1. Polygenic Inheritance
Inherited traits that are controlled by two or more genes
Examples in humans include:
Height
Skin color
Eye color
Intelligence
Polygenic traits tend to exhibit continuous variation with respect to phenotypes in a population.

2. Discontinuous Variation
Phenotypes usually controlled by one pair of genes - single gene inheritance
Has two distinct phenotypic classes that do not overlap

3. Continuous Variation
Phenotypes usually controlled by two or more pairs of genes - polygenic inheritance
Has many phenotypic classes that overlap

4. Continuous Variation

5. Characteristics of Polygenic Inheritance
Traits usually quantified by measurement rather than counting.
Genes (2 or more) contribute to the phenotype in a small but additive way. Some, however, will make no contribution to the phenotype.
Variation is analyzed in populations, not individuals.

7. Additive Model for Polygenic Inheritance
Trait controlled by two or more genes
Dominant alleles make an equal contribution to phenotype
Recessive alleles make no contribution
Genes controlling traits are not linked
Example: 3 genes with two alleles
A, a, B, b, C, c

8. Additive Model for Polygenic Inheritance Trait = Height
Base height = 5 ft.
Each dominant allele add 3 inches to base height
Recessive alleles make no contribution
Genes controlling traits are not linked
Example: 3 genes with two alleles: A, a, B, b, C, c
What is the height for individuals:
AAbbCc ?
aaBbCc ?
AABBCC ?
aaBbcc ?

9. Additive Model for Polygenic Inheritance Trait = Height
Using same genetic information:
What are the possible phenotypes (and their probabilities) for children of the following parents?
AABBCC X aabbcc
AaBbCC X AaBBcc
For Parents: AaBbCc X AabbCc
How many children will be aaBbCC ?
aaBBCc ?
How many children will be 6’3” ?
5’3” ?
6’6” ?

10. No.# of genes vs. No.# of phenotypic classes
Two (2) pairs of genes
1. AABB
2. AABb or AaBB
3. AAbb or aaBB or AaBb
4. Aabb or aaBb
5. aabb
= 5 phenotypic Classes
Phenotypic Classes:

11. No.# of genes vs. No.# of phenotypic classes
3 pairs of genes
AABBCC
AABBCc etc.
AABBcc etc.
AABbcc etc.
AAbbcc etc.
Aabbcc etc.
aabbcc
= 7 phenotypic Classes

12. No.# of genes vs. No.# of phenotypic classes
4 pairs of genes?
= 9 phenotypic Classes

13. Multifactorial Traits
Traits that result from the interaction of one or more environmental factors and two or more genes.

14. Complex Traits
Multifactorial traits in which the relative contributions of environmental vs. genetic factors are not well established.
Examples: Cardiovascular disease, breast cancer, bipolar affective disorder, cleft palate, dyslexia, diabetes mellitus, hypertension, migraines, neural tube defects, schizophrenia, seizure disorders, intelligence

15. Studying Multifactorial Traits
How do genotypes and environmental factors interact? (hard to predict in general population)
Family studies aid in determining such interaction (due to known levels of genetic relatedness)

16. Heritability
Estimates the contribution of genotype on the expressed phenotype
Uses known level of genetic relatedness in families and measured phenotypic variation
Produces a value (H) that estimates the phenotypic variability caused by genetic differences
Example: H=96% for Fingerprint TRC

17. Twin vs. Adoption Studies
Can be used to estimate heritability
Identical Twins - separated at birth
Share same genotype
Do not share same environment
Adoptions (along with natural children)
Share same environment
Do not share same genotype

18. Threshold Model
Used to explain discontinuous distribution of a multifactorial trait (e.g. cleft palate, club foot)
cleft palate club foot

19. Threshold Model
Liability: distributed as a bell-shaped curve (consistent with polygenic trait)
Liability: caused by number of genes (which contribute in an additive fashion) and environmental factors
Phenotypic expression
is achieved only if a
certain threshold is
reached
Frequency