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Genetics Terminology Illustrated:Penetrance and ExpressivityA Presentation for The Angelfish SocietyMay 18, 2008 Do you ever wonder why Some angelfish that are genetically half-black never express the trait? Some times half-black only partially expresses? Pearlscale doesnt always express or fully express?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. You probably keep thinking, “If I keep the water super clean, and feed them only the best food, and dont overcrowd them, theyll all turn.” But no matter what you do, some of them just dont ever develop the half-black coloration.So whats going on? This presentation will talk about two characteristics that affect whether and to what extent some traits are expressed:What is Penetrance? 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 some of them fail to show the trait, then the trait has incomplete penetrance. Lets look at a human example first. Polydactyly in people Polydactyly refers to a trait in which a person has extra fingers and/or toes. This trait is caused by a dominant allele. Babys hand with an extra fingerX-ray of a foot with an extra toe(from Wikipedia)Polydactyly has incomplete penetrance Not everyone who has the allele will show the trait. 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. Based on this study, we would say that polydactyly has 65% penetrance (the percentage that expressed the trait). 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. 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.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. 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.But environmental conditions dont affect expression of all genes. Most angelfish varieties do not exhibit penetrance. For example, if youve 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.So what is going on? Marble (M) and black (D) have full penetrance. They always express. Half-black has incomplete penetrance. It does not always express. Why?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. 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. Now lets look at ExpressivityExpressivity 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. Can you think of any angelfish traits that exhibit expressivity?Half-black Angelfish exhibit expressivity When the half-black trait is expressed, sometimes it doesnt express fully. So we see that half-black exhibits both incomplete penetrance (it sometimes doesnt express) and expressivity (it doesnt always express fully).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. (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 Nortons article: Half-Black AngelfishPhotos and Text by Dr. Joanne Norton FAMA: August 1985, Vol.8, #8 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 angelfishFor 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. These articles are on the TAS website at: http:/theangelfishsociety.org/genetics.htmThe End Now back to the chat room for discussion!
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