Download 10.3

Variations of Inheritance Patterns
Ch. 10.3
Objectives
1.
2.
3.
4.
Describe intermediate inheritance
Multiple alleles
Understand polygenic inheritance
Effect of environment on phenotypes
Variations of Inheritance Patterns
• With Mendel’s pea plants, alleles were quite
simple:
– If a dominant allele present in genotype -->
always present in phenotype
– If two recessive alleles present in genotype -->
present in phenotype
• What about genes where NEITHER gene is
dominant?
“Blue” Chickens?
• A black chicken and a white
chicken can produce blue
offspring in the F1 generation
• Neither black nor white allele
is dominant
• But, this isn’t blending
• Both black and white
phenotypes can reappear in
F2 generation
• This is Intermediate
Inheritance:
– offspring have phenotype that is
intermediate between
phenotypes of parents
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CB CB
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F1
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Since no
dominant
alleles, C is
used for
“color” and
superscripts
used for alleles.
Make the Punnett
squares of the F1
generation, then the
F2
F2
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Multiple Alleles with Blood Types
• 4 options for blood types
• Each parent gives one allele --> 6 possible
combinations
• Blood type AB represents codominance:
– A heterozygote expresses both traits
Blood Type
Possible Genotypes
A
AA or AO
B
BB or BO
AB
AB
O
OO
Blood
Group
Antigens
Antibodies Can give Can receive
Blood to blood from
AB
A and B
None
AB
AB, A, B,
O
A
A
B
A & AB
A&O
B
B
A
B & AB
B&O
O
None
A&B
AB, A,
B, O
O
A bit more on blood types . . .
Polygenic Inheritance
• Intermediate Inheritance and codominance
involve multiple alleles.
• It’s also possible for multiple genes to affect
a character. This is polygenic inheritance:
• Polygenic inheritance:
– Two or more genes affect a single character
• Height and skin color are two examples
• Let’s look at height as an example . . .
Polygenic Inheritance
• Let’s assume 3 genes produce height in humans
– Assume inherited separately (like Mendel’s pea genes)
•
•
•
•
•
•
•
•
A, B, & C are tall alleles
X, Y, & Z are short alleles
Parents each give 3 alleles each
AABBCC would be a tall individual
AXBBCC would be a little shorter
AXBYCC would be even shorter, and so on . . . until
XXYYZZ would be the shortest individual
The potential combinations of alleles (and
phenotypes) for a character increases with the # of
genes that affect that character --> huge range in
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XXYYZZ
AABBCC
Importance of Environment
Genotype never
changes, but
leaves vary in
size, shape, and
color depending
on wind and
sunlight
Identical twins with exact
same genotype accumulate
differences in phenotype due
to different diets, activity
level, exposure to sunlight,
etc.
Enzyme
responsible for
black fur in
Siamese cat is only
active in cooler
temperatures