1 genetics terminology illustrated: degrees of dominance a presentation for the angelfish society...

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Genetics Terminology Illustrated:Degrees of Dominance

A Presentation for The Angelfish SocietyA Presentation for The Angelfish Society

For the General Membership Meeting For the General Membership Meeting

October 21, 2007October 21, 2007Prepared by Tamar StephensPrepared by Tamar Stephens

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Here is the key terminology for this presentation.

• GeneGene• AlleleAllele• HomozygousHomozygous• HeterozygousHeterozygous• DominantDominant• RecessiveRecessive• Partially DominantPartially Dominant

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Purpose of this Presentation

• For predicting outcomes, it is generally easier to For predicting outcomes, it is generally easier to think of an allele as either dominant or recessive.think of an allele as either dominant or recessive.

• In actuality, many alleles for mutations in In actuality, many alleles for mutations in angelfish phenotypes are partially dominant, not angelfish phenotypes are partially dominant, not dominant. dominant.

• The purpose of this presentation is to explain and The purpose of this presentation is to explain and illustrate the difference between dominance and illustrate the difference between dominance and partial dominance.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 A gene is a hereditary unit in a fixed location on a chromosome that contains genetic information for chromosome that contains genetic information for a specific trait in the appearance (phenotype) of an a specific trait in the appearance (phenotype) of an individual. individual.

• An allele is a mutational form of a given gene. An allele is a mutational form of a given gene. Genes commonly have various alternate Genes commonly have various alternate mutational forms, or alleles. For example, dark mutational forms, or alleles. For example, dark and gold are alleles in angelfish. and gold are alleles in angelfish.

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“homozygous” and “heterozygous”

• Since genes occur in pairs – one on each of the paired Since genes occur in pairs – one on each of the paired chromosomes, that means that the two alleles can be chromosomes, that means that the two alleles can be identical or they can be different.identical or they can be different.

• If the two alleles are alike, the individual is homozygous for If the two alleles are alike, the individual is homozygous for the trait. We commonly call this “double dose” for the trait. the trait. We commonly call this “double dose” for the trait. A double dark black angelfish (D/D) is homozygous for A double dark black angelfish (D/D) is homozygous for dark. (“homo” means “same” – so homozygous means the dark. (“homo” means “same” – so homozygous means the alleles are the same)alleles are the same)

• If the two alleles are different, the individual is If the two alleles are different, the individual is heterozygous for the trait. We commonly call this “single heterozygous for the trait. We commonly call this “single dose.” A black lace angelfish (D/+) is heterozygous for dose.” A black lace angelfish (D/+) is heterozygous for dark.dark.

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Dominant versus Recessive

• When an allele expresses itself in the When an allele expresses itself in the phenotype (appearance) in the presence of phenotype (appearance) in the presence of another allele, it is either dominant or another allele, it is either dominant or partially dominant.partially dominant.

• If a single dose of an allele does not express If a single dose of an allele does not express in the phenotype, it is recessive.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

• D (dark)D (dark)• M (marble)M (marble)• Gm (gold marble)Gm (gold marble)• Sm (smokey)Sm (smokey)• Z (Zebra)Z (Zebra)• S (Stripeless)S (Stripeless)• V (Veil)V (Veil)

When heterozygous When heterozygous (single dose), any of (single dose), any of these alleles will these alleles will appear in the appear in the phenotype.phenotype.

That means that they are That means that they are either dominant or either dominant or partially dominant. 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.• g (gold)g (gold)• h (half-black)h (half-black)• a (albino)a (albino)• p (pearl scale)p (pearl scale)

• These alleles do not These alleles do not alter the appearance alter the appearance when heterozygous when heterozygous (single dose). (single dose).

• They are recessive.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 An allele is dominant if the appearance is identical in both a heterozygous (single identical in both a heterozygous (single dose) and homozygous (double dose) state. dose) and homozygous (double dose) state.

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• An allele is partially dominant if you can An allele is partially dominant if you can see the difference between the homozygous see the difference between the homozygous and heterozygous states. and heterozygous states.

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Let’s look at some examples.

• Note: All of the examples in this Note: All of the examples in this presentation compare an allele for a presentation compare an allele for a mutation with the wild type. 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?

• Yes! V/+ makes the tail and fins longer. V/V makes the Yes! V/+ makes the tail and fins longer. V/V makes the tail and fins even longer!tail and fins even longer!

• This means the “V” allele is partially dominant. This means the “V” allele is partially dominant.

Veil tail (V/+) Super veil V/V

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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 Compare D/+ and D/D. So yes! They are different. One dose darkens the fish. Two doses darkens it even further. dose darkens the fish. Two doses darkens it even further. This means that D is partially dominant.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

• A double dose results in much more coverage of the body A double dose results in much more coverage of the body with the smokey coloration. So Sm is partially dominant.with the smokey coloration. So Sm is partially dominant.

Smokey Sm/+ Chocolate Sm/Sm

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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 Distinct differences are visible between S/+ and S/S, so the stripeless allele is partially dominant.S/S, so the stripeless allele is partially dominant.

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Conclusion

• As we saw in this presentation, many of the non-As we saw in this presentation, many of the non-wild alleles that we think of as dominant are wild alleles that we think of as dominant are actually partially dominant. They appear in the actually partially dominant. They appear in the phenotype in single dose, but they have more phenotype in single dose, but they have more effect on the phenotype in double dose. effect on the phenotype in double dose.

• With a practiced eye, you will be able to tell single With a practiced eye, you will be able to tell single

dose apart from double dose for almost all of the dose apart from double dose for almost all of the non-recessive traits!non-recessive traits!

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The End