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Outline
1. Some useful terms
2. What makes one allele dominant over another?
3. Allelic series
4. Codominance and incomplete dominance
5. Dominance is not superiority!
6. Overdominance
7. Underdominance
Some useful terms
• Genotype – the alleles present in an organism
• Phenotype – the traits shown by an organism
• Homozygote – two copies of the same allele
• Heterozygote – copies of two different alleles
What makes one allele dominant over another?
• Usually recessive:
–
–
–
–
Deletion or null allele
Allele that makes non-functional product
Allele that underproduces product
Control mutation that disables an ON switch
• Usually dominant:
– Allele that overproduces product
– Control mutation that disables an OFF switch
• Often dominant or co-dominant:
– Allele that produces a novel product
– Control mutation that introduces a new switch
Lac operon
Lac operon
• A null mutation in the inhibitor is recessive
– If even one copy makes usable inhibitor, LacZ is inhibited
• An inhibitor which no longer recognizes lactose is dominant
– The super-inhibitor will eventually turn LacZ off even if there is also
regular inhibitor present
• A mutation which destroys the inhibitor binding site is dominant
– The uninhibited copy will produce LacZ even if the other is inhibited
• A mutation which damages LacZ is recessive
– The wild-type copy will continue to function
Allelic series
• An allele is dominant or recessive with respect to another allele.
• In cats, one locus controls the number of melanin granules in the hair
shaft
• Black is dominant to Chocolate
• Chocolate is dominant to Cinnamon
Allelic series
• Alleles, not phenotypes, have dominance
• White color in cats can come from a dominant control mutation which
prevents melanin synthesis
• White can also come from a recessive mutation in the melanin gene
• One gene can have multiple effects:
– Dominant-mutation white cats generally deaf
– Recessive-mutation white cats generally normal
– Eye color also affected
Codominance and incomplete dominance
Codominant alleles show the full phenotype of both alleles. A classic
example is the ABO blood type system; alleles A and B are codominant,
while O is recessive to both.
Incompletely dominant alleles show an intermediate phenotype. For
example, sickle cell heterozygotes show some sickling, but not the high
level found in homozygotes.
Codominance often occurs when both alleles produce functional, but
different, proteins. Incomplete dominance is often a dosage effect. Many
genes are codominant on the biochemical level–both products can be
detected–even if the visible phenotype shows dominance.
Dominance is not superiority!
It is easy to think of dominant alleles as superior–after all, ’dominant’ people
and animals tend to boss others around. However, dominance simply says
which phenotype the heterozygote has. It is not, in itself, an advantage or
disadvantage.
Examples:
• Huntington’s disease–dominant is worse than recessive
• Cystic fibrosis–dominant is better than recessive
• Tongue rolling–trait is neutral (as far as we know)
Dominant alleles can be common or rare.
In the absence of selection, dominant alleles have no particular tendency
to increase over recessive ones. This is counterintuitive to many people; if
necessary you should prove it to yourself by experiment.
Overdominance
Watch out! This term sounds as though it’s the same kind of thing as
“dominance”, but it refers to an advantage or disadvantage, not just to
which allele is expressed.
Overdominant alleles are alleles with codominance or incomplete dominance
in which the heterozygote is better than either homozygote.
Almost all commercial grain crops have overdominant alleles so that the
heterozygote is much more impressive than the homozygotes. If you
save seeds and plant the next generation, you will see some disappointing
homozygotes.
Another word for overdominance is “hybrid vigor.”
Underdominance
Again, watch out!
dominance.
Underdominance is not the same kind of thing as
Underdominant alleles are alleles with codominance or incomplete
dominance in which the heterozygote is worse than either homozygote.
A human example is the HLA-DR immune system locus, in which the
heterozygote of alleles 3 and 4 has a much higher risk of childhood diabetes
than either 3/3 or 4/4.
Underdominance
In the African butterfly Pseudacraea eurytus the orange and blue
homozygotes each resemble a local inedible species, but the heterozygote
resembles nothing in particular and is vulnerable to predators.