1. How to do Test Crosses in Angelfish
Presentation for The Angelfish Society
December 9, 2007
By Tamar Stephens

2. Before we get started…
Go to the next page for a quick vocabulary refresher. If you already know the terminology, then move on to the following page.

3. Quick Vocabulary Refresher
Gene – A section of a chromosome that codes for a specific trait. Genes occur in pairs – one on each of 2 matching chromosomes.
Allele – An alternate form of a gene, that codes for a different expression of the trait
Locus – The location on the chromosome for a gene
Homozygous – Both alleles are identical, such as D/D.
Heterozygous – The alleles are different, such as D/g.
Dominant – A trait that is expressed when only one allele for that trait is present.
Recessive – A trait that only expressed when a double dose is present, such as g/g.

4. What is a test cross?
A test cross is done to confirm the genetics of an angelfish by identifying alleles for recessive (“hidden”) traits.
Example: Suppose your friend gave you a bag of gold angelfish. You know that some carry a single allele for pearlscale, and some do not carry it.
Pearlscale is a recessive trait. How do you identify which ones carry the trait so you can breed gold pearl scales?
Gold angelfish (g/g) with possible hidden pearlscale allele

5. How do you do a test cross?
To do a test cross, you can use one of two methods.
Method 1: Cross the fish with an unknown gene with a fish that is homozygous for a recessive trait at that locus. (Recall that homozygous means two identical genes for a trait, such as two gold genes g/g.)
Method 2: Cross the fish with a wild type, then cross their offspring.
Depending on the circumstances, one method may be better than another. We’re going to discuss the first method in this presentation. We’ll discuss the second method in a future presentation.

6. Let’s do an example
Your bag of gold angelfish has fish that are either:
g/g (do not carry pearlscale) or
g/g – +/p (carry one allele for pearl scale)
By sheer luck you happen to have a couple of pearl scales in your fish room already. Let’s say they are a smokey pearlscale (Sm/+ - p/p) and a silver pearlscale (p/p). It doesn’t really matter what type, because all you want to do it identify carriers of pearlscale among the golds.

7. So here is what you do…
Since you happen to have a couple of angels that are both double dose recessive for pearlscale, you can use method one – pair the unknown golds with the ones that you know have the double recessive pearl scale alleles.

8. Set up your crosses
Once your golds are breeding age, you are ready to set up some pairs. (Use a pairing tank, or if you can confirm the sex of any of the golds, just try matching the smokey pearlscale and the silver pearlscale with opposite sex golds.) X
Smokey pearlscale
Gold with possible pearlscale allele

9. Here is the second crossX
Gold with possible pearlscale allele
Silver pearlscale

10. After a pair forms, raise a spawn
Once you have a pair established, simply raise the spawn up until they are big enough to see whether you have any pearlscales (nickel to quarter size).
Looking at the pearlscale trait only, you have two possible results. (next page)

11. If the gold angelfish has a pearlscale allele, you will get 50% pearlscales.
p/p x +/p p +
p/p
Pearlscale
+/p
Nonpearl scale
Results:
50% p/p
Pearlscale
50% +/p
Non Pearlscale p p

12. If the gold angelfish does not have a pearlscale allele here are the results.
p/p x +/+ + +
+/p
Non pearlscale
Results:
100% +/p
Non Pearl Scale p p

13. So to sum it up You have two possible outcomes.
If a gold angelfish does not carry a pearlscale allele, none of the offspring will be pearlscale.
If it does carry a pearlscale allele, 50% will be pearlscale.
Gold pearlscale

14. Why not just let the golds pair up?
You can just let the golds pair up.
If some of the offspring are pearlscale, you know that both members of a pair carry the pearlscale allele.
But…if none of the offspring are pearlscale all you know is that at least one of the pair does not have the pearlscale allele. Maybe neither has it, or maybe just the male or just the female – but you just can’t tell.
If you pair with a pearlscale, you know from the first pairing whether the gold carries a pearl scale allele.

15. Here is a scenario with possible hidden recessive alleles
Q: You get some angels and are told that some of them are heterozygous (single dose) for half-black. How do you identify the ones with half-black?
Does this angel have a hidden half-black allele?

16. You cross with a half-black
A: If you are lucky enough to have one or more half-blacks on hand, you cross the unknown with a half-black. If the offspring include half-blacks, you know that the unknown has a half-black allele.
Half-black

17. Another scenario
Q: You cross two gold marble ghosts (Gm/g – S/+ x Gm/g – S/+).
They give koi, sunset, gold marble and gold offspring.
Some of the gold marble and gold offspring carry a single dose of stripeless.
You want to identify some that have the stripeless allele for future breeding projects. What do you do?
Gold marble ghost pair (Gm/g – S/+)

18. And the test cross
A: Cross the offspring with any other blushing angelfish, such as their blushing siblings. If some of the offspring are blushing, you know the unknown has a single dose of the stripeless allele.
Gold blushing aka Sunset (g/g – S/S)

19. You can probably think of more examples.
But that’s all for now folks!

20. Thank you!
All photos in this presentation were selected from the TAS phenotype library.
Thank you to everyone who has contributed photos to the library, either directly or through your participation in the photo contest.
Where the contributor is known, the name appears on the photo.
The photos in the presentation may only be used for official TAS educational purposes. Ownership and copy write rights belong to the people who took the photos, and their permission must be obtained to use these photos for any other purposes.

21. The End
Back to the chat room for discussion!