Complex Patterns of Heredity...

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Complex Patterns of Heredity...
What Mendel's Laws couldn't explain
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All of the inheritance patterns we have learned
about so far, such as the ones from Mendels
experiments, are often referred to as simple
Mendelian inheritance.
This would include any inheritance which is
controlled by a single pair of alleles, one
dominant, and one recessive.
Many inheritance patterns are far more complex
than those studied by Mendel.
Some of these types of inheritance dont even
involve chromosomes!
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Incomplete Dominance
When inheritance follows the rule of dominance,
then the homozygous dominant, and the
heterozygous individuals all have the same
phenotype.
The allele for tall T the allele for short t
TT
tall
Tt
tall
When traits are inherited in a pattern where the
heterozygous individual is intermediate between
those of the two homozygotes, then the pattern is
called Incomplete Dominance.
tt
short
TT
tall
tt
short
Tt
Average height
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X
Red Snapdragons
White Snapdragons
Pink Snapdragons
When showing incomplete dominance, the alleles
look like this
Rred
R1white
Neither is dominant to the other, so no lower
case letter appears.
Can you predict what happens when you cross two
pink snapdragons?
Do a punnett square crossing RR1 x RR1
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The snapdragons appear in the F2 generation in a
121 phenotypic ratio
The genotypic ratio is also 121
This result supports Mendels law of segregation,
meaning that the alleles combine during
fertilization completely randomly in four
possible ways.
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Codominance Expression of both alleles
Codominance is apparent in a breed of chicken,
where black feathers are B, and white feathers
are W. Both are dominant, so both are expressed
with a capital letter.
When codominant alleles are expressed, both
phenotypes are present equally.
BW
BB
WW
X

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Codominance in Human Blood Types
Humans can have one of four possible blood types
A B O and AB
Human blood is separated into different
classifications because of the varying proteins
contained in each blood type's red blood cells.
These proteins are there to identify whether or
not the blood in the individual's body is it's
own and not something the immune system should
destroy.
The proteins in our blood cells structure is
controlled by three alleles i, IA and IB.
IAIAType A
IBIBType B
iiType O
The i allele is the only recessive allele
IAIBType AB
IA and IB are both codominant
If the recessive allele i is paired with IB or
IA, it's phenotypic expression is hidden. When
the IB and IA are together in a pair, both
proteins A and B are present and expressed.
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Question What are the genotypes of a man and
woman, each with type A, and B blood
respectively, who produce offspring with types A,
B, O and AB?
IA i
Lets not forget to show the offsprings actual
genotypes! Now it becomes obvious what their
genotypes were.
IB i
AB
B
IAIB
IBi
A
O
IAi
ii
RH, or Rhesus factor is inherited separately from
the blood type, and follows the rules of
polygenic inheritance. About 85 of humans
globally contain the Rhesus positive phenotype,
which means they contain the Rh antigen on the
surface of their red blood cells.
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A woman with Type O blood and a man who is Type
AB are expecting a child.  What are the possible
blood types of the offspring? 
This couple can only have offspring that are
types A, and B.
i i
IA IB
Can a person with type AB blood ever produce a
child with type O blood? NO!!!
Blood groupings of humans not only follow the
rules of codominance, but also are determined
through multiple alleles. IA IB i
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While many traits are controlled by a single pair
of alleles, some traits can be controlled by more
than two alleles. These traits are said to be
carried and expressed by multiple alleles.
In the case of rabbit fur color, there are four
different alleles which determine different
phenotypes.
Chinchilla rabbit
Dark-gray colored
Himalayan rabbit
White rabbit
cch dominant to Himalayan and to white
c completely recessive
Cdominant to all other alleles
ch dominant to white
Try predicting how many possible genotypes would
code for chinchilla rabbits.
Remember, while there may be multiple alleles in
a population that determine a trait, each
individual only inherits a pair of these
allelesone from mother, and one from father!
Human blood types are not only codominant, but
multiple allele!
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Sex-Linked Inheritance
Traits controlled by genes located on the sex
chromosomes are called sex-linked traits.
In humans, there are 23 pairs of chromosomes in
all autosomal cells within the body. The 23rd
pair are sex chromosomes.
The male genotype is XY, the female genotype is XX
The presence of the Y chromosome in one of these
karyotypes, makes one of these individuals a
male. When there are two Xs, then it is a female
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The alleles for sex-linked traits are written as
superscripts of the X or Y chromosomedepending
upon which chromosome they are associated with.
Because males only inherit one X chromosome, when
a recessive trait is inherited on the X
chromosome there is no corresponding trait to
mask that trait on the Y chromosomeso the
recessive trait is always expressed
In fruit flies, the allele for eye color is on
the X chromosome.
What would the genotype of a white-eyed female
look like?
XrXr
XRred eyes
What would the genotype of a red-eyed
heterozygous female look like?
XRXr
Xrwhite eyes
What would the genotype of a red-eyed homozygous
dominant female look like?
XRXR
Predict what the genotype of a white-eyed male
would look like.
XrY
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Create a punnett square showing the mating
results of a homozygous dominant female fruit fly
for eye-color, and a male fruit fly with the
recessive trait.
Because there is no second X chromosome in the
male, it cannot be homozygous. Instead, it is
called hemizygous.
What color eyes will the males have?
XRXr
XRY
What color eyes will the females have?
XRY
XRXr
Predict what genotypic combinations would create
white-eyed male flies.
Thomas Hunt Morgan discovered that traits were
linked to sex chromosomes, through his study of
fruit flies.
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Sex-Linked Traits in Humans
Hemophilia is characterized by inability to form
blood clots because of deficiency in a clotting
factor in the blood. The frequency of
hemophiliacs is about 1 in 10,000 males. Affected
individuals suffer from severe bleeding following
a relatively small injury like a cut or a bruise.
Duchenne Muscular Dystrophy affects young boys.
It is characterized by muscular degeneration and
weakness beginning at the age of 3 to 5 years and
progressing rapidly so that affected individuals
are wheel-chair bound by the time they are in
their teens and die in their early twenties
because of severe involvement of their
respiratory muscles.
Red-green color blindness is the inability to
distinguish red from green.
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An individual with normal color vision will see a
5 revealed in the dot pattern.
An individual with Red/Green (the most common)
color blindness will see a 2 revealed in the dots.
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Sex-Linked Pedigree
How can you tell quickly, that this must be a
sex-linked trait?
Only the males have it!
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Polygenic Inheritance Patterns
Some traits, such as skin, eye color and height
in humans vary over a wide range.
These traits are governed by many genes, or
polygenic inheritance.
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Skin Color A Polygenic Trait
Scientists have found that when light-skinned
people mate with dark-skinned people, their
offspring have intermediate skin colors, similar
to what you might expect with incomplete
dominance.
When intermediate skin-colored children mate with
each other, the resulting skin color range from
the light-skin color to the dark-skin color of
the original parents.

X
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Gametes contributed by parents ABC ABc AbC Abc aBC aBc abC abc

ABC 6 5 5 4 5 4 4 3
ABc 5 4 4 3 4 3 3 2
AbC 5 4 4 3 4 3 3 2
Abc 4 3 3 2 3 2 2 1
aBC 5 4 4 3 4 3 3 2
aBc 4 3 3 2 3 2 2 1
abC 4 3 3 2 3 2 2 1
abc 3 2 2 1c 2 1 1 0
ABC dark pigmentation abc light pigmentation
As you can see, this individual offspring would
inherit the darkest pigmentation, whereas this
offspring would inherit the lightest pigmentation.
This makes it easier to understand how two
offspring of biracial ancestry can actually pass
on the darkest, or the lightest pigmentation to
their offspring (even though the odds are against
it! )
What are the odds of this couple producing an
aabbcc individual? AABBCC?
AaBbCc x AaBbCc
1/64
1/64
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Environmental Influences
Sometimes the genetic makeup of an organism
determines only that organisms potential to
develop and function in a particular way.
As the organism develops, many factors can
influence how the gene is expressed, or even
whether the gene is expressed at all.
Influence of external environment
Temperature, nutrition, light, chemicals, and
infectious diseases can influence gene
expression.
Influence of internal environment
Hormones can influence horn size in mountain
sheep.
Arctic Fox