Beyond Mendelian
Inheritance
INCOMPLETE DOMINANCE
-NEITHER ALLELE IS COMPLETELY DOMINANT
OVER THE OTHER
-THE HETEROZYGOUS PHENOTYPE IS
A BLENDING OF THE TWO
HOMOZYGOUS PHENOTYPES
Red flower = RR
Pink flower = Rr
White flower = rr
Sample problem:
Red snapdragon flowers (R) are incompletely
dominant to white snapdragon flowers (r).
A heterozygous flower is crossed with a white flower.
What is the genotypic and phenotypic ratios of the
offspring?
CODOMINANCE
-BOTH THE DOMINANT ALLELES ARE
EXPRESSED IN THE HETEROZYGOUS
INDIVIDUAL.
-USE 2 DIFFERENT CAPITAL LETTERS TO
RERESENT THE DOMINANT ALLELES
Sample problem:
Red and white coat color are co-dominant in cattle.
Two heterozygous cows are crossed. What is the
genotypic and phenotypic ratio of the offspring?
MULTIPLE ALLELES
Several dominant alleles and/or recessive
alleles can be combined to create multiple
phenotypes.
Blood Type
In human blood, type A blood and type B blood
are codominant. However, there is a recessive
allele for type O.
Type A and B show regular dominance over this
recessive allele. Thus, the only way to be type
O is to be homozygous recessive.
BLOOD TYPE
Type A
Type B
Type AB
Type O
CAN RECEIVE FROM:
CAN DONATE TO:
Human Blood Genotypes and
Phenotypes
Phenotype
• 1.
• 2.
• 3.
• 4.
Genotype(s)
Sample Problem #1:
Adam Sandler is homozygous for type A blood.
Megan Fox is heterozygous for type B. If they have
kids what will be the genotypic and phenotypic ratios
of the possible kids?
Sample Problem #2:
Jimmy Fallon has type O blood. Jennifer
Aniston has type AB blood. If they have kids
what will be the genotypic and phenotypic ratios
of the possible kids?
Sample Problem #3:
Channing Tatum has type A blood. Ms. Palmeri
has type B blood. They have a baby with type O
blood. How is this possible? Show your work!
Sex-Linked Traits
• Sex-linked traits are traits that are
controlled by genes on the sex
chromosomes
– The X and Y chromosomes
Are they controlled by both
sex chromosomes?
• Most sex-linked traits are controlled by genes
on the X chromosome.
• This is because an X chromosome is much
larger than a Y chromosome.
• A few traits are suspected to be controlled by
genes on the Y chromosome, however there
is less research about Y-linked traits.
Different Forms of Sex-linked
Inheritance
• There are three different forms of sexlinked inheritance that we will be
examining:
– X-linked recessive inheritance
– X-linked dominant inheritance
– Y-linked inheritance
X-linked Recessive
Inheritance
• X-linked recessive traits are traits resulting
from a recessive allele on the X chromosome.
• There are over 100 different human
conditions that are caused by recessive
alleles found on the X chromosomes.
• X-linked recessive alleles are represented by
a X , superscript lower case letter
X-linked Recessive
Inheritance
• These traits tend to show up in males
more than females.
– Why?
X-linked Dominant Inheritance
• X-linked dominant traits are traits that result
from the presence of a dominant allele on the
X chromosome.
• Unlike X-linked recessive traits, females and
males both require only ONE dominant allele
in order to express the trait.
• X-linked dominant traits are represented by
an X, superscript capital letter or a (+)
Polygenic Traits
• Most of your traits are controlled by the
interaction of many genes.
• Multiple genes working together produce a
continuous distribution in a “Bell Shape”
curve of degrees.
Examples of Polygenic Traits
•
•
•
•
•
•
•
Body Type
Height
Skin Color
Hair color
Eye color
Intelligence
We often see the
famous “Bell Curve”
Individual genes of a
polygenic trait follow
Mendel's laws but
together do not
produce Mendelian
ratios.
Recent studies show …
•
•
•
•
•
•
Hypertension
Diabetes
Cancers
Allergies
Cardiovascular diseases
Behavioral traits (alcoholism and phobias)
…..have some genetic link but also
environmental explanation.
Environmental Effects
Expression of some
genes may be
impacted by
environment
Gene for pigment
production expressed
in cooler regions of
body
Another example of environmental influence:
• Hydrangeas – same genotype, different
environments  different color flowers
Acid pH
Alkaline pH 
• Epistasis
–Action of genes at one loci modify
expression genes at another loci
• Effects often complex
–Examples
• Normal expression of ABO blood type
depends on functional fucosyltransferase 1
– Recessive = no expression/attachment of
antigen to blood cell
• Other examples:
– Anthocynanin coloration in corn
– Coat color in Labrador dogs
Epistasis
One gene affects the expression of a second gene
Example: H gene is epistatic to the ABO gene.
•H protein attaches the A or B protein to the cell
surface
•hh genotype = no H protein
•Without H protein the A or B antigens can not be
attached to the cell
•All hh genotypes have the phenotype of type O
•IAIA hh, IAi hh, IAIB hh, IBIB hh, IBi hh, and ii hh
Epistasis
•R.A.Emerson –
1918
•9:7 ratio
Purple:white corn
•Progeny must
have at least 1
copy of dominant
allele to produce
purple seed
• Epistasis in Labrador Dogs
• Bb or BB  dark (black) pigment
produced
• bb  light (brown) pigment produced
• Ee or EE  deposition of melanin
• ee  deposition of pigment blocked
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
ee
E_
No dark pigment in fur
Dark pigment in fur
Yellow Lab
E_bb
eebb
Yellow fur;
brown nose,
lips, eye rims
eeB_
Yellow fur;
black nose,
lips, eye rims
E_B_
Chocolate Lab
Black Lab
Brown fur,
nose, lips,
eye rims
Black fur,
nose, lips,
eye rims
Pleiotropy
Pleiotropy refers to an allele which has
more than one effect on the phenotype.
This can be seen in human diseases such
as cystic fibrosis or sickle cell anemia.
In these diseases, multiple symptoms can
be traced back to one defective allele.
Pleiotropic effects
• Occurs when an
allele has >1 effect
on phenotype
• Examples are:
– Sickle cell anemia
– Cystic fibrosis