Transcript Genetics Terminology Illustrated III Epistasis

Genetics Terminology Illustrated:
Penetrance and Expressivity
A Presentation for The Angelfish Society
May 18, 2008
by Tamar Stephens
Do you ever wonder why…
• Some angelfish that are genetically halfblack never express the trait?
• Some times half-black only partially
expresses?
• Pearlscale doesn’t 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
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.
So what’s going on?
• This presentation will talk about two
characteristics that affect whether and to
what extent some traits are expressed:
– Penetrance
– Expressivity
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.
• Let’s 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.
Baby’s hand with an extra finger
X-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 halfblack 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 halfblack.
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.
• 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 let’s look at Expressivity
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?
Half-black Angelfish exhibit
expressivity
• 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).
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 Norton’s article:
Half-Black Angelfish
Photos and Text by Dr. Joanne Norton
FAMA: August 1985, Vol.8, #8
Pearlscale
Albino pearlscale angelfish
• 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.
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
The End
• Now back to the chat room for discussion!