Incomplete Dominance Codominance Multiple Alleles In a nutshell!

Review: Dominant/Recessive One allele is dominant over the other (capable of masking the recessive allele) PP = purple Pp = purple pp = white

Review Problem: Dominant/Recessive In pea plants, purple flowers (P) are dominant over white flowers (p) show the cross between two heterozygous plants. P p GENOTYPES: - PP (1); Pp (2); pp (1) - ratio 1:2:1 P p PP Pp PHENOTYPES: Pp pp - purple (3); white (1) - ratio 3:1

Incomplete Dominance A third (new) phenotype appears in the heterozygous that is a blend of the two traits Example are Flower Color in 4 o’clock flowers R = red allele W = white allele (other designations R’ and r) RR = red WW = white RW = pink

Problem: Incomplete Dominance Show the cross between a pink and a white flower. GENOTYPES: R W - RW (2); WW (2) - ratio 1:1 W WW RW PHENOTYPES: - pink (2); white (2) - ratio 1:1

Examples of Incomplete Dominance

Codominance BW = checkered chicken (erminette) BB = Black Chicken The heterozygous condition, both alleles are expressed equally See both traits expressed (spots/strips) BW = checkered chicken (erminette) BB = Black Chicken WW = white chicken

Codominant Human Example Sickle Cell Anemia in humans Cells are sickle-shaped (half-moon) Do not carry oxygen well and can get stuck in veins SS = sickle cells NS = some of each NN = normal cells

Problem: Codominance Show the cross between an individual with sickle-cell anemia and another who is a carrier but not sick. N S GENOTYPES: - NS (2) SS (2) - ratio 1:1 S NS SS PHENOTYPES: - carrier (2); sick (2) - ratio 1:1

Examples of Codominance in Animals Appaloosa horse Roan cow

Multiple Alleles There are more than two alleles for a trait (not just H and h) Hair color HBr = brown, Hbl = blonde, hr = red, hBk = black Eye Color EBr = brown , EHz = hazel, eGr = green, ebl = blue

An Example of Multiple Alleles – Human Blood Type Blood Types? Type A, Type B, Type AB, Type O Blood Alleles – There are three A, B, O (in book – IA, IB, i) We all get a combination of two of these alleles

Rules for Blood Type A and B are codominant and both are dominant over O IA IA (homozygous form) and IA i (heterozygous form)= Type A IB IB (homozygous form) and IB i (heterozygous form) = Type B IAIB = Type AB ii = Type O