Genetics Terminology IllustratedEpistasis

A Presentation for The Angelfish SocietyA Presentation for The Angelfish SocietyMay 20, 2007May 20, 2007

by Tamar Stephensby Tamar Stephens

Have you ever seen…

• A smokey gold?A smokey gold?• Or a zebra gold?Or a zebra gold?• Or a half-black gold angelfish?Or a half-black gold angelfish?

Well, why not?Well, why not?

That’s because of something called “epistasis”

• So what is epistasis?So what is epistasis?

What is Epistasis?

• Epistasis come from the Greek – Epistasis come from the Greek – – ““epi” means “upon”epi” means “upon”– ““histani” means “to place”histani” means “to place”

• So it means to place upon or to So it means to place upon or to stand upon.stand upon.

Definition

““A nonreciprocal interaction between two non-A nonreciprocal interaction between two non-alternative forms of genes in which one gene alternative forms of genes in which one gene suppresses the expression of another suppresses the expression of another affecting the same part of the organism” affecting the same part of the organism” (American Heritage Dictionary)(American Heritage Dictionary)

Wow! Is that crystal clear to you?Wow! Is that crystal clear to you?

Here’s a simpler definition

““The situation in which the alleles at one The situation in which the alleles at one gene cover up or alter the expression of gene cover up or alter the expression of alleles at another gene.” (alleles at another gene.” (GeneticsGenetics, Weaver , Weaver and Hedrick, Wm. C. Brown and Hedrick, Wm. C. Brown publishers,1989)publishers,1989)

How is epistasis different from dominance?

Dominance is when an allele suppresses the Dominance is when an allele suppresses the expression of another allele at the same expression of another allele at the same locus. 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 Epistasis is when an allele at one locus masks (covers up) or alters the expression of an (covers up) or alters the expression of an allele at a different locus. allele at a different locus.

Pair of Chromosomes

“A” Locus A

“B” Locus B

a

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 The allele for black (D) and the allele for gold (g) are

alternate forms of a gene at the same locus locus. alternate forms of a gene at the same locus locus. If an angelfish has the genotype D/g, D is dominant If an angelfish has the genotype D/g, D is dominant

and the angelfish will be black. and the angelfish will be black. Black + gold blackD/D + g/g D/g

In fact, gold is recessive to all of the alleles at that locus

• Wild type: Wild type: +/g = silver+/g = silver• Dark: Dark: D/g = blackD/g = black• Marble: Marble: M/g = marbleM/g = marble• Gold Marble:Gold Marble: Gm/g = gold marbleGm/g = gold marble• Gold Gold g/g = gold g/g = gold

But in double dose (g/g) gold is epistatic to some But in double dose (g/g) gold is epistatic to some alleles at other loci. Let’s look at a few.alleles at other loci. Let’s look at a few.

Here is an example of epistasis• Gold in double Gold in double

dose is epistatic to dose is epistatic to smokey. smokey.

• Thus g/g – Sm/+ Thus g/g – Sm/+ and g/g – Sm/Sm and g/g – Sm/Sm both look like a both look like a gold angelfish. gold angelfish.

• The double dose of The double dose of gold prevents the gold prevents the expression of the expression of the smokey trait. 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. Cross the gold angelfish with a wild type silver.

(You need to know the genetics of the silver – it cannot have any hidden (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 alleles! If must be wild-caught, or directly descended from wild-caught

angels that have not been interbred with any domestics.) 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 +/+ gg x +/+ 100% (+/g) 100% (+/g)

All of the offspring are silver angels,

each with one recessive gold allele.

If the gold angelfish has one allele for smokey…Cross Gold x SilverCross Gold x Silver

g/g - Sm/+ x +/+ - +/+ 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 +/+ - +/+ g/g – Sm/Sm x +/+ - +/+ 100% +/g – Sm/+ smokey offspring100% +/g – Sm/+ smokey offspring

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

Gold is epistatic to zebra!

• g/g – Z/+ = goldg/g – Z/+ = gold• g/g – Z/Z = goldg/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 Gold (g/g) is epistatic to half-black (h/h).to half-black (h/h).

Thus: g/g - h/h = goldThus: 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 Each one of these genotypes will be gold!genotypes will be gold!

• g/g – Sm/+g/g – Sm/+• g/g – Sm/Smg/g – Sm/Sm• g/g – Z/+g/g – Z/+• g/g – Z/Zg/g – Z/Z• g/g – h/hg/g – h/h• g/g – Sm/+ - Z/+g/g – Sm/+ - Z/+• Etc.Etc.

Is half-black epistatic to pearlscale?

• I read on a web site a few years ago that I read on a web site a few years ago that half-black was epistatic to pearl scale.half-black was epistatic to pearl scale.

• A few months ago, I asked on the TAFII A few months ago, I asked on the TAFII forum if anyone had ever seen a half-black forum if anyone had ever seen a half-black pearlscale, and….(next slide)pearlscale, and….(next slide)

Half-black pearlscale• The next time I The next time I

checked that thread, I checked that thread, I found several photos found several photos of half-black of half-black pearlscale!pearlscale!

So is half-black So is half-black epistatic to pearlscale? epistatic to pearlscale?

Obviously, the answer Obviously, the answer is no, because both is no, because both traits are present and traits are present and fully expressed.fully expressed.

Stripeless• Stripeless is epistatic to the gene that codes Stripeless is epistatic to the gene that codes

for stripes in wild type angelfish. for stripes in wild type angelfish.

+/+

Wild type angelfish (silver with stripes)

The addition of one stripeless allele modifies the expression of the wild type by suppressing expression of the stripes.

+/+ - S/+

Silver ghost (no stripes)

That’s all Folks!

The End