Genetics%20Terminology%20Illustrated%20Epistasis

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Genetics Terminology IllustratedEpistasis

• A Presentation for The Angelfish Society
• May 20, 2007
• by Tamar Stephens

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Have you ever seen

• A smokey gold?
• Or a zebra gold?
• Or a half-black gold angelfish?
• Well, why not?

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Thats because of something called epistasis

• So what is epistasis?

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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.

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Definition

• A nonreciprocal interaction between two
  non-alternative 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?

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Heres 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)

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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 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.
A Locus A
a
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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.

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.)
Pair of Chromosomes
A Locus A
a
B Locus B
B
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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 gold ? black D/D g/g ? D/g
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In fact, gold is recessive to all of the alleles
at that locus

• Wild type /g silver
• Dark D/g black
• Marble M/g marble
• Gold Marble Gm/g gold marble
• Gold g/g gold
• But in double dose (g/g) gold is epistatic to
  some alleles at other loci. Lets look at a few.

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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?
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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.)

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What are the possible outcomes?
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If the gold angelfish has no alleles for smokey

• gg x / ? 100 (/g)

All of the offspring are silver angels, each with
one recessive gold allele.
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If the gold angelfish has one allele for smokey

• Cross Gold x Silver
• g/g - Sm/ x / - / ?

50 of the offspring are /g - /
50 of the offspring are /g Sm/ smokey
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If the gold angelfish has two alleles for smokey

• g/g Sm/Sm x / - / ?
• 100 /g Sm/ smokey offspring

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• Can you think of another example where g/g is
  epistatic to another allele?

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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!
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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!
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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.

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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)

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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.

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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)
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Thats all Folks!
The End