Download Genetics_Illustrated_-_Dominance

Genetics Terminology Illustrated:
Degrees of Dominance
A Presentation for The Angelfish Society
For the General Membership Meeting
October 21, 2007
Prepared by Tamar Stephens
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Here is the key terminology for this
presentation.
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Gene
Allele
Homozygous
Heterozygous
Dominant
Recessive
Partially Dominant
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Purpose of this Presentation
• For predicting outcomes, it is generally easier to
think of an allele as either dominant or recessive.
• In actuality, many alleles for mutations in
angelfish phenotypes are partially dominant, not
dominant.
• The purpose of this presentation is to explain and
illustrate the difference between dominance and
partial dominance.
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First – let’s do a quick review of
some vocabulary
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“gene” and “allele”
• A gene is a hereditary unit in a fixed location on a
chromosome that contains genetic information for
a specific trait in the appearance (phenotype) of an
individual.
• An allele is a mutational form of a given gene.
Genes commonly have various alternate
mutational forms, or alleles. For example, dark
and gold are alleles in angelfish.
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“homozygous” and “heterozygous”
• Since genes occur in pairs – one on each of the paired
chromosomes, that means that the two alleles can be
identical or they can be different.
• If the two alleles are alike, the individual is homozygous for
the trait. We commonly call this “double dose” for the trait.
A double dark black angelfish (D/D) is homozygous for
dark. (“homo” means “same” – so homozygous means the
alleles are the same)
• If the two alleles are different, the individual is
heterozygous for the trait. We commonly call this “single
dose.” A black lace angelfish (D/+) is heterozygous for
dark.
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Dominant versus Recessive
• When an allele expresses itself in the
phenotype (appearance) in the presence of
another allele, it is either dominant or
partially dominant.
• If a single dose of an allele does not express
in the phenotype, it is recessive.
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Can you think of any angelfish alleles
that appear in the phenotype when
present in a single dose?
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Here are some
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D (dark)
M (marble)
Gm (gold marble)
Sm (smokey)
Z (Zebra)
S (Stripeless)
V (Veil)
When heterozygous
(single dose), any of
these alleles will
appear in the
phenotype.
That means that they are
either dominant or
partially dominant.
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Can you think of any alleles that
do not appear in the phenotype
when present in a single dose?
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These alleles do not alter the
appearance of wild type in single
dose.
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g (gold)
h (half-black)
a (albino)
p (pearl scale)
• These alleles do not
alter the appearance
when heterozygous
(single dose).
• They are recessive.
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So what is the different between
dominant and partially dominant?
• An allele is dominant if the appearance is
identical in both a heterozygous (single
dose) and homozygous (double dose) state.
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• An allele is partially dominant if you can
see the difference between the homozygous
and heterozygous states.
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Let’s look at some examples.
• Note: All of the examples in this
presentation compare an allele for a
mutation with the wild type.
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Is “veil” dominant or partially
dominant?
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Can you see the difference between
V/+ single dose versus V/V double
dose?
Veil tail (V/+)
Super veil V/V
• Yes! V/+ makes the tail and fins longer. V/V makes the
tail and fins even longer!
• This means the “V” allele is partially dominant.
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Is “dark” (D) dominant or partially
dominant?
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Does D look different in single
versus double dose?
• Compare D/+ and D/D. So yes! They are different. One
dose darkens the fish. Two doses darkens it even further.
This means that D is partially dominant.
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What about Smokey? Dominant or
partially dominant?
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Compare Sm/+ with Sm/Sm
Smokey Sm/+
Chocolate Sm/Sm
• A double dose results in much more coverage of the body
with the smokey coloration. So Sm is partially dominant.
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What about Stripeless? Dominant or
partially dominant?
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Compare S/+ with S/S
• Distinct differences are visible between S/+ and
S/S, so the stripeless allele is partially dominant.
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Conclusion
• As we saw in this presentation, many of the nonwild alleles that we think of as dominant are
actually partially dominant. They appear in the
phenotype in single dose, but they have more
effect on the phenotype in double dose.
• With a practiced eye, you will be able to tell single
dose apart from double dose for almost all of the
non-recessive traits!
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The End
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