Download Incomplete Dominance Codominance Multiple alleles and lethal

Incomplete Dominance
Codominance Multiple Alleles, Blood
Types and lethal genes
Begins
Okay, it’s time to look critically at
new situations and how they give us
different results than we are used to
from Mendelian genetics. Now from
my movie, you probably know that I
have a particular interest in blood.
In this presentation, I will discuss the
genetics of blood, but first I have to
teach you about the other genetic
concepts. These concepts are
necessary to understand the genetic
rules that govern blood types.
INCOMPLETE DOMINANCE
Previously, when we studied Mendelian
genetics, there was no in-between variety
when genes where combined for the
organism. The phenotype was always a
display of either the dominant or
recessive allele, with no other alternative.
This is called COMPLETE DOMINANCE
Incomplete dominance is
different and involves a mixing of the two
traits to form an intermediate variety. To
the side you see the red flower which is
homozygous RR and the white flower
which is homozygous ww.
Heterozygote is pink Rw in-between two
homozygotes
You will notice that there are some differences in the approach
to this problem of incomplete dominance. One of these, is the
use of two different letters as opposed to the Mendelian
convention of using only the upper and lower case of one letter.
The Reason is that neither is dominant over the other, they are
both incompletely dominant
In snapdragon plants, red flowers are incompletely dominant to white flowers. They
heterozygote displays the pink phenotypes. Determine the result of a cross between
a red and white snapdragon. ( That’s the f1)
STEP 1 Statements
Let R represent the red allele
Let w represent the white allele
Let RW represent the
intermediate pink heterozygote
STEP 2 PARENTAL CROSS
P1 Ô RR X Ю WW
STEP #3 SHOW THE
R
R
GAMETES
W
W
STEP #4 PUNNETT
SQUARE AND
ANALYSIS
Phenotype analysis
4/4 PINK
FLOWERS
Pretty weird huh? This seems to go against all the Mendelian
genetics stuff. It’s like there is a blending of traits in future
generations– something Mendel proved wrong. Well if you take
all these pink flowers though, none of them will breed true and
produce more pink flowers. Take a look at a result of the cross of
two pink flowers below. The red and white reappear like
Mendelian genetics predicts. There is thus no continuous
blending of traits.
STEP #4 PUNNETT SQUARE AND ANALYSIS
STEP 1 Statements
Let R represent the red allele
Let w represent the white allele
Let RW represent the
intermediate pink heterozygote
R
R
W
RR
RW
Phenotype analysis
¼ RED FLOWER
½ PINK FLOWER
STEP 2 PARENTAL CROSS
W
P1 Ô RW X Ю RW
STEP #3 SHOW THE
R
W
WW
¼ WHITE FLOWER
RATIO IS 1:2:1
GAMETES
R
RW
W
This is different than the usual 3:1
Mendelian phenotype ratio. It is however,
the same as the genotype ratio. Here the
heterozygotes show a different phenotype!!
So now you have something new to add to your
genetics bag of tricks. You have the monohybrid
3:1 ratio and the dihybrid 9:3:3:1 ratio. Now if
you see a 1:2:1 ratio, you have incomplete
dominance.
But wait, there is more! There is another situation that is
almost the same as incomplete dominance called
CODOMINANCE. The punnett squares and problem however
is tackled in exactly the same way that we did the incomplete
dominance problem.
The difference is only in the description of how the genes
interact.
Hi there. I’m your pilot
And I am your COPILOT
Take me
somewhere
warm!
Lets go on vaca!
Take me to
Talledega!
CODOMINANCE
With CODOMINANCE neither gene is dominant to the other. They
both act and are both expressed. It’s kind of like the pilot and Copilot.
Both actually fly the plane, yet they remain two people – they are not
a blend of the two.
I am a roan cow. Roan is an example of
codominance as I have genes making
red coat and genes making white coat
that are both expressed.
STEP 1
In cattle red and white coat colour are
CODOMINANT. If a red cow and white
bull are mated, determine the offspring.
phenotype
Let R represent Red allele
Let W represent white allele
Let RW represent roan
STEP 2
P1 Ô WW X Ю RR
4/4 roan
cattle
You may not be aware of
this, but I have a baby. Since
Bella is human and I am a
vampire that makes the
baby a mixture of the two
and although this is make
believe, you could take it as
an example of codominance.
The baby would have
characteristics of both a
human and a vampire.
You forgot to
mention that you
had to change me
into a vampire in
order for me to
deliver the baby
and save my life.
Now I can’t have
any more children
‘cause I’m a
vampire, thanks to
you Edward.
Let’s see if you can use the genetics to help you in a business
venture. In this case you are to buy Andalusian Fowl raise them
and sell them at a profit. The most valuable birds are blue, like the
variety shown below
That’s right. I’m blue and cost
$500
So you buy a breeding pair of these blue fowl,
that’s a rooster and a chicken, and you get the
following offspring: Initial outlay $1000
When you mate them together
you get ten eggs. When they
hatch you would think you would
get ten fowl worth an estimated
$5000. That is a great profit but
you still have to consider your
time, and the cost of feeding and
maintaining your fowl.
Unfortunately, things aren’t that simple
and when the eggs hatch you get:
3 worthless
black
2 worthless
mottled
5 blue
Well, that’s not too bad. A least you can sell the 5 blue and
gross $2500 for your effort. The question you should have is why
didn’t your mating pair produce all high quality blue fowl?
Take a look at the numbers – 3/10 - 2/10 - 5/10
Codominance numbers would be ¼ - ½ - ¼ 1:2:1
With an experimental probability, the results are consistent with
predicted codominance for 10. That is 2.5 homozygotes
5 heterozygotes and 2.5 homozygotes.
3 approx. 2.5
homozygote black
5 approx. 5
heterozygote blue
1:
2 approx. 2.5
homozygous mottled
2:
1
It would be a lot better for your business if you were able to
produce 100% blue offspring and you have the means to do it.
You just have to understand the genetics in your breeding. Take
the homozygous black and mate it with the homozygous
mottled
STEP 2
X
BB
STEP 1
x
bb
STEP 4
B
B
b
Bb
Bb
b
Bb
Bb
B= black
b = mottled
Bb= blue
Phenotype
100% blue
It’s magical!
Sometimes there are more than two possible
alleles that can be used in the expression of a
characteristic. So you might have three alleles.
Maybe the allele for B is dominant, the allele for A
dominant and the allele for o recessive.
This creates a problem with the convention used in these problems. If you
always use a capital for the dominant and a lower case for the recessive, how do
you deal with three alleles – two dominant and one recessive?
In order to avoid the confusion, you could connect these
alleles to the same characteristic with a letter indicating
each allele as a superscript.
I A is dominant
note capital A for dominant
–
I B is dominant
–note capital B for dominant
I o recessive
or also i
This is exactly what happens with human
blood types. There are multiple alleles, some
codominant, others recessive. There is a
dominant A allele, a dominant B allele and a
recessive O allele.
Blood type A is
genotype IA I A or IA Io
Blood type O is
genotype IO I O
Blood type B is
genotype IB I B or IB Io
Blood type AB is
genotype IA I B
Some of you may know that blood also has an Rh factor making
it either Rh positive or Rh negative. This will be dealt with once
you get confident with the ABO blood types.
Lets do some blood type genetics problems
A type AB man marries a type O woman. What
are the possible blood types for their children?
STEP 2
STEP 4 PUNNETT
P1 Ô IA IB X Ю IOIO
IA
PHENOTYPES
IB
STEP 3-GAMETES
IA
IB
IO
½ TYPE A
½ TYPE B
IO
IO
IAIO
I BI O
IO
IAIO
I BI O
GENOTYPES
½ heterozygous
type A
½ heterozygous
type B
Let’s review and apply something new. Remember you
learned how to do a test cross? How would you do a
test cross to determine the genotype of a type B male?
The test cross takes the dominant phenotype and mates it with
the recessive. If you get a recessive offspring, the dominant
phenotype is heterozygous. If you do not produce any recessive
offspring, the dominant phenotype is homozygous.
P1 Ô IB IB X Ю IOIO
OR
P1 Ô IB IO X Ю IOIO
IB
IB
IO
I BI O I BI O
IO
I BI O I BI O
IB
PHENOTYPE
100% 4/4 Type B
IO
IO
IBIO IOIO
50% Type B
IO
IBIO IOIO
50% Type O
So if I tell you that a type B man is mated with a
type O woman and they have a type O baby, you
will know the genotypes of the parents, right?
The male must be IBIO and the female
type O has only the IOIO possibility.
The proof is on the punnett squares on the previous
slide.
I have blood type O. Can I have a
type AB baby Edward Cullen?
No you cannot have
a type AB baby
Jessica. Here is the
punnett showing it is
not possible.
Jessica
?
?
IO
?IO ?IO
IO
?IO ?IO
No substitution of any letter will
result in IAIB in this punnett
Okay, lets try another problem. I was
watching one of those daytime talk
shows and they were trying to
determine if Bert was the father of
Debby’s baby and if Bert was the father
of Suzy’s baby.
Bert fathered my child
Bert has blood type AB and Debby has blood type B. Debby’s child has blood type O.
Could Bert be the father?
Bert
Debby
IA
IB
IB
IAIB IBIB
IB
IAIB IBIB
Do the
paternity
test!!
Bert
Type
AB or B
child
NO
type O
Debby
IB
IO
No we don’t always
need an expensive
paternity test . A
simple blood test
will do sometimes.
IA
IB
IAIB IBIB
IAIO IBIO
Type AB,
B or A
NO type
O child
possible
Bert cannot be the father of
Debby’s child. There is no possible
way that he could produce the
type O child with his genetics! The
proof is in the punnett squares!
Okay, so Bert cannot be the father of Debby’s baby. We don’t actually
need to do the paternity test. But what about Suzy. Could Bert be the
father of Suzy’s child? Here’s the 411 (information)
Bert has blood type AB and Suzy has blood type A.
Suzy’s child has blood type B. Suzy’s mother has
blood type O. Could Bert be the father?
Calculate Suzy’s blood type first
A
I
?
Suzy’s
mom
IO
IAIO ?IO
IO
IAIO ?IO
So the question now becomes if Suzy is heterozygous
type A, and the child is type B can Bert be the father?
It’s absurd, of course
because Bert is a
muppet! Why do I
watch these shows?
What a waste of
time.
Bert
IA
IB
Suzy
IA
IAIA IAIB
IO
BO
IAIO I I
Once again there
is NO Possibility
of Bert producing
a type O child.
He cannot be the
father!
Sometimes a paternity test is needed. If the blood types do not
eliminate an individual and he could be the father, then a paternity
test would be necessary because there is always the possibility of
another male , with the same blood type, being responsible for
fathering the child.
Remember when Bella got
attacked and lost a lot of blood.
Well we used biological
knowledge about blood types to
save her. Since we did not know
her blood type and we had to
immediately give her a blood
transfusion from another
person, we used blood type O.
Blood type O is considered to be
the universal donor as the red
blood cells do not have and
antigens on the surface that
could be rejected by the immune
system
I got tested after and found out that my actual
blood type is AB and I am therefore the
universal acceptor. I can take any type of
whole blood without rejection.
Whole blood
Blood type compatibility is something us vampires
are very interested in . It is more complicated than it
first appears. Of course it is critical to understand it
in the medical profession as a unit of incompatible
blood transfused between a donor and recipient will
cause a severe acute hemolytic reaction with
hemolysis (RBC burst) renal failure and shock as a
result. Death is a definite possibility.
This is for whole blood
transfusions only.
The O universal donor
relationship is reversed for
plasma products. AB type
plasma can be transfused to
all patients, while O- types
are the universal plasma
recipients.
Now it’s time to talk about the Rh factor.
Rh positive is dominant and represented by +.
Rh negative is recessive and represented by -.
A person who is Rh positive can have the genotype ++ or +-
A person who is Rh negative can only have the genotype -The Rh factor is another element to consider with blood type
compatibility. If you are Rh positive, you need to have Rh
positive blood for a transfusion. The same applies to the Rh
negative people.
Now let me compare the vampire blood line to
the Rh factor in the human blood type. If you
assume that Edward, the male vampire,
is Rh positive and his woman, Bella, is Rh
negative then when they have a baby , there is
a possibility of the baby being Rh positive.
Bella would reject an Rh positive baby if the
blood mixed with her own. So Rh negative
mothers are dangerous to Rh positive babies
because the mother’s immune system
produces antibodies that pass into the baby’s
blood stream. In order to keep her baby safe
from this, she has to get a shot of RhoGAM
that combines with all the Rh positive antigens
that may pass into into her blood. This
prevents her antibodies from being able to
react to Rh positive cells
+
-
-
+-
--
-
+-
--
½ Rh positive
½ Rh negative
In the case of us vampires, however, our special
blood proteins mean that in order to save the
baby and yourself you would have to become a
vampire!!
There is another ratio you should be aware of. There
is something called a lethal allele and if an organism
gets the homozygous combination, they die in
embryonic development and are never born.
Take a look at these results.
They are not a 3: 1 ratio nor
are they a 1:2:1 ratio. They
appear to be a 1:2 ratio.
Here is your
answer. Those
with YY die as
an embryo.
Y
Y
y
YY
Yy
y
Yy
yy
dead
2/3 are yellow
1/3 is non yellow
Now we need to talk about the last issue. There is something
called sex linkage, where one sex primarily males seems to get a
particular genetic trait.
The gene for red green colour blindness is located on the X
chromosome. This is an interesting situation as boys have XY
chromosomes and the Y does not match, nor provide a second allele.
Males inherit the recessive defective
gene only from their mothers as
their fathers must give them the Y in
order for them to be boys.
The males are HEMIZYGOUS Xc Y if they get
the recessive colour blind gene or XC Y if they
get the dominant.
I can’t tell red from
green. It’s your X gene
mom!
Women can be carriers of colour blindness
and they will pass it on to their sons in a
50/50 manner
XC
Y
XC
XCXC
XCY
Xc
XCXc
XcY
PHENOTYPE
2/2 girls normal
Colour
blind
½ boys colour blind
Are you colour blind? See if you can see the numbers on the colour plates
This plate does not have a number. It’s the way a
number plate would appear if you were colour blind
So if you encounter a situation where a disproportionate number of males are
afflicted with something, it’s probably due to X linked sex linkage. It also will skip
generations so be prepared for potential questions. Another common x linked
question is hemophilia (blood clotting disorder) or fruit fly eye colour white and red.
Xr
XR
NEWSFLASH
Wild type in
fruit flies
means they are
heterozygous.
XR
XR
Y
Problem: red eye female fruit fly mates a male
white eye and produce all red eye offspring.
What does this mean?
White eyes are recessive to red
XRXr XRY
XRXr XRY
Y
P1 cross Xr Y x XRXR
Take a red eye female from the f1 and mate it
with a red eye male from f1 and you get 112
females with red eyes and 54 males with white
eyes and 58 males with red eyes.
F1 cross XRY x XR X r
XR
XRXR XRY
Female 100% red
Xr
XRXr XrY
Male 50% red
and 50% white
Affects the males – you
should suspect X linked
sex linkage.
Thomas Morgan discovered these
sex linked fruit fly genes
My dad was a hemophiliac but no one in my family
got the disease. What does that tell you about my
genetics and should I be worried about my baby
with Edward having the disorder?
That’s not good Bella. If
your Dad had hemophilia
that means that you are a
carrier of the allele. Look at
the punnett below.
Xh
P1 Xh Y x XHXH
XH
XH
Y
XH Xh
XHY
XH X h
XH Y
Female carrier
Female carrier
100% of
females are
carriers of the
recessive
hemophilia
gene
How can I be a carrier? My brothers didn’t get
the hemophilia gene. What does that mean
about my baby with Edward?
1.First, you got the gene from your hemopiliac dad. All girls
get one X from their mom and one from their dad. X h is the
only one he had to give to you
2.Your concern is well warranted about your baby. Since you
are a carrier, if you have a son, there’s a 50/50 chance he will
be hemophiliac. That is a concern for a baby human-vampire
hybrid. Daughters will not be hemophiliac
XH
No hemophilia gene
I carry the
hemophilia gene
P1
XH Y
x
X HX h
Y
XH
XH XH XH Y
Xh
XHXh XhY
Normal boy
Hemopiliac boy!!
Hemophiliac vampire
baby boys.....Well
that’s enough for me
to think about.......
OMG! Is this what
my life has come
to?
Hey Edward. Since we are together, can we have
a cat? I found this really cool cat. It’s called a
calico cat and it has a whole bunch of colours on
its coat. I think it’s really cool and it’s supposed
to be good luck. Lets keep it
There is a genetic
reason why there
are all these
colours on the
cat fur. Let me
explain.
For tortoiseshell and calico cats, the gene for coat
colour is located on the X chromosome. The two
colour alleles that can be present are orange and
black, both being codominant. They are
represented by XO- orange allele and XB. – black
allele. We have to use different letters because
neither is recessive.
For female cats you can have the
following combinations
XBXB = black female
XOXO = orange female
XOXB = tortoiseshell female
For normal male cats, you
can have the following
combinations
XBY= black male
XOY= orange male
Take a look at one of the cells of a
calico female cat. Look at the
nucleus and observe the dark
area on the side of the nuclear
membrane. This structure you
see is an inactivated X
chromosome called the Barr
body.
These alleles are activated or
inactivated randomly in cells.
XO
Orange fur
XB
Black fur
Activate XO
orange
Activate XB
black
The male cats are hemizygous at this locus and
they are only capable of producing two
different colour cats.
Only female cats can be tortoiseshell (calico) cats.
Female cats can also be black or orange.
Let me show you a sample cross between
these cats that often appear in questions.
male
female
P1 XBY x XOXO
THOUGHT CHALLENGE 1
What would you expect in
the F1 if you reversed the
colours of the parent cats?
XB
Y
PHENOTYPES
100% Calico females
100% calico females
100% black males
XO
XOXB XOY
THOUGHT CHALLENGE 2
What cross will produce both
orange and black males?
XO
XOXB
Female must be calico - male does not matter
XOY
100% orange males
Male calico cats are possible if they have chromosomal
abnormalitities. In extremely rare circumstances you
could have non-disjunction of the chromosomes alike to
Klinefelter syndrome in humans and produce a calico
male
XBXOY= calico male genetic abnormality
So if you see a calico cat, it is a female
unless in the very rare instance of a
genetic abnormality!
Wow! Can you just
stop now.
Hemophiliac vampire
baby boys..calico
cats... I’ve really had
enough today. I’ve got
to go, so see you later.
Incomplete dominance ..Sex
linkage
Ends