E XC E P T I O N S T O T H E R U L E S

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)

A A

a Aa Aa

a Aa Aa100 % 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 1BB = Dominant 2AB = Dominant 1 & Dominant 2

EXAMPLE OF CODOMINANCE:COLOR OF CORNSNAKES

CORNSNAKES

AA

AB

BB

ANOTHER EXAMPLE OF CODOMINANCE:ROAN COWS AND 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

• IA, IB, i• IA & IB alleles are co-dominant• i allele recessive to both

EXAMPLE OF MULTIPLE ALLELES: HUMAN BLOOD TYPE

Alleles: IA=A protein IB=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

a a

A

a

A a

A

a

Aa x Aa

Aa

aa aa

Aa

50% dwarf:50% normal or 1:1

AA

aa

Aa

67% dwarf:33% normal or 2:1

Aa

lethal

dominantinheritance

dwarf 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_B_C_bbC_bbC__ _cc_ _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 traitEach gene may be inherited separately

SYMBOLS OF POLYGENIC INHERITANCE

A, a = gene 1B, b = gene 2Etc…

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 colorThere 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 chromosomesGender influences phenotype

SYMBOLS OF SEX-LINKED INHERITANCE

XH = dominant alleleXh = recessive alleleY = no allele for this trait

EXAMPLES:

Hemophilia

Red-green color-blindedness

Male pattern baldness

EPISTASIS

Different genes interact… One gene affects the expression of anotherEx. E = dark pigment, e = no pigmentB = How dark pigment isEEbb or Eebb = chocolate browneebb, 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 plantsEven temperature affects coloration on Siamese

cats!