EXCEPTIONS TO
MENDELIAN
GENETICS
Since Mendel worked his magic,
scientists have learned much more
about heredity. Not all traits are
inherited in the simple
dominant/recessive way.
Mendelian Genetics
DOMINANT/RECESSIVE
TRAITS
• Traits that are determined by a single
gene with just 2 alleles.
One dominant & the other recessive
• Examples:
- Freckles / No Freckles
-Dimples / No dimples
(only 2 possible phenotypes)
Exceptions to Mendel:
• Intermediate Inheritance*
• Codominance*
• Multiple Allelism*
• Polygenic Traits
• Linked Traits
• Sex-linked Traits*
* Punnett square problems
Intermediate Inheritance
• Also known as “Incomplete dominance” and
“Blending Inheritance”
• 1 gene, 2 alleles but neither is completely
dominant.
• Produces an intermediate phenotype in the
heterozygous individual (looks like blending)
Purebred
X
Purebred
=
3 possible
phenotypes
Intermediate Inheritance
Punnett square
• Since neither allele is completely dominant
over the other, then each allele is given its
own letter.
• Red = R
White = W
RW
RR
WW
X
=
Intermediate Inheritance
Punnett square
• Red = R
White = W
• Cross two pink flowers (RW)
R
R
RR
W
W
RW
WW
RW
Codominance
-Both alleles contribute to the
phenotype.
-Neither allele is dominant or
recessive, nor do the alleles blend
(like in intermediate inheritance)
Ex: In certain varieties of chicken, the
allele for black feathers is codominant
with the allele for white feathers.
- Heterozygous chickens are speckled
with both black and white feathers.
The black and white colors do not
blend, but appear separately.
CODOMINANT ALLELES
• Ex. –
SICKLE-CELL ANEMIA
• A PERSON WITH 1 NORMAL
ALLELE & 1 SICKLE CELL
ALLELE WILL HAVE BOTH
TYPES OF BLOOD CELLS
• Punnett Squares are done
just like Intermediate
Inheritance
CODOMINANT ALLELES
Heterozygous person has
increased resistance to malaria
than a homozygous person
Multiple Alleles
–Genes that are controlled by more than two
alleles
•An individual can’t have more than 2 alleles.
However, more than two possible alleles can exist
in a population.
ie: a rabbit's coat color is
determined by 1 gene that has
at least four different alleles.
• Different combinations of alleles result in the
colors shown here.
KEY
C=
full color; dominant
to all other alleles
cch = chinchilla; partial
defect in pigmentation;
dominant to
ch and c alleles
ch = Himalayan; color in
certain parts of the
body; dominant to
c allele
chhc
ch,cCc
h
ch
AIbino:
Chinchilla:
Himalayan:
cc CC,
cc
c,hCc
, or
cch
c,hhor
cch
c
Full color:
, or
Cc
c = albino; no color;
recessive to all other
alleles
Blood Types—Multiple Alleles and Codominance
Human blood types are determined by genes that follow the
CODOMINANCE pattern of inheritance. There are two dominant
alleles (IA and IB) and one recessive allele (i).
Blood Type
(Phenotype)
Genotype
Can donate blood to:
Can receive blood from:
O
ii
A,B,AB and O
(universal donor)
O
AB
IAIB
O, AB
A,B,AB and O
(universal receiver)
A
IAIA or IAi
AB, A
O,A
B
IBIB or IBi
AB,B
O,B
POLYGENIC TRAITS
• TRAITS THAT ARE
DETERMINED BY
MORE THAN ONE
GENE (poly = many)
• HEIGHT, WEIGHT,
SKIN COLOR, EYE
COLOR
• Creates great
variation in the
phenotypes.
POLYGENIC TRAITS
POLYGENIC TRAITS
*Characteristics of any organism are
determined by the interaction between
genes and the environment.
- Diseases such as breast cancer, stroke,
schizophrenia, diabetes, heart disease
can be genetic
- non-genetic risk factors include poor
diet, low exercise, alcohol & drug abuse
HOMEWORK
On the back of your notes sheet,
do the 2 problems!!
Then finish Vocab and STUDY
FOR QUIZ!
LINKED GENES
• GENES THAT ARE FOUND TOGETHER ON THE
SAME CHROMOSOME.
• THEY ARE LINKED TOGETHER AND SO DO NOT
SORT OUT INDEPENDENTLY OF EACH OTHER.
THEY STAY TOGETHER (UNLESS CROSSING-OVER
BREAKS THEM UP)
• Linked genes most likely account for such
phenomena as red hair being strongly associated
with light complexioned skin among humans. If
you inherit one of these traits, you will most likely
inherit the other.
SEX-LINKED TRAITS
• TRAITS THAT ARE
DETERMINED BY
GENES ON ONE OF
THE SEX
CHROMOSOMES,
USUALLY THE X
CHROMOSOME.
• COLORBLINDNESS &
HEMOPHILIA ARE
EXAMPLES
SEX-LINKED TRAITS
• X -linked TRAITS
ARE SEEN MORE IN
MALES THAN IN
FEMALES. THIS IS
BECAUSE THE Y
CHROMOSOME DOES
NOT CARRY THE
SAME GENES AS THE
X. THERE IS NO
BACK-UP ON THE Y.
SEX-LINKED PUNNETTS
The punnett square below shows how a woman who is a
carrier for color-blindness passes the trait to her son,
but not her daughters.
Pedigrees
• A diagram of family relationships that uses
symbols to represent people and lines to
represent genetic relationships.
• Easier to visualize relationships within families,
particularly large extended families.
• Used to determine how genetic diseases are
inherited. (dominant, recessive, sex-linked, etc.)
How to read a pedigree…
-squares represent males
-circles represent females
-Horizontal lines connecting
a male and female represent
mating
-Vertical lines extending
downward from a couple
represent their children
These represent
generations. Oldest on
the top of the Pedigrees.
How to read a pedigree…
-Shaded in shapes means the
person shows the trait.
Depending on the pedigree,
you might see ½ shape
shaded. This means they are
a carrier for the trait.
(Heterozygous)
3 types: Autosomal Dominant
Autosomal Recessive
Sex-Linked Recessive
Genetic Disorders: Can be traced with Pedigrees
• Mutant alleles often
recessive
• Heterozygotes (carriers)
basically normal
• Diseases usually
seen in homozygous
recessive individuals
• Most people carry 5-15
defective recessive alleles
low odds for double recessive
genotype (in offspring)
UNLESS PARTNERS ARE RELATED
Genetic Disorders: Can be traced with Pedigrees
Dominant disorders:
• Affected children have an
affected parent.
• Heterozygotes are
affected.
• 2 affected parents can
produce unaffected
children.
• 2 unaffected parents
will not have affected
children.
Genetic Disorders: Can be traced with Pedigrees
Recessive disorders
• can go several generations
before popping up.
• Heterozygotes (carriers)
have a normal phenotype
• Most affected children
have normal parents
• 2 affected parents will have
affected children
Genetic Disorders: Can be traced with Pedigrees
Sex-linked disorders:
• An affected son can have
normal parents, but the
mother is a carrier.
•In order for a daughter to
be affected, her father must
be affected and her mother
must either be affected or
a carrier.
• If a woman is affected, all
her sons will be too.