Download 14-Incomplete Dominance and Codominance

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This is a special type of inheritance in which
there are no forms of a particular gene which
are totally dominant nor totally recessive.
An analogy: if you place a piece of blue glass
over a piece of yellow glass and hold it up to
the light, you see green glass – an color
intermediate between the two.
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In traits that follow incomplete dominance,
we see something unusual in heterozygous
individuals.
Heterozygous = a blending of the two
different traits.
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ex. snapdragon flower color:
Red – RR
White - WW
Pink – RW (white and red = pink)
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In order to show that neither form of the
gene is totally dominant, we do not use
any lowercase letters. Instead, we can use
two different capital letters.
In snapdragons, flower color
shows incomplete dominance.
There are genes for red color or
white color. Show a cross of a
pink-flowered snapdragon with
a red-flowered snapdragon.
R
R
R
W
RR
RW
RR
RW
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Another way to write the gene letters for
incomplete dominance is using two of the
same capital letter, with a ’ (prime) mark to
distinguish the two.
Red blood cells can come in
two shapes: normal round (R)
or sickled (R’). Show a cross of
someone with sickled cells
with someone with round
cells.
R
R
R’
RR’
RR’
R’
RR’
RR’
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An inherited disorder in which the red blood cells
are shaped like crescent moons (sickle shape).
Sickled cells cannot carry oxygen properly so body
tissues may be damaged. Sickled cells don’t move
easily through blood capillaries because of their
shape. Symptoms: fatigue, pain, may have
shortened life span.
Seen mainly in African American populations.
Trait show incomplete dominance.
◦ RR = all round RBC
◦ RR’ = some sickled, some round RBC
◦ R’R’ = all sickled RBC
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Four types: A, B, AB, O
Determined by certain proteins which can be
found on the surface of the cell membrane of
red blood cells.
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A protein: type A blood
B protein: type B blood
both A and B proteins: type AB blood
neither A nor B proteins: type O blood
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Genes for blood type A and B show codominance with each other. But both A and B
are totally dominant over O.
Type O blood: genes OO
Type AB blood: genes AB
Type B blood: genes BB or BO
Type A blood: genes AA or AO
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Remember, every chromosome has
sections on it called genes. Each gene
contains a genetic code for the
production of a protein for one
particular trait.
Every chromosome of every pair, from
chromosome number 1 through
number 23, has genes.
image from University of Washington, Department of Pathology
(www.pathology.washington.edu)
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So even the X chromosome (of the 23rd
pair) has genes on it.
We know some of the genes found on the
X chromosome:
◦ colorblindness
◦ male pattern baldness
◦ hemophilia
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Genes found on the X chromosome are
called X-linked genes. We say they code
for X-linked traits.
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X-linked traits are seen more commonly in
males than in females.
This is because females have two X
chromosomes and therefore have 2 of every
gene on the X chromosome. If a female
inherits one “bad” copy of a recessive Xlinked gene, she often will have a “good”
dominant copy on her matching X
chromosome. Therefore she would show
the dominant trait and would be “normal”
for that trait. In order to show the “bad”
trait, she would have to inherit two “bad”
copies of the gene, which is much less
likely.
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XN
XN
Xn
XN
XN Xn
XN Xn
XN XN
XN Y
XN Xn
Xn Y
Y
Xn
XN
Y
XN Y
Xn
XN Y
XN
XN Xn
Xn
Xn Xn
Bald father, normal mother.
All children normal hair.
Y
XN Y
Xn Y
normal dad,
normal
(carrier)
mom; all
female
children
normal; but
half of sons
will have
baldness.
bald dad,
normal
mom;
half of
daughters
and half of
sons will
have
baldness.
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Colorblindness is also caused by a recessive
X-linked gene. What would happen if a
woman with Cc (XCXc) and man with a c gene
on his X chromosome (XcY) had children?
Xc
Y
XC
XCXc
XCY
Xc
XcXc
XcY
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Each day, nearly 600 babies are born in the
U.S. with some type of disorder. Some of
these disorders are inherited.
Errors in chromosome number
◦ sometimes people are born with more or fewer
than 46 chromosomes.
◦ this happens when the sperm or egg cell does not
have exactly 23 chromosomes.
◦ Addition or deletion of more than 1 chromosome
is usually fatal (but there are exceptions to this).
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Trisomy 21 is a
genetic disorder in
which the individual
inherited an extra 21st
chromosome.
The resulting
syndrome is called
Down syndrome.
Symptoms include
some degree of
retardation, heart
problems and certain
physical
characteristics.
image from University of Washington, Department of Pathology
(www.pathology.washington.edu)
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This is a disorder
in which males
have inherited
extra X
chromosomes.
The picture shows
a person with
49XXXXY.
Symptoms of the
disorder include
tall stature,
infertility,
abnormal sexual
structures.
image from University of Washington, Department of Pathology
(www.pathology.washington.edu)
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Males who
inherited an
extra Y
chromosom
e
Symptoms
can include
tall stature
and acne.
image from University of Washington, Department of Pathology
(www.pathology.washington.edu)
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A certain recessive gene on the X
chromosome can cause a rare disorder called
hemophilia.
Hemophilia is a disorder in which a person’s
blood does not clot properly.
Bleeding from a cut or bruise may take hours
to stop. Special medication is available to
help stop bleeding.
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Genetic disorders can also be caused by
genes found on the autosomes
◦ Dyslexia – genetic disorder (possibly on
chromosome 6 and/or 15) involving word
blindness; people may have trouble
writing/reading certain letters or parts of
words. Caused by a dominant gene.
◦ PKU – phenylketonuria. Recessive genetic
disorder on chromosome 12 which prevents
the normal use of protein in food. Chemical
byproducts from abnormal breakdown can
harm brain cells. Today babies are tested for
PKU soon after birth. Treated with dietary
modifications (ex. a diet low in phenylalanine).
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◦ Cystic fibrosis (CF) – a recessive genetic disorder
found on chromosome 7 which, in its classic and
most severe form, involves the production of
large amounts of sticky mucus in the lungs;
pancreas problems; salty sweat.
◦ Huntington’s disease – a dominant genetic
disorder found on chromosome 4. This is a
brain disorder which usually doesn’t strike until
a patient is in his 30’s or 40’s. Symptoms can
include difficulties with movement, thinking,
behavior and emotions.
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“It runs in the family” – genetic disorders can
run in families, if a particular gene is passed
on through generations.
Genetic Counseling involves people meeting
with a counselor who is specially trained to
help couples know their chances of having a
child with a certain disorder.
◦ can involve discussions, Punnett squares, family
histories, blood tests, etc.
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The baby of a young couple died when it
was very young, from cystic fibrosis.
The couple went to a genetic counselor to
help them understand the genetics behind
their baby’s condition and their chances of
having another child with the same disorder.
If both parents are Ff, there is a 25% chance
of having a child with cystic fibrosis.
Genetic counseling allows families the
opportunity to make certain choices in their
family planning.
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