Transcript How_to_do_Test_Crosses

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
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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?
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A test cross is done to confirm the
genetics of an angelfish by
identifying alleles for recessive
(“hidden”) traits.
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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?
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Gold angelfish (g/g) with
possible hidden pearlscale
allele
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How do you do a test cross?
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To do a test cross, you can use one of two methods.
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2.
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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.
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Let’s do an example
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Your bag of gold angelfish has fish that are
either:
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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.
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So here is what you do…
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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
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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
Gold with
possible
pearlscale
allele
X
Silver
pearlscale
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After a pair forms, raise a spawn
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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/p
Pearlscale
+/p
Nonpearl
scale
Results:
50% p/p
Pearlscale
p
p/p
Pearlscale
+/p
Nonpearl
scale
50% +/p
Non Pearlscale
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If the gold angelfish does not have a
pearlscale allele here are the results.
p/p x +/+
p
p
+
+
+/p
Non
pearlscale
+/p
Non
pearlscale
+/p
Non
pearlscale
+/p
Non
pearlscale
Results:
100% +/p
Non Pearl Scale
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So to sum it up
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You have two possible
outcomes.
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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?
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You can just let the golds pair up.
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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.
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Here is a scenario with possible
hidden recessive alleles
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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
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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
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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
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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.
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But that’s all for now folks!
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Thank you!
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All photos in this presentation were selected from the TAS
phenotype library.
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Thank you to everyone who has contributed photos to the
library, either directly or through your participation in the photo
contest.
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Where the contributor is known, the name appears on the photo.
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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!
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