a
Aa
Aa
dwarf
dwarf
aa
aa
A
a
A
a
AA
Aa
lethal
Aa
aa
50% dwarf:50% normal or 1:1 67% dwarf:33% normal or 2:
POLYGENIC TRAIT
More than 1 gene controls the trait
Each gene may be inherited separately
Symbols of polygenic traits same as
“normal” Mendelian genetics:
 A, a = gene 1
 B, b = gene 2
 Etc…
POLYGENIC INHERITANCE
• Some phenotypes determined by
additive effects of 2 or more genes
on a single character
• phenotypes on a continuum
• human traits
•
•
•
•
•
skin color
height
weight
intelligence
behaviors
EXAMPLE OF A POLYGENIC TRAIT
Eye color
There are genes for
Tone of pigment (what color it is)
The amount of pigment
Position of pigments (look at people’s eyesthere are many different patterns in the iris)
SEX-LINKED INHERITANCE
Alleles carried on sex chromosomes
Gender influences phenotype
SYMBOLS OF SEX-LINKED
INHERITANCE
XH = dominant allele
Xh = recessive allele
Y = no allele for this trait
XH
XH
Y
Xh
EXAMPLES:
Hemophilia
Red-green color-blindedness
Male pattern baldness
X-INACTIVATION…
If a male is XY, female is XX, how can females get
“double” the amount of “X” chromosome DNA?
 The answer? The second “X” is turned off in females
 = “Dosage compensation” or “X inactivation”
ENVIRONMENTAL INFLUENCES
Diet, exercise, stress…
Sunlight, water alter phenotypes in plants
Even temperature affects coloration on Siamese
cats!