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.

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

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

Continuous Variation

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.

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

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 ?

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” ?

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

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

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

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

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

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)

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

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

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

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