Solved by a verified expert:Thus far we have studied genes that have
only two possible alleles. This isn’t
always the case. Sometimes these
multiple alleles will express a hierarchy of dominance. In other cases, there may be co-dominance
(where both alleles are fully expressed, resulting in a different phenotype, in
a heterozygous situation). Co-dominance
differs somewhat from incomplete dominance (where the heterozygote is an
intermediate, resulting in incomplete expression of the two alleles).
EXAMPLE:
In humans, one form of blood typing
involves the ABO grouping. This is coded
for by a
single gene, with multiple alleles that are
designated as follows:
IA = A antigen IB=
B antigen i = neither
antigen
When IA is present in an
individual, an A antigen is present on their red blood cells. If the individual
were homozygous for this allele, they would have type A blood. When IB is present in an
individual, a B antigen is present on their red blood cells. If the individual were homozygous for this
allele, they would have type B blood. In
situations where an individual has one of each of the co-dominant alleles, they
will have BOTH types of antigens (i.e., type AB blood.)
When an individual has one of the co-dominant
alleles along with the recessive allele, then the dominant allele is expressed
(e.g., IAi would have the A antigens and be type A.) If an individual has two recessive alleles
(ii), they have neither A nor B antigens and are called type O (for zero).
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