Genetics Terminology Illustrated: Penetrance and Expressivity

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Genetics Terminology IllustratedPenetrance and
Expressivity

• A Presentation for The Angelfish Society
• May 18, 2008
• by Tamar Stephens

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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?

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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.

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So whats going on?

• This presentation will talk about two
  characteristics that affect whether and to what
  extent some traits are expressed
• Penetrance
• Expressivity

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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.

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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 finger
X-ray of a foot with an extra toe (from Wikipedia)
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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).

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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.
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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.

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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.

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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?

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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.

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Now lets look at Expressivity
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Expressivity

• 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?

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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).

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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
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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
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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.
  
• These articles are on the TAS website at
  http//theangelfishsociety.org/genetics.htm

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The End

• Now back to the chat room for discussion!