How to do Test Crosses in Angelfish

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How to do Test Crosses in Angelfish

• Presentation for The Angelfish Society
• December 9, 2007
• By Tamar Stephens

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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.

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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.

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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
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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. Were going to discuss the
  first method in this presentation. Well discuss
  the second method in a future presentation.

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Lets 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. Lets
  say they are a smokey pearlscale (Sm/ - p/p) and
  a silver pearlscale (p/p). It doesnt really
  matter what type, because all you want to do it
  identify carriers of pearlscale among the golds.

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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.

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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
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Here is the second cross
X
Gold with possible pearlscale allele
Silver pearlscale
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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)

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If the gold angelfish has a pearlscale allele,
you will get 50 pearlscales.
p/p x /p
p

p/p Pearlscale /p Nonpearl scale
p/p Pearlscale /p Nonpearl scale
Results 50 p/p Pearlscale 50 /p Non
Pearlscale
p
p
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If the gold angelfish does not have a pearlscale
allele here are the results.
p/p x /


/p Non pearlscale /p Non pearlscale
/p Non pearlscale /p Non pearlscale
Results 100 /p Non Pearl Scale
p
p
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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
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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.
• Butif 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 cant tell.
• If you pair with a pearlscale, you know from the
  first pairing whether the gold carries a pearl
  scale allele.

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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?
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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
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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/)
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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)
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You can probably think of more examples.

• But thats all for now folks!

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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.

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The End

• Back to the chat room for discussion!