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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 =
THIRD phenotype!!– Ex. Snap dragons or 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 W
R
W
R R
R W
R W
W W
R = Red
W = White
RW = Pink
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
• 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
X
=
Codominance – Hair Color in CattleCross a Homozygous Red Hair Cow (RR) with a
Homozygous White Hair Bull (WW)
RR x WW
R = red hair
W = white hair
RW = roan
R R
W
W
R R
R R
W W
W W
Codominance - What About This?What happens when you cross a Roan Cow (RW)
with a Roan Bull (RW)?
RW x RW
R = red hair
W = white hair
RW = roan
R W
R
W
R R
R W
R W
W W
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
IB 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 Parent 1
Allele from Parent 2
Genotype of offspring
Blood types of 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
Blood Type O is universal donor – Why?
Blood Type AB is universal recipient – Why?
IA IA IA IB IA iIA IB IB IB IB Ii i
Technical Genotype
Human Blood Type – A Codominant Multiple Allele Trait
Blood Type O is universal donor – Why?
Blood Type AB is universal recipient – Why?
BloodGroup(Phenotype) OGenotypes
AntibodiesPresent inBloodRed Blood Cells
Reactions When Blood from Groups Below Is Mixed with Antibodies from Groups at Left
A B AB
O
A
B
AB
ii
IAIB
IBIB
or
IBi
IAIA
or IAi
CarbohydrateA
CarbohydrateB
Anti-A
Anti-B
Anti-AAnti-B
—
Pleiotropy
• One gene influences several characteristics– Ex. Sickle Cell Anemia – abnormal
hemoglobin, hemoglobin crystallizes, different blood cell shape – all of this affects different parts of the body since blood flow is reduced
Polygenic Inheritance
• Many traits are produced by the interaction of several genes
• Polygenic inheritance = 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 and height