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.
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 WhiteRESULTING PHENOTYPE
Black Fur
Grey Fur
White Fur
where
B = allele for black &
W = allele for whiteAnd here's an example with Codominance:
GENOTYPES
BB = Homozygous Black
BW = Heterozygous
WW = Homozygous WhiteRESULTING PHENOTYPE
Black Fur
Black & White Fur
White Fur
where
B = allele for black &
W = allele for white
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?An excellent example of multiple allele inheritance is human blood type. Blood type exists as four possible phenotypes: A, B, AB, & O.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.
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:
IA IB i |
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.
IAIA IAi |
Type A Type A |
IBi |
Type B |
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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.
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.
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. 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.
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 ....
Step #1 - "homozygous A" = IAIA, & "heterozygous B" = IBi 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). 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. |
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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. |