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Genetics Terminology Illustrated
Epistasis
A Presentation for The Angelfish Society
May 20, 2007
by Tamar Stephens
Have you ever seen…
• A smokey gold?
• Or a zebra gold?
• Or a half-black gold angelfish?
Well, why not?
That’s because of something called
“epistasis”
• So what is epistasis?
What is Epistasis?
• Epistasis come from the Greek –
– “epi” means “upon”
– “histani” means “to place”
• So it means to place upon or to
stand upon.
Definition
“A nonreciprocal interaction between two nonalternative forms of genes in which one gene
suppresses the expression of another
affecting the same part of the organism”
(American Heritage Dictionary)
Wow! Is that crystal clear to you?
Here’s a simpler definition
“The situation in which the alleles at one
gene cover up or alter the expression of
alleles at another gene.” (Genetics, Weaver
and Hedrick, Wm. C. Brown
publishers,1989)
How is epistasis different from
dominance?
Dominance is when an allele suppresses the
expression of another allele at the same
locus.
Pair of Chromosomes
“A” Locus
A
a
“A” and “a” are alleles to
each other. That means they
are alternate forms of the
gene at the “A” locus.
If “A” suppresses the
expression of “a” then “A”
is dominant to “a.”
Epistasis involves two gene pairs
Epistasis is when an allele at one locus masks
(covers up) or alters the expression of an
allele at a different locus.
Pair of Chromosomes
“A” Locus
A
a
“B” Locus
B
B
If the “A” allele at the A
locus alters or masks the
expression of the “B” allele
at the B locus, then A is
epistatic to B.
(The B locus could be on
the same chromosome as the
A locus, or it could be on a
different chromosome.)
Here is an example of dominance.
The allele for black (D) and the allele for gold (g) are
alternate forms of a gene at the same locus locus.
If an angelfish has the genotype D/g, D is dominant
and the angelfish will be black.
Black
D/D
+
+
gold
g/g


black
D/g
In fact, gold is recessive to all of the
alleles at that locus
•
•
•
•
•
Wild type:
Dark:
Marble:
Gold Marble:
Gold
+/g = silver
D/g = black
M/g = marble
Gm/g = gold marble
g/g = gold
But in double dose (g/g) gold is epistatic to some
alleles at other loci. Let’s look at a few.
Here is an example of epistasis
• Gold in double
dose is epistatic to
smokey.
• Thus g/g – Sm/+
and g/g – Sm/Sm
both look like a
gold angelfish.
• The double dose of
gold prevents the
expression of the
smokey trait.
This gold angelfish could have none,
one, or two alleles for smokey.
How would you find out?
To check for a hidden gene…
Cross the gold angelfish with a wild type silver.
(You need to know the genetics of the silver – it cannot have any hidden
alleles! If must be wild-caught, or directly descended from wild-caught
angels that have not been interbred with any domestics.)
X
What are the possible outcomes?
If the gold angelfish has no alleles
for smokey…
gg x
All of the offspring
are silver angels,
each with one
recessive gold allele.
+/+  100% (+/g)
If the gold angelfish has one allele
for smokey…
Cross Gold x Silver
g/g - Sm/+ x +/+ - +/+ 
50% of the offspring are
+/g – Sm/+ smokey
50 % of the offspring
are +/g - +/+
If the gold angelfish has two alleles
for smokey…
g/g – Sm/Sm x +/+ - +/+ 
100% +/g – Sm/+ smokey offspring
• Can you think of another example where
g/g is epistatic to another allele?
Gold is epistatic to zebra!
• g/g – Z/+ = gold
• g/g – Z/Z = gold
This zebra angelfish can have one allele for gold – but
if it had two alleles (g/g), the gold would suppress the
expression of zebra and it would be gold!
Have you ever seen a half-black
gold?
Gold (g/g) is epistatic
to half-black (h/h).
Thus: g/g - h/h = gold
This half-black could have one hidden
allele for gold. But if it had a double dose
of gold, it would be gold!
So we have seen that g/g is epistatic
to smokey, zebra, and half-black!
Each one of these
genotypes will be gold!
• g/g – Sm/+
• g/g – Sm/Sm
• g/g – Z/+
• g/g – Z/Z
• g/g – h/h
• g/g – Sm/+ - Z/+
• Etc.
Is half-black epistatic to pearlscale?
• I read on a web site a few years ago that
half-black was epistatic to pearl scale.
• A few months ago, I asked on the TAFII
forum if anyone had ever seen a half-black
pearlscale, and….(next slide)
Half-black pearlscale
• The next time I
checked that thread, I
found several photos
of half-black
pearlscale!
So is half-black
epistatic to pearlscale?
Obviously, the answer
is no, because both
traits are present and
fully expressed.
Stripeless
• Stripeless is epistatic to the gene that codes
for stripes in wild type angelfish.
The addition of
one stripeless
allele modifies the
expression of the
wild type by
suppressing
expression of the
stripes.
+/+
Wild type angelfish
(silver with stripes)
+/+ - S/+
Silver ghost
(no stripes)
That’s all Folks!
The End