Extending Mendelian Genetics 20 October, 2004 Text Chapter 14

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Extending Mendelian Genetics20 October, 2004
Text Chapter 14
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Incomplete Dominance
Alleles of some genes show incomplete dominance.
In these cases, the heterozygote has an
intermediate phenotype - a mixture of the
phenotypes conferred by its alleles.
Is this an example of blending? Why or why not?
No. Two pink individuals can mate to produce
red, white offspring.
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Codominance
In a set of codominant alleles, the heterozygote
does not show an intermediate phenotype. The
phenotypes of both alleles are individually
expressed.
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What is a Dominant Allele?

• Usually, dominant alleles are recipes for
  functional proteins.

• Recessive alleles are altered recipes for
  non-functional proteins.

Think about flower color in pea
plants. Substrate (colorless)
Product (purple) The P
allele is a recipe for a functional enzyme. The
p allele is a recipe for a non-functional enzyme.
Purple is dominant because one copy of a
functional recipe is enough.
Enzyme P
In the analogous situation in snapdragons, one
copy is not enough, And an intermediate
phenotype is seen.
At the molecular level, both functional and
non-functional proteins are present. This is
more like codominance.
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Epistasis
In epistasis, the alleles of one gene alter the
expression of alleles of another gene.
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Polyfactorial Characters
Most interesting characters are influenced not by
one or two genes, but by dozens or hundreds. In
this example, alleles at three loci control skin
color.
The distribution shown here is characteristic of
quantitative characters. These characters vary
continuously, rather than in a few discrete
states.
Environmental factors can also influence these
polyfactorial characters
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Pleiotropy
Pleiotropy is the converse of the concept of
polyfactorial characters. In this case, one
allele causes many different phenotypes.
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Pedigree Analysis
Pedigree analysis is a useful tool for studying
inheritance in families.
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Mendelian Inheritance in Humans
In humans, many disorders follow Mendelian
patterns of inheritance.
Cystic fibrosis, sickle cell disease, Tay-Sachs
syndrome, and many others are inherited as
recessive alleles. The recessive allele codes
for a non-functional protein. Usually, one
functional allele is enough, so heterozygotes are
asymptomatic.
Why are cystic fibrosis and sickle cell alleles
so common? In both cases, heterozygotes are
protected from other diseases - recessive CFTR
alleles protect against typhoid infection, while
HbS alleles protect against malaria.
Lethal dominant alleles are (for obvious reasons)
less common. Huntingtons disease is caused by a
dominant allele whose effect on phenotype is not
obvious until after age 40.
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