Blood Grouping and Inheritance

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Blood Grouping and Inheritance

• An example of how we show multiple alleles and
  codominace

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Genotypically speaking blood is quite similar to
the other genes we have discussed.
It uses a capital letter to represent a dominant
trait
It also uses a lower case letter to represent a
recessive trait
i
I
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i
I

• From a dominant and recessive allele we can
  assume that we have 3 possible combinations
• II Homozygous dominant
• Ii Heterozygous
• ii Homozygous recessive

But with blood there is more than just two
possible alleles to consider
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When we talk about the ABO blood groups there are
actually 3 alleles that are responsible for the
different groups
The O trait has the first trait and is the lone
recessive allele and is written as i
The other two traits, A and B are both
dominant so they both use the capital letter I
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PROBLEM - But with I written as is there is no
way to decipher if it is an A allele or a B allele
SOLUTION For each separate trait the allele
will have an accompanying superscript letter.
IA
IB
This is not done to confuse you The capital I
tells you the trait is a dominant allele and the
superscript just helps you figure out which
dominant allele it is.
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So instead of just three possible genotypes, II
Homozygous dominant Ii Heterozygous ii
Homozygous recessive
We actually have 6 possible genotypes
IAIA
IAIB
IBIB
ii
IBi
IAi
And four possible Phenotypes
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Hopefully that helps you out