GS/BIO212
BIO212
GENETICS
LECTURE 3
LEVELS OF DOMINANCE
1
Re-cap of Mendel’s laws
• 1. Mendel’s law of dominance: When an
organism has two different alleles for a
trait, one allele dominates.
•
2. Mendel’s law of segregation: During
gamete formation by a diploid organism,
the pair of alleles for a particular trait
separate, or segregate, during the
formation of gametes (as in meiosis).
•
3. Mendel’s law of independent
assortment: The members of a gene pair
separate from one another independent
of the members of other gene pairs.
(These separations occur in the formation
of gametes during meiosis.)
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LEVELS OF DOMINANCE
• Mendel’s experiments and analysis established that genes exist in
alternate forms-ALLELEs
• This suggested a simple dichotomy between ALLELE’s, one allele
(DOMINANT) contributes to the phenotype and the other
(recessive) with no effect unless its in homozygous state.
• Research showed that genes can exist in more than two allelic
states.
• Each allele can have a different effect on the phenotype; so ITS NOT
THAT SIMPLE & clear cut!!!!
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INCOMPLETE DOMINANCE
• A dominant Allele will have the same phenotypic effect in
heterozygotes as in homozygotes (Tt & TT are
indistinguishable).
• In Incomplete dominance, a heterozygote has a phenotype
different from that of either of its associated homozygotes.
• i.e with incomplete dominance, a cross between organisms
with two different phenotypes produces offspring with a third
phenotype that is a blending of the parental traits.
• E.g. The flower color in snapdragon (Antirrhinum majus)
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Incomplete dominance
• White & Red varieties are
Homozygous for different
alleles of a color determining
gene.
• When crossed they produced
a heterozygous with Pink
flowers (an intermediate
color.)
• The Allele for red color is
therefore incompletely or
partially dominant over the
white allele.
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Incomplete dominance
• Likely Explanation:
• the intensity of pigmentation in
this species depends on the
amount of a product specified by
the color gene.
• i.e if the allele R specifies Red
color and r does not, RR
homozygote will have a deeper
color red than Rr as the amount
of color product is twice in RR
than in Rr.
Phenotype Genotype Amount of Gene
product
Red
RR
2x
Pink
Rr
X
white
rr
0
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Codominance
• This is another exception to
the principle of simple
dominance.
• Both Alleles of a gene are
fully expressed in the
phenotype.
• E.g. 1. red cows crossed with
white will generate roan cows.
Roan refers to cows that have
red coats with white blotches.
• Rr x Rr = 1/4 red, 1/2 roan, 1/4
white
• NB: F2 Phenotype Ratio of
Rr x Rr = 1:2:1
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codominance
• E.g. 2. The MN blood group in humans; one allele directs the
synthesis of the M-glycoprotein (Antigen) while the other
Allele directs the synthesis of the N-glycoprotein (Antigen).
• Homozygous MM produce only M-antigen
• Heterozygous MN produce both M and N antigens
• Homozygous NN produce only N-antigen
• Neither Allele is dominant, or even partially dominant over the
other.
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Multiple Alleles
• In certain cases, more than two alleles exist for a particular
characteristic.
• Even though an individual has only two alleles, additional alleles
may be present in the population: multiple alleles
• multiple alleles: 3 or more alleles
• multiple alleles occurs in human blood types where blood
groups are determined by a single gene with 3 possible alleles:
A, B, or O.
• Red blood cells can contain two antigens, A and B; the presence
or absence of these antigens results in 4 blood types: A, B, AB,
and O.
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Multiple alleles
• Antigen: a type of carbohydrate attached to RBC’s
• Antibody: part of your body’s defence system; antibody B
“attacks” antigen B
• If a person’s red blood cells have antigen A, the blood type is A.
If a person’s red blood cells have antigen B, the blood type is B.
• If the red blood cells have both antigen A and antigen B, the
blood type is AB.
• If the red blood cells have neither antigen A nor antigen B, the
blood type is O.
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Multiple alleles
• The alleles for type A and type B blood are codominant; that is,
both alleles are expressed fully.
• However, the allele for type O blood is recessive to both type A
and type B.
• Because a person has only two of the three alleles, the blood
type varies depending on which two alleles are present.
• E.g. if a person has the A allele and the B allele, the blood type is
AB.
• If a person has two A alleles, or one A and one O allele, the blood
type is A. If a person has two B alleles, or one B and one O allele,
the blood type is B. If a person has two O alleles, the blood type
is O.
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ABO Blood type Allele representation
• The Gene responsible for the producing the A and B antigens is
normally denoted by the letter “I”
• The 3 Alleles are thus represented as IA, IB, i
• IA allele specifies production of A-antigen
• IB allele specifies production of B-antigen
• i allele does not specify an antigen
• There are 6 possible genotypes, but there are 4 distinguishable
phenotypes, A, B, AB & O blood types.
• All 3 alleles are found in appreciable frequencies in human
population, therefore I-gene is said to be Polymorphic.
• In US: A=41%, B=11%, AB=4% and O=44%
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ABO blood x-match
• Type AB is called the
universal recipient, and Type
O is called the universal
donor. Why?
• NB: Blood typing for A & B
antigens is completely
independent of blood typing
for M & N antigens
Type
Can Donate to
Can Receive from
A
A, AB
O, A
B
B, AB
O, B
AB
AB
O, A, B, AB
O
A, B, AB, O
O
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ABO Blood type
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Allelic series: COAT COLOR IN RABBITS
• The coat color in rabbits is also a
classical example of a gene with
multiple alleles.
• The color determining gene: c has 4
alleles
• c(albino), ch (himalayan), cch
(chinchilla) and c+ (wildtype)
• Most rabbits in the wild are
homozygous for the c+ allele and this
called the wildtype.
• The other alleles are the altered forms
(mutants) of the wildtype.
• Heterozygotes combinations resulting
from crosses between homozygotes
showed some hierachy of dominance;
• c+ > cch > ch > c
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Allelic series: COAT COLOR IN RABBITS
• The wildtype is completely dominant
over the other 3 alleles in the series,
Chinchilla is partially dominant over
Himalayan and Himalayan is
completely dominant over the albino
allele.
• NB: plausible explanation is that the
c-gene controls a step in the
formation of black pigment in the
fur.
• Wildtype is fully functional, chinchilla
and himalayan partiallfunctional.
• Albino is non-functionalnull/amorphic
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Lecture 3 Summary
• Complete dominance
• Incomplete dominance
• The flower color in snapdragon (Antirrhinum majus)
• Codominance
• Coat color in cows
• MN blood grouping system
• Multiple allelism
• ABO Blood grouping system
• Allelic Series
• Rabbits coat color
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