1. Genetics Terminology Illustrated: Degrees of Dominance
A Presentation for The Angelfish Society
For the General Membership Meeting
October 21, 2007
Prepared by Tamar Stephens

2. Here is the key terminology for this presentation.
Gene
Allele
Homozygous
Heterozygous
Dominant
Recessive
Partially Dominant

3. 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.

4. First – let’s do a quick review of some vocabulary

5. “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.

6. “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.

7. 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.

8. Can you think of any angelfish alleles that appear in the phenotype when present in a single dose?

9. Here are some 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.

10. Can you think of any alleles that do not appear in the phenotype when present in a single dose?

11. These alleles do not alter the appearance of wild type in single dose.
g (gold)
h (half-black)
a (albino)
p (pearl scale)
These alleles do not alter the appearance when heterozygous (single dose).
They are recessive.

12. 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.
An allele is partially dominant if you can see the difference between the homozygous and heterozygous states.

13. Let’s look at some examples.
Note: All of the examples in this presentation compare an allele for a mutation with the wild type.

14. Is “veil” dominant or partially dominant?

15. Can you see the difference between V/+ single dose versus V/V double dose?
Super veil V/V
Veil tail (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.

16. Is “dark” (D) dominant or partially dominant?

17. 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.

18. What about Smokey? Dominant or partially dominant?

19. 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.

20. What about Stripeless? Dominant or partially dominant?

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

22. Conclusion
As we saw in this presentation, many of the non-wild 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!

23. The End