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

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)

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

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

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

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

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

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