1. Mendelian Inheritance
Most human traits are polygenic; meaning they are controlled by more than one gene
Pedigrees are tools used by geneticists to trace a trait back through a family line or help predict traits of offspring

2. Simple Recessive Heredity
An individual must have two copies of an allele to express a certain trait Ex
Cystic fibrosis
Tay-Sachs Disease
Phenylketonuria (PKU)

3. Simple Dominant Heredity
Follows Mendelian rules of dominance; an individual only needs one allele for the trait to be visible in the phenotype
EX:
Tongue curling
Hitchhiker’s thumb
Huntington’s disease

4. Complex Patterns of Inheritance
Incomplete dominance – a heterozygous individual has a blend or mix of the parental phenotypes; pink flowers
Codominance – both alleles are expressed in the phenotype of a heterozygous individual; checkered chickens

5. Multiple phenotypes from multiple alleles – a trait is controlled by more than 2 versions of the gene; ABO blood typing
Sex determination – determined by acquiring an X or a Y chromosome from the male parent, the female parent donates only an X

6. Sex-linked inheritance – a trait located on the sex chromosomes usually referring to traits on the X chromosome
Polygenic (traits) inheritance – multiple genes code for 1 trait, like eye color

7. Environmental Influences
Factors, other than genetics, that affect an organism’s phenotype
External factors: nutrition, light, chemicals, temperature, infection
Internal factors: hormones, structural differences

8. Changes in Chromosome number
When humans have a number of chromosomes other than 46
EX:
Down syndrome – trisomy 21
Turner’s syndrome – XO – only one X chromosome
Klinefelter’s syndrome – XXY