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12.4 Notes
The Mendelian inheritance pattern is rare in nature. Most traits have more than 2 alleles
or are controlled by more than 1 gene.
Polygenic Inheritance (Multiple genes): When several genes (not just one) affect a
trait. Example: eye color is controlled by several genes.
Multiple alleles: When there are more than 2 alleles for a trait.
Example: Blood types A, B or AB or O.
IA
IB
i
IA
IA IA
A
IA I B
AB
IAi
A
IB
IA IB
AB
IB I B
B
IB i
B
i
IAi
A
IB i
B
ii
O
Codominance: When both traits show up (both are fully expressed). If an animal
has a gene for black fur and gene for white fur, he has both black and white hair
(not gray).
B=Black W = White BW= Black and White
25% Black; 25% White; 50% Black and White
B
B
BB
W BW
W
BW
WW
Incomplete Dominance: When both traits blend because neither trait completely
dominates over the other.
B= Black W = White BW = Gray
25% Black; 25% White; 50% Gray
B
B
BB
W BW
W
BW
WW
Environment affects phenotype: Temperature and nutrition, for example, can
affect how genes of fur color or height are expressed.
Linkage – genes (traits) that are close together on one chromosome are less likely to
separate during meiosis and are said to be linked.