NON-MENDELIAN GENETICS
Incomplete Dominance, Co-Dominance, and Sex-linked dominance
INCOMPLETE DOMINANCE
INCOMPLETE DOMINANCE
Two alleles – dominant and recessive
Genotypes are the same as simple Mendeliangenetics: AA, Aa, aa
Some phenotypes are different than simple Mendelian genetics
When an organism is homozygous dominant, they look dominant
When an organism is homozygous recessive, they look recessive
When an organism is heterozygous, they are a blend of dominant and recessive
INCOMPLETE DOMINANCE
Example – flower color
Red petals are dominant to white petals
A flower with red petals is crossed with a flower that
has white petals
The offspring will have pink petals
INCOMPLETE DOMINANCE
If two heterozygous flowers are crossed, they
will give a 1:2:1 genotypic ratio AND a 1:2:1
phenotypic ratio
Genotypic
Phenotypic
INCOMPLETE DOMINANCE
If two heterozygous flowers are crossed, they
will give a 1:2:1 genotypic ratio AND a 1:2:1
phenotypic ratio
Genotypic
1 RR
2 Rr
1 rr
Phenotypic
1 red
2 pink
1 white
CO-DOMINANCE
CO-DOMINANCE
The alleles are both dominant; neither is
recessive
Both alleles are FULLY expressed in the
offspring
Co-dominant alleles are written as capital
letters with superscripts
AB
CO-DOMINANCE
Example: In chickens, the alleles for feather
color are co-dominant (letter for the feather
color trait is F). One allele is for white feathers,
the other is for black feathers
CO-DOMINANCE
Example: In chickens, the alleles for feather
color are co-dominant (letter for the feather
color trait is F). One allele is for white feathers,
the other is for black feathers
Black feathers = FB
White feathers = FW
CO-DOMINANCE
Example: In chickens, the alleles for feather
color are co-dominant (letter for the feather
color trait is F). One allele is for white feathers,
the other is for black feathers
Black feathers = FB
White feathers = FW
Genotype Combinations
CO-DOMINANCE
Example: In chickens, the alleles for feather
color are co-dominant (letter for the feather
color trait is F). One allele is for white feathers,
the other is for black feathers
Black feathers = FB
White feathers = FW
Genotype Combinations
FBFB
FWFW
FBFW
CO-DOMINANCE
Example: In chickens, the alleles for feather
color are co-dominant (letter for the feather
color trait is F). One allele is for white feathers,
the other is for black feathers
Black feathers = FB
White feathers = FW
Genotype Combinations
FBFB = a chicken with only black feathers
FWFW = a chicken with only white feathers
FBFW = A chicken with BOTH white AND black feathers
CO-DOMINANCE
CO-DOMINANCE – BLOOD TYPES
Human blood types are an example of co-dominant traits with multiple alleles (3)
Type O is recessive
Types A and B are codominant with each other (neither dominates over the other)
They are both fully dominant to type O
The dominant alleles are written with the letter I and a superscript. The recessive allele is a lowercase i
CO-DOMINANCE – BLOOD TYPES
Type A –
Type B –
Type AB –
Type O -
CO-DOMINANCE – BLOOD TYPES
Type A – IAIA or IAi
Type B – IBIB or IBi
Type AB – IAIB
Type O - ii
CO-DOMINANCE – BLOOD TYPES
CO-DOMINANCE – BLOOD TYPES
Example cross
One parent is heterozygous for type A blood
The second parent is heterozygous for type B blood
CO-DOMINANCE – BLOOD TYPES
Example cross
One parent is heterozygous for type A blood
The second parent is heterozygous for type B blood
CO-DOMINANCE – BLOOD TYPES
What is the genotypic
ratio?
What is the phenotypic
ratio?
CO-DOMINANCE – BLOOD TYPES
What is the genotypic
ratio?
25% IAIB
25% IAi
25% IBi
25% ii
What is the phenotypic
ratio?
CO-DOMINANCE – BLOOD TYPES
What is the genotypic
ratio?
25% IAIB
25% IAi
25% IBi
25% ii
What is the phenotypic
ratio?
25% AB 25% A 25% B 25% O
BLOOD TYPE EXAMPLES
A child is born to a mother who is homozygous
for type B blood and a father who is
heterozygous for type A blood. What is the
probability the child will have type A blood?
Type B blood?
A child is born to a mother who has type AB
blood and a father who has type O blood. What
are the possible blood types for the child?
SEX-LINKED DOMINANCE
SEX-LINKED
Traits that are found on the X chromosome are
known as “sex-linked” since they are controlled
by one of the sex chromosomes
Since females have two X chromosomes, they
are more likely to be heterozygous carriers
(having one recessive X and one normal X)
SEX-LINKED
Since males only have one X chromosome
(from their mother), they are more affected
than females by recessive sex-linked traits
They either have the recessive trait or they don’t
(they cannot be heterozygous)
Examples: color-blindness and hemophilia
COLOR BLINDESS
SEX-LINKED
Example: A child is born to a mother who is a
carrier for hemophilia, and a father who does
not have the disease. Hemophilia is a recessive
sex-linked disease. What are the chances that
the child will be:
A boy with hemophilia?
A boy without hemophilia?
A girl with hemophilia?
A girl without hemophilia?
SEX-LINKED
SEX-LINKED
PEDIGREE
Used to trace the inheritance of a trait through
several generations
Uses symbols to illustrate inheritance
PEDIGREES
PEDIGREE TYPES
Autosomal Dominant
Affected offspring cannot have non-affected parents
Examples
Huntington’s Disease
Achondoplasia
(dwarfism)
PEDIGREES
Autosomal Recessive
Non-affected parents can have affected offspring
(parents are carriers)
If both parents are affected,
they MUST have affected
offspring
Examples
Cystic Fibrosis
Albinism
Tay-Sachs Disease
PEDIGREES
X-Linked Dominant
Affected fathers must have affected daughters
Examples: Fragile X Syndrome (causes autism)
PEDIGREES
X-Linked Recessive
High incidence of affected males (since they have only one X
chromosome)
Examples
Hemophilia
Red/Green Color-blindness
PEDIGREES
PRACTICE
PRACTICE
Identify this pedigree as autosomal recessive,
autosomal dominant, X-linked dominant, or X-
linked recessive
PRACTICE
Identify this pedigree as autosomal recessive,
autosomal dominant, X-linked dominant, or X-
linked recessive
PRACTICE
Identify this pedigree as autosomal recessive,
autosomal dominant, X-linked dominant, or X-
linked recessive