exceptions to the rules non mendelian inheritance
TRANSCRIPT
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…
CODOMINANCE
• Again, no “recessive” allele• Both "dominant" traits appear together in
the phenotype of hybrid organisms.
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
PLEIOTROPY
• Most genes are pleiotropic • one gene affects more than one phenotypic
character• 1 gene affects more than 1 trait• dwarfism (achondroplasia) • gigantism (acromegaly)
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 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
SYMBOLS OF SEX-LINKED INHERITANCE
XH = dominant alleleXh = recessive alleleY = no allele for this trait
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”