VARIATIONS OF
INHERITANCE
Dominant Controlled Inheritance
(aka “simple”) (quick review)
•So far, we have been looking at traits that
are controlled by the dominant allele.
•A homozygous dominant or
heterozygous genotype will result in the
dominant trait being expressed.
•F = purple flower, f = white flower
•FF and Ff both result in a purple flower.
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Incomplete(Intermediate)
Inheritance
•Scientists observed that some traits
simply did not show this “dominance
only” form of inheritance.
•Some phenotypes appear to be a mixture
of the two parent phenotypes.
•When neither allele for a trait is dominant,
it is called incomplete inheritance.
Incomplete Inheritance
Let’s look at an example using FUR COLOR(or “F”).
• FB = allele for black dog fur
• FW = allele for white dog fur
• What happens if we cross one homozygous black dog
and one homozygous white dog ?
B
B
F F
x
W
W
F F
Incomplete Inheritance
FB
FB
FW
FBFW
FBFW
FW
FBFW
FBFW
All the
offspring have
a heterozygous
genotype.
Incomplete Inheritance
•Assume this trait is controlled by
incomplete inheritance.
•The phenotype for the offspring
would be be brown (or tan) colored
fur.
•Since neither allele is dominant, they
both get blended together.
Incomplete Inheritance
Now, what would be the outcome if we
crossed two of these brown-colored
dogs?
Incomplete Inheritance
FB
FB
FW
The result would
be…
FBFB
FBFW
25% black fur
50% brown fur
FW
FBFW
FWFW
25% white fur
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Codominance Inheritance
•Sometimes a heterozygous
individual will have a phenotype that
isn’t a complete mix of the parental
traits.
•Instead, the offspring will express
both traits of the parents at the
SAME TIME.
•When this happens it is called
codominance.
Codominance Inheritance
Let’s look at an example.
•FR = allele for red roses
•FW = allele for white roses
•What happens if we make the following cross?
FRFR
FWFW
X
Codominance Inheritance
FR
FR
FW
FRFW
FRFW
FW
FRFW
FRFW
Once again, all of
the offspring have
a heterozygous
genotype.
Codominance Inheritance
•The phenotype in this case would be roses that
are red with white markings (or white with red
markings).
Codominance
Codominance Inheritance
•Intermediate and codominance are similar
because they both produce a third phenotype
when heterozygous.
•They are different because intermediate shows
a blend or mix of the parent traits.
•While codominance can produce both parent
traits simultaneously.
P1/P2
F1
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Polygenic Inheritance
•We’ve already seen the effects of
dominance-only(simple), intermediate,
and codominance inheritance.
Polygenic Inheritance
•However, none of them can explain a trait
such as human skin color with so many
different variations.
Polygenic Inheritance
•Or different oak trees that
produce various amounts of
acorns.
Polygenic Inheritance
•The only way to get such large
variation is to have multiple genes
controlling the trait.
•Human height is just such an
example of polygenic inheritance.
•Let’s say height is controlled by three
hypothetical genes: A, B, & C.
Polygenic Inheritance
•A person with the
genotype AABBCC would
be very tall.
• A person with the
genotype aabbcc
would be very short.
Polygenic Inheritance
•A person with AABBCc would be
taller than someone with AABbCc.
•The combinations of phenotypes
increases with the number of genes
that affect that trait.
Polygenic Inheritance
•The result is a very large number of
combinations (Some characters are even
affected by dozens of genes!)
• First Iris Layer Pigment
AA = Produce lots of pigment
Aa = Produce some pigment
aa = Do not produce pigment
• Second Iris Layer Pigment
BB = Produce lots of pigment
Bb = Produce some pigment
bb = Do not produce pigment
Environmental Influence
•The genotype does not always predict an
individual’s phenotype.
•Many times the environment has a strong
impact on how the genes get expressed.
•Example: Koreans (both North and South)
all share the same common ancestry.
Environmental Influence
•North Korean children are on
average much shorter.
•Why the difference?
Environmental Influence
38th
Parallel
Environmental Influence
•Malnutrition is widespread in N.
Korea due to a repressive
government.
•This variation in diet has lead to a
difference in phenotype (height)
that is not genetically based.
Environmental Influence
•This means that many genes
are not rigidly set.
•The amount of gene
expression is largely
dependent on an individual’s
environment.
Arctic Fox
• Winter Fur
• Summer Fur
Sex-Linked Inheritance
•A normal human has 23 pairs of
chromosomes in each of his/her
cells.
•The last pair are called sex
chromosomes.
Sex-Linked Inheritance
•Females have two X chromosomes.
Sex-Linked Inheritance
•Males have one X chromosome and one Y
chromosome.
Sex-Linked Inheritance
•The X chromosome is
much larger than the Y
chromosome.
Sex-Linked Inheritance
•Therefore, more genes are found on the
X chromosome than the Y chromosome.
•Males, having only one X chromosome,
are more likely to express sex-linked
disorders.
•Let’s use a real example to see why.
Sex-Linked Inheritance
•Human red-green colorblindness is a sex-
linked trait that is controlled by a recessive
gene found on the X chromosome.
Sex-Linked Inheritance
•N = non-color blind allele
•n = color blind allele
•A woman with either the genotype XN XN or XN
Xn would have normal vision.
•A woman with Xn Xn would be colorblind.
Sex-Linked Inheritance
•Males, however, can only have XN Y or
Y since the gene is not found on the Y
chromosome.
•XN Y = normal vision male
•Xn Y = colorblind male
Xn
Sex-Linked Inheritance
•Here’s an example.
•A non-colorblind male and a
heterozygous female (called a
“carrier”) are going to have a child.
•Are they likely to have a colorblind
child?
Sex-Linked Inheritance
Male = XN,Y
Female = XN,Xn
XN
Y
XN
XNXN
XNY
Xn
XNXn
Non-color blind
female and male
Color Blind Male
XnY
Non-color blind
carrier female
Sex-Linked Inheritance
•The predicted results…
•This couple has a 0% chance of having a
colorblind daughter.
•However, half of their sons are expected
to be colorblind.
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