Transcript Beyond Dominant and Recessive

Beyond Dominant and
Recessive Alleles
The whole story . . .
• Not all phenotypes are dictated by dominant
and recessive alleles (in fact, very few are).
• Some alleles are neither dominant nor
recessive. Most phenotypes are controlled
by the following mechanisms:
• Incomplete Dominance
• Codominance
• Multiple Alleles
• Polygenic Traits
Incomplete Dominance
• When one allele is not completely
dominant over another and the
heterozygous phenotype is an
intermediate (blending) between the
two homozygous phenotypes.
Example: WW = White Flowers
RR = Red Flowers
RW = Pink Flowers
Codominance
• Similar to incomplete dominance, except that
no new phenotype is created (not a blending
of the other phenotypes).
• Instead, both of the homozygous phenotypes
are being expressed simultaneously
Example: CBCB= Black feathers
CWCW = White feathers
CBCW = has some Black and some
White feathers
Multiple Alleles
• Occurs when there is more than 2
possible alleles that can be inherited
from the parents. (Each parent only
has 2 alleles, but in this case there
are several possible types of alleles.)
ABO Blood Groups
•There are 3 possible alleles for blood type:
•IA, IB, and i
•Alleles IA and IB are codominant, while i is
recessive.
Polygenic Traits
• In polygenic traits, more than one
gene is used to determine the
genotype and phenotype of a
particular trait (ex. skin color).
• This allows for an extremely large
variety of phenotypes.