Mr kousen is    Water Man

MULTIPLE ALLELES

It makes absolutely no sense whatsoever to continue if we don't know what the word "allele" means.


allele = (n) a form of a gene which codes for one
possible outcome of a phenotype

For example, in Mendel's pea investigations, he found that there was a gene that determined the color of the pea pod.  One form of it (one allele) creates yellow pods, & the other form (allele) creates green pods.

Get it? Two possible phenotypes of one trait (pod color) are determined by two alleles (forms) of the one "color" gene.




SOME BACKGROUND

When the gene for one trait exists as only two alleles & the alleles play according to Mendel's Law of Dominance, there are 3 possible genotypes (combination of alleles) & 2 possible phenotypes (the dominant one or the recessive one).

Using the pea pod trait as an example, the possibilities are like so:
 
GENOTYPES
Homozygous Dominant (YY)
Heterozygous (Yy)
Homozygous Recessive (yy)
RESULTING PHENOTYPE
Yellow
Yellow
Green

where 
Y = the dominant allele for yellow & 
y = the recessive allele for green

If there are only two alleles involved in determining the phenotype of a certain trait, but there are three possible phenotypes, then the inheritance of the trait illustrates either incomplete dominance or codominance.

In these situations a heterozygous (hybrid) genotype produces a 3rd phenotype that is either a blend of the other two phenotypes (incomplete dominance) or a mixing of the other phenotypes with both appearing at the same time (codominance).

Here's an example with Incomplete Dominance:
 
GENOTYPES
BB = Homozygous Black
BW = Heterozygous
WW = Homozygous White
RESULTING PHENOTYPE
Black Fur
Grey Fur
White Fur

where 
B = allele for black &
W = allele for white

And here's an example with Codominance:
 
GENOTYPES
BB = Homozygous Black
BW = Heterozygous
WW = Homozygous White
RESULTING PHENOTYPE
Black Fur
Black & White Fur
White Fur

where 
B = allele for black &
W = allele for white



THE DEALS ON MULTIPLE ALLELES

Now, if there are 4 or more possible phenotypes for a particular trait, then more than 2 alleles for that trait must exist in the population.  We call this "MULTIPLE ALLELES".

Let me stress something.  There may be multiple alleles within the population, but individuals have only two of those alleles.

Why?

Because individuals have only two biological parents.  We inherit half of our genes (alleles) from ma, & the other half from pa, so we end up with two alleles for every trait in our phenotype.

An excellent example of multiple allele inheritance is human blood type. Blood type exists as four possible phenotypes: A, B, AB, & O.

There are 3 alleles for the gene that determines blood type.
(Remember: You have just 2 of the 3 in your genotype --- 1 from mom & 1 from dad).

The alleles are as follows:
 
ALLELE
IA
IB
i
CODES FOR
Type "A" Blood
Type "B" Blood
Type "O" Blood

Notice that, according to the symbols used in the table above, that the allele for "O" (i) is recessive to the alleles for "A" & "B".

With three alleles we have a higher number of possible combinations in creating a genotype.
 
GENOTYPES
IAIA
IAi
RESULTING PHENOTYPES
Type A
Type A
IBIB
IBi
Type B
Type B
IAIB
Type AB
ii
Type O

Notes:



SAMPLE QUESTIONS

Let me inform you that in my time teaching this fabulous subject of biology & this incredibly fun unit on genetics, the only multiple allele questions I have ever seen have been about the human blood type trait.  So included here, for your academic pleasure, are some examples of these types of questions.  Work out the problems on paper & then click to see the solutions.

(I realize that paper is old-fashioned in the world o' internet, but I
haven't become technically savvy enough to do it any other way .... yet.)


1. A woman with Type O blood and a man who is Type AB have are expecting a child.  What are the possible blood types of the kid?  {answer}

2. What are the possible blood types of a child who's parents are both heterozygous for "B" blood type?  {answer}

3. What are the chances of a woman with Type AB and a man with Type A having a child with Type O?  {answer}

4. Determine the possible genotypes & phenotypes with respect to blood type for a couple who's blood types  are homozygous A & heterozygous B. {answer}

5. Jill is blood Type O.  She has two older brothers (who tease her like crazy) with blood types A & B.  What are the genotypes of her parents with respect to this trait?  {answer}

6. A test was done to determine the biological father of a child.  The child's blood Type is A and the mother's is B.  Dude #1 has a blood type of O, & dude #2 has blood type AB.  Which dude is the biological father?  {answer}




 

Well, that's all I have to say about that .... hope it was helpful.
 

biotopics page

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TOP SECRET ANSWER AREA

1. A woman with Type O blood and a man who is Type AB have are expecting a child.  What are the possible blood types of the kid?

Solve this using the symbols for blood type alleles & the good old punnett square.  Step #1, figure out the genotypes of ma & pa using the given info. "Woman with Type O" must be ii, because that is the one & only genotype for Type O. "Man who is AB" must be IAIB, again because it is the one & only genotype for AB blood.
So our cross is: ii x IAIB.  The proper p-square would look like this:
 
As you can see, our results are as follows:
50% of kids will be heterozygous with blood Type A
50% will be heterozygous with blood Type B
on to #2

2. What are the possible blood types of a child who's parents are both heterozygous for "B" blood type?
Step 1 - determine genotypes of parents using info in the question. 
Heterozygous means one dominant & one recessive allele.  Since they are Type "B", the dominant allele is IB, & the only recessive allele for blood type is "i".  So the both parents are IBi, & the cross is IBi x IBi.
Step 2 - our friend the punnett sqaure:
 
There is a 75% (3 of 4) chance that the child will be Type B,
and a 25% (1 of 4) chance that the child will inherit Type O (ii).
on to #3



3. What are the chances of a woman with Type AB and a man with Type A having a child with Type O?

OK, no sweat.  The genotypes of the parents are kindly supplied to us, so it's just of matter of using the punnett square correctly.  But wait a minute, we don't know whether the dad is homozygous A (IAIA) or heterozygous A (IAi).  Hmmmm .... 
Well, let's just go with what we know.  We'll just use a "?" for the unknown allele.
Correct use of a p-square should lead you to something that resembles this:
 
Remember, the question is "what are the chances of a child with TypeO"? 
As you can see, none of the squares are gonna be "ii", so there is no chance of a child with Type O. 
If one parent is AB, there is no chance any child can be O because the AB parent doesn't have a "i" to pass on.
on to #4



4. Determine the possible genotypes & phenotypes with respect to blood type for a couple who's blood types  are homozygous A & heterozygous B.

Step #1 - "homozygous A" = IAIA, & "heterozygous B" = IBi
Step #2 - Punnett Square Time !
 
Alright, the results are in.
50% (2 of 4 squares) are IAIB & those kids would have AB blood.
The other 50% (2 of 4 squares) are IAi, those kids will have Type A blood.
on to #5

5. Jill is blood Type O.  She has two older brothers (who tease her like crazy) with blood types A & B.  What are the genotypes of her parents with respect to this trait?
With some careful thinking we don't even need to do the p-square thing.  Jill is Type O, meaning her genotype is "ii".  This means that her parents each have at least one "i" in their genotype (since she inherited one from each parent).
Since one brother is Type B, one of the parents must have the IB allele, making that parent IBi. 
And since the other brother is Type A, the other parent must have the IA allele & have a genotype of IAi. 
There you have it.  Jill's parents are IAi & IB , and her brothers shouldn't be so mean.

on to #6


6. A test was done to determine the biological father of a child.  The child's blood Type is A and the mother's is B.  Dude #1 has a blood type of O, & dude #2 has blood type AB.  Which dude is the biological father?
Well well, a real brain teaser type.  Sherlock Holmes deal question.  First sort out the given facts.  Then do 2 punnet squares, each dude crossed with the mom to see what the possible offspring could or couldn't be with respect to blood type.
Fact 1 - The child is either IAIA or IAi.
Fact 2 - Mom has "B" blood, so she is either IBIB or IBi.  BUT, if she were IBIB the child would have inherited a IB from her & couldn't have a blood type of just A (which he does).  So mom must be IBi.
Fact 3 - Dude #1 has type O blood, so his genotype must be "ii", becasue that is the only genotype that codes for "O" blood.
Fact 4 - Dude #2 is type AB.  The only genotype for AB blood is IAIB, so that's what he is.
Now, let's determine the possible blood types of kids produced by the mom with each of the dudes.
 
Woman x Dude #1
IBi x ii
Possible offspring of
Woman & Dude #1

50% Type B,
50% Type O

Woman x Dude #2
IBi x IAIB
Possible offspring of
Woman & Dude #1

25% chance AB child
25% chance A child
50% chance B child
 

 

The child is Type A, so dude #1 cannot be the father.  Dude #2 could be.

The question can be answered more "quickly" be realizing that since the child's blood type is different from his mom, that allele must have come from dad.  So the "A" allele that makes him Type A came from dad, & dude #2 is the only one with that allele in his genotype (IAIB).  Dude #1 doesn't have any IA alleles, he's type O (ii).

 

 BACK

Type AB is an example of codominance.  The IA allele & the IB allele are "equal".  What I mean is that neither one dominates the other.  Instead, when inherited together in the genotype, they appear together in the phenotype.  Wa-la !  Codominance.

 
 

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