PHENOTYPES ARE THE RESULTS OF THE INTERACTIONS

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PHENOTYPES ARE THE RESULTS OF THE INTERACTIONS
OF GENOTYPES AND ENVIRONMENTS
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These are different phenotypes produced by
mutations of the bar eye gene in Drosophila.
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Note the change in the number of eye facets at
different temperatures

This graph is an example of a norm of
reaction ---a graph or table showing the
relationship of genotypes and phenotypes under
different environmental conditions and/or at
different levels of organization.
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Polydactyly, a dominant trait with variable
expressivity.
LF/RF LT/RT

Note the variation in both the number of extra
digits (arrows) and their expression on different
hands of the same individual (above, right).
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Afflicted progeny from normal parents.

Polydactyly also shows variable or
incomplete penetrance.
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The ABO blood group antigens are formed by
adding different sugar molecules to a basic H
structure or H substance. In type O, no
sugars are added. The A and B alleles encode
distinct enzymes which add the different sugars.
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SUPPOSE YOU COULDNT MAKE ANY H-SUBSTANCE AT ALL,
WHAT WOULD BE YOUR (APPARENT) BLOOD TYPE?
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SUPPOSE YOU COULDNT MAKE ANY H-SUBSTANCE AT ALL,
WHAT WOULD BE YOUR (APPARENT) BLOOD TYPE?
Ans You would be type O regardless of your
I-allele genotype. Such an epistatic mutation
is known in human populations, though it is
rare. It assorts independently of the I-gene
and is recessive. The h/h genotype encodes
the Bombay phenotype.
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A pedigree showing the Bombay phenotype. What is
(are) the real ABO genotype(s) of the affected
individual ?
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AnsShe can be AB and her mate A/O. She gives
her B allele to the first progeny, her A to the
second and her B to the third. Her mate gives
his A allele to the first progeny and his O
allele to the other two. She can also be B/O
and her mate A/O . In that case, she would again
give her B allele to the first progeny (her
mate would give his A allele), her O allele
to the second progeny (her matewould give his
A allele) and her B allele to the third
progeny (her mate would give his O allele). Work
this out for yourself.
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Historical note There is another gene,
secretor, which allows the ABO blood antigens
to leak into saliva and various other body
fluids. Homo- zygotes for this recessive allele
can literally be blood typed from their
spit. Genotype blood type in blood
blood type in saliva IAIA Se/se
A O
(obviously a mistake) IAIA se/se
A A
When it was first discovered, it was not
immediately realized that secretor was
variable in the human population, and many
individuals could not be accurately blood typed
from their saliva. Given that type O is the
universal donor, this mistake could have been
lethal. Blood from these false Os could
have been transfused into people with
incompatible blood types. Many would have
died. Luckily, this potentially disastrous
mistake was discovered before the Army could
inaccurately blood type thousands of WWII
draftees from their saliva. Likewise, the
mistake was fortunately discovered before the
blood typing of semen became an established
forensic technique, for many guilty rapists would
have gone free!
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These individuals show the phenotype of
achondrioplastic dwarfism. This condition is
usually dominant and varies less in penetrance
and expressivity than polydactyly.
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The dilute gene (D or d) regulates the
expression of the Chestnut and Bay color
patterns in horses. The genetic regulation of
the piebald color pattern in beagles is not
well understood.
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Testicular feminization syndrome, an X-linked
recessive. These individuals obviously have a
female phenotype, but are genetically male
(XY)---they have no receptors to bind androgens.
Thus, the tfs allele prevents the expression of
the Y chromosome.
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A lethal allele, that encoding yellow coat color
in mice. Note the modified ratio in yellow x
yellow matings. Why are 2/3 of the progeny
yellow?
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The manx trait in domestic cats is another
example of the action of a lethal allele.
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Genotype Appearance
Survivorship AA normal
yes AAY
yellow
yes AYAY ?
no OO
normal
yes OOM manx
yes OMOM
highly deformed no Is the
affected phenotype in each case dominant or
recessive?
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Genotype Appearance
Survivorship AA normal
yes AAY
yellow
yes AYAY ?
no OO
normal
yes OOM manx
yes OMOM
highly deformed no Ans If
survivorship is considered, both are recessive.
If appearance is considered, neither trait
is dominant.
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Expression of Huntingtons disease, a dominant
phenotype.
Note that even by age 40 only about half the
people with the allele for this disease have
developed symptoms. People with a parent who has
this disease very often have to make
reproductive decisions before they know if they
are carrying the disease allele themselves.