Polygenic Inheritance

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: 1 2 3 4 5

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
1 2 3 4 5 6 7

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