Complex Patterns of Inheritance Beyond Mendel’s rules of simple dominant/recessive inheritance

Incomplete Dominance Neither allele is dominant; heterozygote is a blend of phenotypes of the two homozygotes. Red + White = Pink

Genotype = CC; phenotype = chestnut

Genotype = WW; phenotype = white

Genotype = CW; phenotype = palomino

Practice incomplete dominance with Four O’Clocks (flowers) P = RR x WW Red parent plant R R W RW RW White parent plant W RW RW Genotype of F1 = 100% RW Phenotype of F1 = All pink!

R W R RR RW W RW WW F1 = RW x RW Pink parent plant Pink parent plant Genotype ratio of F2 = 1RR : 2RW: 1WW Phenotype ratio of F2 = 1 red : 2 pink: 1 white

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

Genotype: RW

Genotype = RR; phenotype = red

Genotype = WW; phenotype = white

Genotype = RW; phenotype = roan

Show the cross between two roan cows (use R and W). What is the probability that two roan cows will have at least one roan offspring? 50% What is the probability that two roan cows will have a white offspring? 25% R W RR RW R W RW WW

Multiple Alleles More than 2 alleles possible for a trait Example: Blood type in humans Three alleles possible (A, B, and O), each person only has two of those alleles Four phenotypes (blood types) possible: A, B, AB, and O A and B are codominant; O is recessive Alleles notated as IA, IB, i

Notice the new allele notation! Type AB: IAIB Type O: ii Type A: IAIA or IAi Type B: IBIB or IBi Notice the new allele notation!

Polygenic Traits Trait is controlled by interaction between 2 or more genes Examples: skin color, eye color, height, hair color Results in a continuum of expressed phenotypes

Lethal Recessive Alleles Having two mutated copies of an essential gene may be deadly E.g., Cystic Fibrosis, Sickle-cell Anemia, Tay-Sachs Disease Heterozygote survivse because it has one nonmutated form of the gene Heterozygote is carrier for disorder Heterozyous condition is sometimes linked with beneficial traits e.g., sickle cell carrier and malaria resistance

Other Complex inheritance patterns still to come: Sex-linked traits More about blood Environmental effects Chromosomal disorders