Extending Mendelian
Genetics
19 October, 2005 Text Chapter 14
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.
Codominance
In a set of codominant alleles, the heterozygote does not show an
intermediate phenotype. The phenotypes of both alleles are individually
expressed.
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)
Enzyme P
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.
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.
Epistasis
In epistasis, the alleles of
one gene alter the
expression of alleles of
another gene.
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
Pleiotropy
Pleiotropy is the converse of
the concept of polyfactorial
characters. In this case, one
allele causes many different
phenotypes.
Pedigree Analysis
Pedigree analysis is a useful
tool for studying inheritance
in families.
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.