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Genetics Terminology Illustrated: Penetrance and Expressivity A Presentation for The Angelfish Society May 18, 2008 by Tamar Stephens.

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Presentation on theme: "Genetics Terminology Illustrated: Penetrance and Expressivity A Presentation for The Angelfish Society May 18, 2008 by Tamar Stephens."— Presentation transcript:

1 Genetics Terminology Illustrated: Penetrance and Expressivity A Presentation for The Angelfish Society May 18, 2008 by Tamar Stephens

2 Do you ever wonder why… Some angelfish that are genetically half- black never express the trait?Some angelfish that are genetically half- black never express the trait? Some times half-black only partially expresses?Some times half-black only partially expresses? Pearlscale doesn’t always express or fully express?Pearlscale doesn’t always express or fully express?

3 Have you ever bred half-black angelfish? If you have, you will have seen that some of the young fish never develop the half-black coloration, and some only develop it partially.If you have, you will have seen that some of the young fish never develop the half-black coloration, and some only develop it partially. You probably keep thinking, “If I keep the water super clean, and feed them only the best food, and don’t overcrowd them, they’ll all turn.”You probably keep thinking, “If I keep the water super clean, and feed them only the best food, and don’t overcrowd them, they’ll all turn.” But no matter what you do, some of them just don’t ever develop the half-black coloration.But no matter what you do, some of them just don’t ever develop the half-black coloration.

4 So what’s going on? This presentation will talk about two characteristics that affect whether and to what extent some traits are expressed:This presentation will talk about two characteristics that affect whether and to what extent some traits are expressed: –Penetrance –Expressivity

5 What is Penetrance? Penetrance is the percentage of individuals with a particular genotype that express the trait.Penetrance is the percentage of individuals with a particular genotype that express the trait. If every individual with a particular genotype expresses the trait for that genotype, the trait is said to have 100% penetrance.If every individual with a particular genotype expresses the trait for that genotype, the trait is said to have 100% penetrance. If some of them fail to show the trait, then the trait has incomplete penetrance.If some of them fail to show the trait, then the trait has incomplete penetrance. Let’s look at a human example first.Let’s look at a human example first.

6 Polydactyly in people Polydactyly refers to a trait in which a person has extra fingers and/or toes.Polydactyly refers to a trait in which a person has extra fingers and/or toes. This trait is caused by a dominant allele.This trait is caused by a dominant allele. Baby’s hand with an extra finger X-ray of a foot with an extra toe (from Wikipedia)

7 Polydactyly has incomplete penetrance Not everyone who has the allele will show the trait.Not everyone who has the allele will show the trait. Sometimes it seems to skip generations.Sometimes it seems to skip generations. In one study, 65% of people with the dominant allele were born with extra fingers and/or toes. The rest had normal fingers and toes.In one study, 65% of people with the dominant allele were born with extra fingers and/or toes. The rest had normal fingers and toes. Based on this study, we would say that polydactyly has 65% penetrance (the percentage that expressed the trait).Based on this study, we would say that polydactyly has 65% penetrance (the percentage that expressed the trait).

8 Half-black Angelfish have incomplete penetrance In any given spawn of half-blacks (h/h), some of the offspring will never develop the half-black coloration.In any given spawn of half-blacks (h/h), some of the offspring will never develop the half-black coloration. So we say that the half- black trait has incomplete penetrance.So we say that the half- black trait has incomplete penetrance. This pair of half-black angelfish show the trait fully expressed. All of the offspring from this pair of half-black angelfish are genetically half-blacks (h/h). But some of the offspring will not exhibit the half-black trait.

9 Does environment affect expression of half-black? Experienced breeders will tell you that if you provide the fry with excellent water quality, uncrowded conditions, and feed them well on baby brine shrimp, a lot of them will express the half-black coloration.Experienced breeders will tell you that if you provide the fry with excellent water quality, uncrowded conditions, and feed them well on baby brine shrimp, a lot of them will express the half-black coloration. If they are overcrowded, underfed, in poor quality water, many may not develop the half-black coloration at all.If they are overcrowded, underfed, in poor quality water, many may not develop the half-black coloration at all. So yes, environment affects expression of half- black.So yes, environment affects expression of half- black.

10 But environmental conditions don’t affect expression of all genes. Most angelfish varieties do not exhibit penetrance. For example, if you’ve raise marble angelfish, you will know that they will always look like marbles, regardless whether they are crowded, poorly fed, or experience poor water quality.Most angelfish varieties do not exhibit penetrance. For example, if you’ve raise marble angelfish, you will know that they will always look like marbles, regardless whether they are crowded, poorly fed, or experience poor water quality. The same with black angelfish – they always look black. Poor water quality or overcrowding may result in ragged, stunted fins – but the color of the fish will still be black.The same with black angelfish – they always look black. Poor water quality or overcrowding may result in ragged, stunted fins – but the color of the fish will still be black.

11 So what is going on? Marble (M) and black (D) have full penetrance. They always express.Marble (M) and black (D) have full penetrance. They always express. Half-black has incomplete penetrance. It does not always express.Half-black has incomplete penetrance. It does not always express. Why?Why?

12 Genetics is actually rather complex Chromosomes are tightly coiled DNA. At any given time, most of the genes are buried in the coil in an “off” position.Chromosomes are tightly coiled DNA. At any given time, most of the genes are buried in the coil in an “off” position. Traits such as half-black that have incomplete penetrance are influenced by other genes that act as triggers to activate them. The “trigger” genes produce proteins that will activate an incompletely penetrant trait, perhaps by simply uncoiling a segment of DNA and exposing a gene to an “on” position..Traits such as half-black that have incomplete penetrance are influenced by other genes that act as triggers to activate them. The “trigger” genes produce proteins that will activate an incompletely penetrant trait, perhaps by simply uncoiling a segment of DNA and exposing a gene to an “on” position.. A well-nourished, unstressed fish is more likely to have all the necessary genes acting in concert, and thus express the trait.A well-nourished, unstressed fish is more likely to have all the necessary genes acting in concert, and thus express the trait.

13 Now let’s look at Expressivity

14 Expressivity Expressivity is a variation in the strength of a trait.Expressivity is a variation in the strength of a trait. If the trait varies in how much it expresses in a given genotype, then we say it exhibits expressivity.If the trait varies in how much it expresses in a given genotype, then we say it exhibits expressivity. Can you think of any angelfish traits that exhibit expressivity?Can you think of any angelfish traits that exhibit expressivity?

15 Half-black Angelfish exhibit expressivity When the half-black trait is expressed, sometimes it doesn’t express fully.When the half-black trait is expressed, sometimes it doesn’t express fully. So we see that half-black exhibits both incomplete penetrance (it sometimes doesn’t express) and expressivity (it doesn’t always express fully).So we see that half-black exhibits both incomplete penetrance (it sometimes doesn’t express) and expressivity (it doesn’t always express fully).

16 Example of incomplete expression of half-black trait This angelfish has an incomplete half-black pattern, limited to a wide black band in the center of the tail.This angelfish has an incomplete half-black pattern, limited to a wide black band in the center of the tail. (Typically a half-black will have the body stripes of a silver angelfish. The body stripes in this fish are faded temporarily due to moving the fish to a photo tank.) (Typically a half-black will have the body stripes of a silver angelfish. The body stripes in this fish are faded temporarily due to moving the fish to a photo tank.) Photo from Dr. Joanne Norton’s article: Half-Black Angelfish Photos and Text by Dr. Joanne Norton FAMA: August 1985, Vol.8, #8

17 Pearlscale Pearlscale is another angelfish trait that has shows expressivity. Pearlscale is a recessive trait that affects the shape of the scales, making them more reflective of light. The pearlscale pattern does not always fully express. Like half-black, its expression is reported to be environmentally influenced. Albino pearlscale angelfish

18 For more on half-blacks… Dr. Joanne Norton has written two excellent articles on her experiences with half-black, back when it was a new variety of angelfish in the US.Dr. Joanne Norton has written two excellent articles on her experiences with half-black, back when it was a new variety of angelfish in the US. These articles are on the TAS website at: articles are on the TAS website at:

19 The End Now back to the chat room for discussion!Now back to the chat room for discussion!


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