Genetics Terminology Illustrated Epistasis

1. Genetics Terminology IllustratedEpistasis A Presentation for The Angelfish Society May 20, 2007 by Tamar Stephens

2. Have you ever seen… • A smokey gold? • Or a zebra gold? • Or a half-black gold angelfish? Well, why not?

3. That’s because of something called “epistasis” • So what is epistasis?

4. 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.

5. 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?

6. 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)

7. 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

8. 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

9. 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

10. 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. Let’s look at a few.

11. 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?

12. 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?

13. 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.

14. 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

15. If the gold angelfish has two alleles for smokey… g/g – Sm/Sm x +/+ - +/+  100% +/g – Sm/+ smokey offspring

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

17. 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!

18. 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!

19. 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.

20. 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)

21. 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.

22. 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)

23. That’s all Folks! The End