Incomplete Dominance,
Codominance, Sexlinked and Polygenic
Inheritance
Incomplete Dominance
• Offspring have an appearance somewhat in
between the phenotypes of the two parents
– “Mixed”
– Blended
R
R
W
RW
RW
F1 generation
W
RW
RW
All Rr = pink
(heterozygous pink)
Incomplete Dominance
• Incomplete dominance occurs when two or
more alleles influence the phenotype
• Neither allele is
completely dominant
over the other
Incomplete Dominance
Codominance
• BOTH alleles are expressed equally in
heterozygous individuals
• Neither allele is dominant over the other
– Example: blood type
• Determined by whether or not you have A or B
proteins
• IA = A protein
• IB = B protein
• I = no protein
• Another example
Is the speckled chicken
Codominance
Codominance Problem
• Example:homozygous male Type B (IBIB)
•
x
heterozygous female Type A (IAi)
IB
IB
IA
IAIB
IAIB
i
IBi
IBi
1/2 = IAIB
1/2 = IBi
Another Codominance Problem
Example: male Type O (ii)
x
female type AB (IAIB)
IA
IB
i
IAi
IBi
i
IAi
IBi
1/2 = IAi
1/2 = IBi
Codominance
• Question:
If a boy has a blood type O and his sister
has blood type AB, what are the genotypes
and phenotypes of their parents?
boy - type O (ii) X girl - type AB (IAIB)
Codominance
• Answer:
IA
IB
i
i
IAIB
ii
Parents:
genotypes = IAi and IBi
phenotypes = A and B
Sex-linked Traits
• Traits (genes) located on the
sex chromosomes
• Sex chromosomes are X and Y
– XX genotype for females
– XY genotype for males
• Many sex-linked traits carried
on X chromosome
Sex – Linked Traits
• Example: Colorblindness
– If the mother carries the colorblindness gene
on her X chromosome, her son could get it.
– As long as one X chromosome
X
y
is ok, a female will not
cX
c y
X
X
C
express the trait
X
X
XX
Xy
Sex-linked Traits
Example: Colorblindness
Sex Chromosomes
Colorblindness
XX chromosome - female
Xy chromosome - male
Polygenic Inheritance
• Inheritance pattern controlled by two or more
genes
• Genes can be on the same chromosome or on
different chromosomes
• Genotypes are written as AA, Aa, or aa except
you will have three or more genes each with a
dominant and recessive trait
• Skin color is influenced by the additive effects of
melanin by three to six genes. Eye color is also
a result of melanin added from multiple genes.
Polygenic Inheritance
a pedigree
Polygenic Inheritance of Skin Color
Gametes
ABC
ABc
ABC
AABBCC
AABBCc
AbC
Abc
aBC
AABbCc
AaBBCC
aBc
ABc
AbC
Abc
aBC
AAbbcc
aaBBCc
aBc
abC
abc
aaBbcc
The table shows typical
ranges of skin color
combinations. Other
polygenic influenced
conditions include Spina
bifida and Cleft palate.
abC
abc