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Patterns of Intermediate Inheritance
Exceptions to Mendel’s Principles
• Mendel’s 3 principles
provide us with an
important foundation in
building our knowledge
of genetics.
• However, there are
many types of
inheritance patterns that
do not follow the
principle of dominance.
• There are several types
of intermediate
inheritance patterns:
–
–
–
–
–
Co-dominance
Multiple Alleles
Incomplete Dominance
Polygenic Traits
Multifactorial Inheritance
1
Co-dominance
• A trait for which both alleles contribute equally
to the phenotype.
– Example: Coat color in cows and horses
– When a homozygous red cow is crossed with a
homozygous white cow they produce a
heterozygous cow that is red and white (roan).
Genotype: WW
Genotype: RR
Genotype: RW
2
Practice Cross - Codominance
• A horse that is roan is a blend
of red and white colors that
result when a red horse is
crossed with a white horse.
Red (R) horses breed true;
and white (W) horses breed
true.
• What is the genotype of a
red horse?
– RR
• What is the genotype of a
white horse?
– WW
• What is the genotype of a
roan horse?
– RW
• What is the probability
that two roan horses will
have at
least one roan offspring?
– 50%
• What is the probability
that two roan horses will
have a white offspring?
– 25%
R
W
R
RR
RW
W
RW
WW
3
Multiple Alleles
• When a gene has more than two possible alleles.
• ONLY TWO alleles are inherited.
– Example: Rabbit Coat Color is controlled by 4 alleles - C, cch,
ch, and c
chc or chch
cchcch, cchch, cchc
CC, Ccch, Cch, Cc
cc
4
Practice cross: Multiple Alleles
•
Practice cross: In rabbits there are four
different color alleles:
–
–
–
–
•
C is dominant over all
cch is dominant to ch & c
ch is dominant to c
C is recessive to all
C = wild type color
cch = chinchilla color
ch = Himalayan color
c = albino
Each rabbit can inherit only two alleles. A wild
colored rabbit is crossed with an albino and in
the offspring there is a Himalayan rabbit.
5
Show the cross between a wild type and
albino rabbit that has a Himalayan baby.
• What are the genotypes of
the parents?
C
ch
c
Cc
cch
c
Cc
cch
– Parent one : cc
– Parent two : Cch
• What is the probability that
they will have an offspring
that is Himalayan?
– 50%
• What is the probability that
they will have an offspring
that is albino?
– 0%
• What is the probability that
they will have an offspring
that is wild colored?
– 50%
Mult. Alleles
& 6
Co-dominance
Incomplete Dominance
•
Results in a heterozygous phenotype
that is a blend of the two homozygous
phenotypes.
– Examples: mirabilis (four o’clock) plants,
coat color in horses
– When a homozygous chestnut horse is
crossed with a homozygous white horse,
they produce a heterozygous tan (palomino)
horse.
7
Genotype: cc
(Chestnut)
Genotype: ww
Genotype: cw (Palomino)
8
Practice Cross – Incomplete
Dominance
• In Four O’Clocks, the
gene for red flowers (r) is
incompletely dominant to
the gene for white flowers
(w).
• The heterozygous
condition results in pink
flowers.
• Are the F1 flowers purebred
or hybrid?
– hybrid
• What is the phenotype of the
F1 flowers?
– pink
r
r
w
rw
rw
w
rw
rw
• Red Plant Genotype?
– rr
• White Plant Genotype?
– ww
9
Polygenic
Humans
& Plants
• A trait that is controlled
by the interaction
between 2 or more
genes.
– Examples: skin color, eye
color, height, hair color
– Results in a continuum of
expressed phenotypes.
10
Multifactorial Inheritance
• The phenotype is a result of an interaction between your
genotype and certain environmental factors.
• The expression of most all genes is influenced by
environmental conditions.
• Examples:
– Diabetes
– Height
– Heart Disease
• Can inherit a predisposition to all of these diseases/characteristics.
• Their development (phenotype) is influenced by environmental
factors such as proper nutrition, exercise, quality medical care, etc.
Enviro
Factors
11
Examples in plants:
• In many plants, the
presence of light
triggers the
production of
chloroplasts.
• Chemicals that
stimulate
germination are only
produced in the
presence of certain
temperatures.
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12
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