EXCEPTIONS TO THE RULES NON MENDELIAN INHERITANCE
DOMINANCE: IT’S NOT JUST A GOOD IDEA… IT’S THE LAW! “Complete Dominance” = Dominant alleles cannot be “hidden” AA or Aa will both show the “A” trait Recessive alleles can be masked, however… aa the only genotype to show the recessive trait…
CONFOUNDING OBSERVATION? Sometimes the heterozygous offspring have a trait that isn’t exactly the trait of either purebred parent Translation? Gregor Mendel was either twice the genius people give him credit for being… OR… He was lucky to have chosen the pea plant!
INCOMPLETE DOMINANCE VS. CODOMINANCE “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. AA = Dominant 1 BB = Dominant 2 AB = Dominant 1 & Dominant 2 AA aAaAa aAaAa
IS INCOMPLETE DOMINANCE REAL ? On the macroscopic level, snapdragons are pink… In the microscopic level, they are red and white (just like codominance)
ANOTHER EXAMPLE OF CODOMINANCE: ROAN COWSAND HORSES Red x White Red and White hairs
MULTIPLE ALLELES Phenotype affected by more than 2 alleles 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 carbohydrate I B =B carbohydrate i=O (no carbohydrate)
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)
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
POLYGENIC TRAIT More than 1 gene controls the trait Each gene may be inherited separately Symbols of polygenic traits same as “normal” Mendelian genetics: 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)
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 XHXH XhXh XHXH Y
EXAMPLES: Hemophilia Red-green color-blindedness Male pattern baldness
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!
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