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Beyond Mendelian Genetics
14.3-14.4
Extending Mendelian genetics
• Mendel worked with a simple system
– most traits are controlled by a single gene
– each gene has only 2 alleles, 1 of which
is completely dominant to the other
• The relationship between
genotype & phenotype
is rarely that simple
Spectrum of Dominance
• Alleles can show different degrees of
dominance and recessiveness in relation to
each other
Complete
Dominance
Incomplete
Dominance
Codominance
Complete Dominance
Complete
Incomplete
Co
• Mendel’s pea experiments
= Complete Dominance
– One allele completely
dominant over the other
– Phenotypes of
heterozygote and dominant
homozygote are
indistinguishable
• E.g. Pp and PP flowers both
appear purple
wild type
allele producing
functional protein
mutant
allele producing
malfunctioning
protein
homologous
chromosomes
Codominance
Complete
Incomplete
Co
• 2 alleles affect the phenotype in separate,
distinguishable ways
– 2 alleles produce two different, functional
Antigen (protein
proteins
and carbohydrate
= glycoprotein)
– human ABO blood groups
–
http://www.youtube.com/watch?v=KXTF7WehgM8
ABO Blood Groups
• Multiple Alleles
(3 alleles - IA, IB, i)
– IA & IB alleles are codominant
• Alleles code for
enzyme that
connects
glycoprotein antigen
on the surface of RBC
– i allele is recessive to
both
“I” stands for the
enzyme that adds the
antigen to the RBC
IAIA
or IAi
IBIB
or IBi
IAIB
ii
Incomplete dominance
• Heterozygotes have
phenotype that is
intermediate between 2
parental phenotypes
– RR = red flowers
– rr = white flowers
– Rr = pink flowers
• Heterozygotes produce less
red pigment than the red
homozygotes
Complete
Incomplete
Co
Incomplete Dominance = Blending?
• If blending theory was
correct, would you be
able to produce a
pure red or white
carnation from a pink
one?
– No (this would take ∞
crosses!)
Pleiotropy
• So far we have assumed 1 gene
= 1 phenotypic character
• In reality most genes can have
more than one effect on
phenotype
– E.g. Sickle Cell Anemia is
mutation in one gene but
produces multiple symptoms
Extending Mendelian Genetics for
Two or More Genes
• So far we have considered the effects of the
alleles of a single gene.
• But, there are two situations where two or
more genes are involved in determining a
particular phenotype.
– Epistasis
– Polygenic Inheritance
Epistasis
• One gene alters the phenotypic expression of
another gene (2 genes affecting 1 character)
B_C_
bbC_
_ _cc
– coat color in mice = 2
separate genes
• B,b:
more pigment (black=B)
or less (brown=b)
• C,c:
pigment (C) or
no pigment (c)
• cc = albino,
no matter B allele
• 9:3:3:1 becomes 9:3:4
Epistasis in Labrador Retrievers
• 2 genes: (E,e) & (B,b)
– pigment (E) or no pigment (e)
– pigment concentration: black (B) to brown (b)
E–bb
eebb
eeB–
E–B–
Polygenic Inheritance
• Phenotypes determined by the additive
effects of 2 or more genes on a single
character
– phenotypes on a continuum
– human traits
•
•
•
•
•
skin colour
eye colour
height
weight
intelligence
Any Questions?
• Genetics Practice 2: Beyond the Basics
• Inheritance of Blood Types Worksheet
• Textbook Questions:
– Pg. 146 #1,3,4,5,6,12
– Pg. 180 #4,9