Complex Patterns of Inheritance Exceptions to Mendels rules

Complex Patterns of Inheritance Exceptions to Mendel’s rules: not simple dominant/recessive inheritance

Incomplete Dominance The phenotype of the heterozygote is INTERMEDIATE. Red + White = Pink

Red parent plant R’ R R RR’ RR’ White parent plant R’ All pink offspring!

Codominance Both alleles are expressed! Red + White = Both Red and White (NOT pink!)

Pleiotropy The ability of a gene to affect an organism in many ways, not just in one way. • Example: Human disease PKU (phenylketonuria) • Causes mental retardation and reduced skin & hair pigmentation • Caused by mutation in gene for creating the enzyme that breaks down the amino acid phenylalanine • This mutation makes the person unable to break down phenylalanine, leading to toxic levels that can damage the body in many ways. • PKU is manageable with medications and by following a diet free of phenylalanine.

PKU patient

Epistasis A gene at one location affects the expression of a gene at another location. Coat color in mice: BB & Bb (black) bb (brown) Gene that allows color to be deposited: CC & Cc (color is present) cc (albino)

Multiple Alleles This is a gene that is controlled by more than two forms of an allele • Blood Types: A, B, AB, O • A and B are codominant • O is recessive to both

Type A: IAIA or IAi Type AB: IAIB Type B: IBIB or IBi Type O: ii

Polygenic Traits Trait controlled by two or more GENES (not alleles) Aa. Bb. Cc x Aa. Bb. Cc • Skin Pigmentation • Height

Sex-Linked Traits Humans have 46 chromosomes 22 pairs autosomes (pairs 1 -22) 1 pair sex chromosomes (pair #23) Males: XY Females: XX Sex-Linked traits are located on the X chromosome. These traits tend to affect males more than females. WHY? ? Baldness

Colorblindness is a sex-linked trait. People with normal vision should see a light image of a red/brown sailboat.