1. Variations of Inheritance Patterns
Ch. 10.3

2. Objectives Describe intermediate inheritance Multiple alleles
Understand polygenic inheritance
Effect of environment on phenotypes

3. 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?

4. “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

5. P F1 F2 Make the Punnett squares of the F1 generation, then the F2
Since no dominant alleles, C is used for “color” and superscripts used for alleles. P
CB CB
CW CW F1
Make the Punnett squares of the F1 generation, then the F2
CB CW F2
CB CB
CB CW
CB CW
CW CW

6. 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 O OO

7. Antigens Antibodies Can give Blood to Can receive blood from AB
Group
Antigens
Antibodies
Can give
Blood to
Can receive blood from AB
A and B
None
AB, A, B, O A B
A & AB
A & O
B & AB
B & O O
A & B
A bit more on blood types . . .

8. 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 . . .

9. 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 heights!

10. XXYYZZ
AABBCC

11. Importance of Environment
Genotype never changes, but leaves vary in size, shape, and color depending on wind and sunlight
Enzyme responsible for black fur in Siamese cat is only active in cooler temperatures
Identical twins with exact same genotype accumulate differences in phenotype due to different diets, activity level, exposure to sunlight, etc.