Transcript Extending Mendelian Genetics for two or more genes

EXTENDING MENDELIAN
GENETICS FOR TWO OR
MORE GENES
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Epistasis (“stopping”) – a gene at one locus alters the phenotypic
expression of a gene at a second locus
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Example: Mice…
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Black is dominant to brown (B and b)
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A second gene determines whether or not pigment will be
deposited in the hair. Dominant allele (C) results in color deposition.
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If a mouse is homozygous recessive (cc), the color is not deposited,
so the mouse is white.
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The gene for pigment deposition is said to be epistatic to the gene
that codes for black or brown pigment
EPISTASIS
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Quantitative Characters – characters that vary in a population
along a continuum (in gradations)
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Polygenic inheritance – an additive effect of two or more genes
on a single phenotypic character
POLYGENETIC INHERITANCE
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Example: skin pigmentation (is controlled by at least three genes)
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Dark-skin allele for each gene = A,B,C each contributing one unit of
darkness to the phenotype, and are dominant to the alleles a,b,c
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AABBCC would be very dark, aabbcc would be very light
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Alleles have a cumulative effect, so AABbcc is the same
intermediate shade as AaBbCc… both contribute 3 units to skin
darkness
POLYGENETIC INHERITANCE
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When a phenotype for a character depends on environment as
well as genotype
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In humans, for example
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Height (nutrition)
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Build (exercise)
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Skin darkness (Sun-tanning)
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Intelligence (experience)
NATURE AND NURTURE
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Norm of Reaction = a range of phenotypic possibilities due to
environmental influences
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Multifactoral = many factors, both genetic and environmental,
collectively influence phenotype
NATURE AND NURTURE