EXCEPTIONS TO THE RULES NON MENDELIAN INHERITANCE

OBSERVATION? Sometimes the heterozygous offspring have a trait that isn’t exactly the trait of either purebred parent

INCOMPLETE DOMINANCE Hybrid (heterozygote) is a “blend” of two parent phenotypes Dominant allele isn’t completely so…

INCOMPL. DOM SYMBOLS AA (red) x aa (white) AA aAaAa aAaAa 100 % Pink

EXAMPLE OF INCOMPLETE DOMINANCE: SNAPDRAGONS

CODOMINANCE Again, no “recessive” allele Both "dominant" traits appear together in the phenotype of hybrid organisms.

CODOMINANCE SYMBOLS AA = Dominant 1 BB = Dominant 2 AB = Dominant 1 & Dominant 2

EXAMPLE OF CODOMINANCE: COLOR OF CORNSNAKES

CORNSNAKES AA AB BB

ANOTHER EXAMPLE OF CODOMINANCE: ROAN COWSAND HORSES Red x White  Red and White Spotted

MULTIPLE ALLELES 2 alleles affect the phenotype equally & separately not blended phenotype human ABO blood groups 3 alleles I A, I B, i I A & I B alleles are co-dominant i allele recessive to both

EXAMPLE OF MULTIPLE ALLELES: HUMAN BLOOD TYPE Alleles: I A =A protein I B =B protein i=O (no protein)

TYPES OF BLOOD A B O AB Anything strange about this one?

PLEIOTROPY Most genes are pleiotropic one gene affects more than one phenotypic character 1 gene affects more than 1 trait dwarfism (achondroplasia) gigantism (acromegaly)

ACROMEGALY: ANDRÉ THE GIANT

Aa x aa INHERITANCE PATTERN OF ACHONDROPLASIA aa A a Aa A a Aa x Aa Aa aa Aa 50% dwarf:50% normal or 1:1 AA aa Aa 67% dwarf:33% normal or 2:1 Aa  lethal dominant inheritance dwarf

EPISTASIS One gene completely masks another gene coat color in mice = 2 separate genes C,c: pigment (C) or no pigment (c) B,b: more pigment (black=B) or less (brown=b) cc = albino, no matter B allele 9:3:3:1 becomes 9:3:4 B_C_ bbC_ _ _cc

EPISTASIS IN LABRADOR RETRIEVERS 2 genes: (E,e) & (B,b) pigment (E) or no pigment (e) pigment concentration: black (B) to brown (b) E–B–E–bbeeB–eebb

POLYGENIC TRAIT More than 1 gene controls the trait Each gene may be inherited separately

SYMBOLS OF POLYGENIC INHERITANCE A, a = gene 1 B, b = gene 2 Etc…

POLYGENIC INHERITANCE Some phenotypes determined by additive effects of 2 or more genes on a single character phenotypes on a continuum human traits skin color height weight intelligence behaviors

EXAMPLE OF A POLYGENIC TRAIT Eye color There are genes for Tone of pigment (what color it is) The amount of pigment Position of pigments (look at people’s eyes- there are many different patterns in the iris)

EYE COLOR CONTINUED

SEX-LINKED INHERITANCE Alleles carried on sex chromosomes Gender influences phenotype

SYMBOLS OF SEX-LINKED INHERITANCE X H = dominant allele X h = recessive allele Y = no allele for this trait

EXAMPLES: Hemophilia Red-green color-blindedness Male pattern baldness

EPISTASIS Different genes interact…  One gene affects the expression of another Ex. E = dark pigment, e = no pigment B = How dark pigment is EEbb or Eebb = chocolate brown eebb, eeBb, eeBB = yellow… color is masked completely

X-INACTIVATION… If a male is XY, female is XX, how can females get “double” the amount of “X” chromosome DNA? The answer? The second “X” is turned off in females = “Dosage compensation” or “X inactivation”

ENVIRONMENTAL INFLUENCES Diet, exercise, stress… Sunlight, water alter phenotypes in plants Even temperature affects coloration on Siamese cats!