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Beyond Dominant and
Recessive Alleles
Beyond Dominant and Recessive
Alleles
• There are important exceptions to
Mendel’s discoveries
• Not all genes show simple patterns of
dominant and recessive alleles
• Many traits are controlled by multiple
alleles or multiple genes
Incomplete Dominance
• Cases in which one allele is NOT
completely dominant over another
• Heterozygous phenotype is somewhere in
between the two homozygous phenotypes
– Get a mixture/blending of the parents’ traits
– Ex. four o’ clock plants and flower colors
Incomplete Dominance – Four O’
Clock Flowers
R = Red
W = White
RW = Pink
Incomplete Dominance – Four O’
Clock Flowers
R = Red
W = White
RW = Pink
Incomplete Dominance - What
About This?
What happens when you cross a Pink Four O’
Clock (RW) with another Pink Four O’Clock (RW)?
RW x RW
R
R = Red
W
W = White
RW = Pink
RR
RW
R
RW
W
WW
Incomplete Dominance
• In people, hypercholesterolemia –
dangerous amounts of cholesterol in blood
– is an example of incomplete dominance
– HH = normal amounts of cholesterol
– hh = about 5 times as much cholesterol in
blood
– Hh = about 2 times as much cholesterol in
blood
Codominance
X
=
• Both alleles contribute to the
phenotype of organism
– See both parents’ traits in offspring
– Ex. chickens and feather color
• Allele for black feathers is codominant with
allele for white feathers. Chickens with both
alleles are black and white speckled
– Ex. cattle and hair color
• Allele for red hair is codominant with allele for
white hair. Cattle with both alleles are roan, or
pinkish brown in color because the coat is a
mixture of both red and white hairs
Codominance – Hair Color in Cattle
Cross a Homozygous Red Hair Cow (RR) with a
Homozygous White Hair Bull (WW)
RR x WW
R
RW
R
RW
R = red hair
W = white hair
RW = roan
W
RW
W
RW
Codominance - What About This?
What happens when you cross a Roan Cow (RW)
with a Roan Bull (RW)?
RW x RW
R
RR
W
RW
R = red hair
W = white hair
RW = roan
R
RW
W
WW
Multiple Alleles
• Many genes have more than two alleles
and are therefore said to have multiple
alleles
• An organism doesn’t have more than two
alleles, but more than two possible alleles
exist
Human Blood Type – A Multiple
Allele Trait
There are three different alleles for human blood type:
Blood types
For simplicity,
we call these
IA
A
B
I
B
i
O
Each of us has two ABO blood type alleles, because we each inherit one blood type allele
from our biological mother and one from our biological father. A description of the pair
of alleles in our DNA is called the genotype.
Since there are three different alleles, there are a total of six different genotypes at the
human ABO genetic locus.
Allele from Allele from Genotype of Blood types of
Parent 1
Parent 2
offspring
offspring
A
A
AA
A
A
B
AB
AB
A
O
AO
A
B
A
AB
AB
B
B
BB
B
B
O
BO
B
O
O
OO
O
Technical Genotype
IA IA
IA IB
IA i
IA IB
IB IB
IB I
ii
Blood Type O is universal donor – Why?
Blood Type AB is universal recipient – Why?
Polygenic Traits
• Many traits are produced by the interaction of
several genes
• Polygenic traits = many, many different
phenotypes possible!
• Traits controlled by two or more genes =
polygenic traits
– Ex. 3 genes involved in making reddish-brown
pigment in eyes of fruit-flies. Different combinations
of these 3 genes produce different eye colors
– Ex. Human skin color possibly controlled by 4 genes