Understanding Genetics
~Written by
Marla
Davis Robinson
I'd like to add some ideas that may help everyone
(scientists and glider-slaves) be on the same page. I am not going to
explain the genetics of color in Sugar Gliders, that subject is (as you
can see) under investigation and not fully understood. Molecular markers
were developed at the end of last year, and within a short amount of
time the genome will be sequenced; this still will not tell us all about
the genetics of color, only crosses (breeding) can do that. I will say
that in sugar gliders, (as in most mammals) color is probably controlled
by many genes; genes for both the actual color of the pigment/melanin
molecules and what's called "deposition" (putting that pigment into
skin/hair/eye cells) which allows for lots of variation.
Mutation is the source of ALL variation in color, so what I am going to
try to do is correct/clarify the misinformation that has been posted
here about how inheritance/mutation in genes works.
I see a lot of confusion here due to the way certain words are used in
common language and the way they're used in science.
EX: "mutation". Mutation is used as both a verb and noun in biology. A
mutation in biology (as stated in the first line of the first post) is
just a CHANGE in a genetic locus. So it's used as a verb when describing
the fact that there has been a change in the DNA sequence, "There was a
mutation in gene {G1}, substituting one nucleotide for another to form
gene {G2}" and as a noun to describe the result "The {a} allele is a
mutation of the {A} allele of the gene for pigment". That's all.
There is no additional requirement/understanding that once it is passed
on to offspring it is no longer considered to be a mutation. IT IS STILL
A MUTATION, only then it is an inherited one. A difference between what
the DNA sequence was, and what it is now. Whether a mutation is
heritable and passed on to offspring is a separate consideration, it
does not change it from being a mutation in the first place.
The word mutation is used in common conversation to refer to a BAD
physical change that occurs during the post-birth lifetime of an
organism, that then affects the physical condition of that organism
itself. True mutations more often affect offspring, because they happen
in germ cells or during development. Like the "limb in the middle of
your forehead" comment from the first post (more accurately, something
like this is called a developmental defect), & like many popular culture
icons (Killer Tomatoes, Swamp Thing, Teenage Mutant Ninja Turtles ),
which were "mutated" from "normal" things into "mutants" by radiation or
toxic chemical waste.
THERE IS NO SUCH NEGATIVE CONNOTATION TO THE TERM MUTATION IN GENETICS.
It is neither a "good" or a "bad" thing, it's JUST A CHANGE.
There are also major differences in scale that are being obscured in
this thread. Change/mutation can be at the level of the chromosome
(which in genetic terms is huge and contains 10000's of genes), or just
a single base-pair substitution/insertion/deletion of the DNA (2
molecules, 1/1000 of an average single gene), this small-scale mutation
happens all the time, it's happening in your body right now! Sometimes
they're good, sometimes they're bad (cancer) and sometimes they have no
effect at all. Large-scale/chromosomal mutations are more rare, b/c they
affect whole suites of genes and are much more likely to have negative
effects, but they do happen.
ALL ORGANISMS HAVE MUTATIONS, by definition, we're all "mutants", b/c
through evolution of the genetic code since the beginning of life, we
all have many "changes" that have been made to our DNA. I am a pigment
mutant! I have blonde hair, which is a homozygous recessive genetic
trait, and a MUTATION, b/c my Homo sapiens ancestors (and yours) long
ago all had dark hair, and somewhere along my lineage there was a
mutation (which has occurred and re-occurs throughout human history in
different populations) that led to light yellow hair. My husband loves
my mutation! And I his mutant blue eyes, which are a homozygous
recessive genetic trait that is a mutation! There's nothing judgmental
about the correct, biological use of the word.
There is no reason to be defensive about whether the beautiful
cream-fur/burgundy-eye gliders, or any specially-colored gliders are the
result of a "mutation" or not. They are, whether it was long ago in the
ancestors of their parents, or during fertilization/gestation of this
pregnancy (which is possible, I'll explain below) makes no difference.
It is still a mutation, neither a good or bad thing, just a change from
an original genetic state. You can distinguish whether it's a NEW
mutation (happened in this organism) or an OLD mutation (happened in a
previous generation), but they're all mutations.
2.Why "albinism is not a mutation because it exists/has existed in all
animals & is a heritable recessive genetic trait" statements are
incorrect:
1st.The opposite of "mutation" is not "normal". There really is no
opposite, only "ancestral and derived", i.e. you're not the opposite of
your parents/ancestors, you're the descendant/derived of them.
2nd. Albinism does exist in all animals, but that is NOT because the
"mutation happened a long time ago and has now evolved to be "normal".
There are many different causes (genotypes) and forms (phenotypes) of
albinism, they are not all 1 thing. Some of the genes responsible for
some kinds of albinism arose via mutation long ago and have been passed
on, but some happen and are happening RIGHT NOW!
Albinism is common because the underlying basis of coloration (i.e.
pigmentation) is shared by all animals, and these mechanisms are even
more similar among mammals b/c we're very closely related to each other,
and changes (mutations) in the genes for pigmentation are common (b/c
not fatal to the embryo) and so arise OVER AND OVER AGAIN,
spontaneously, especially in captive-bred organisms, where breeding is
much more frequent & survival isn't dependant on camouflage.
Thus, the mutation could be inherited, but could also have occurred this
year! It could've been a mutation in the DNA of a parent's germ
(sperm/egg) cells, that then went on to make a baby, or even during
development of the babies in question, through a mutation (just a
change!) in the DNA sequence during development that leads to lack of
pigment and/or pigment deposition. In both of these scenarios, these
parents are unlikely to ever have albino offspring again.
For an albino's offspring, this change may or may not be heritable, it
depends on whether it was a mutation in the "germ" (reproductive) cells'
DNA or "somatic" (non-reproductive) cells' DNA.
If it is a somatic cell mutation, IT IS NOT HERITABLE, it just means a
change in the genes of skin cells results in lack of pigment, but that
albino will have offspring the color of a combo of its parents because
it cannot pass on the mutation, as its germ cells do not have the
mutation.
If instead it is a mutation in the germ cells, then the trait will be
passed on to the albino's offspring, but will only be expressed (i.e.
show up in the "phenotype", the outward appearance of the organism) if
the offspring are "homozygous recessive" for the trait. This concept
seems to be confusing as well, so let's explore it:
Genes are the instructions for how to build proteins. Recessive means
that the gene does not make a protein, or it makes a non-functional
version of the protein. A dominant gene makes a functional protein. You
only need one FUNCTIONAL (dominant) gene to make enough of most needed
proteins, so to express the recessive phenotype, an organism has to have
only the recessive {a} gene, not the dominant {A} in order that no
protein is made. How does this work? See below:
A "gene" is named for the trait it gives instructions for (like the
gene(s) for pigmentation) and an "allele" is one of the different
version/forms of that gene. Ex: I'm using{A}and {a} as alleles of the
gene for pigmentation. Why?->
Diploid" means we have 2 sets of genes (1 from each parent). If we
designate the gene for pigment as {A}, then the recessive form (which
does not make the pigment, due to mutation(s) that occurred recently OR
in the distant past) is {a}. This is because genes are assigned names
based on the expression of the non-functional (also called the
"knock-out") version, in this case "albinism".
Having 2 of the same version (allele) of a gene is being "homozygous",
having different versions is "heterozygous". So the "genotype" (what
genes you have, as opposed to "phenotype", what genes you show) {AA} is
homozygous, as is {aa}. {Aa} is heterozygous.
"Homozygous recessive", therefore, means you're genotype {aa}, which has
a phenotype of albino. Homozygous dominant means you're genotype {AA},and
your phenotype is pigmented. Heterozygous means you have a genotype of {Aa},
and a phenotype of pigmented, because 1 copy/version/allele of the gene
is enough to make the protein to give you color. Being {aa} for a gene
does NOT mean that gene isn't the result of a mutation, it means you
have 2 copies of a version of a gene that doesn't make a protein that
the functional/dominant {A} version makes, THAT'S ALL.
As far as offspring, during the formation of "gametes" sex/germ cells
like sperm & eggs, the "diploid" genotype is divided, so in 4 eggs or
sperm from an {Aa} parent, 2 will be {A} and 2 will be {a}. This means
that through INHERITANCE [see above for how parents can make different
offspring NOT through inheritance]{AA} parents can only make {A}
gametes, {aa} parents can only make {a} gametes, but {Aa} parents can
make EITHER {A} or {a} gametes.
Gametes are then combined through fertilization to make a new diploid
baby, which (barring a NEW mutation) will follow the same rules,
previously outlined, of genotype/phenotype.
Punnett Squares allow you to attempt to predict the likelihood of
offspring genotype & phenotype, knowing the above ratios. But it is not
a guarantee, it's a probability, like when you say you have a 50:50
chance of flipping a coin and getting heads vs. tails. This is true, and
with many repeat tosses of the coin it will work out, but if we only
toss it twice? Birth sex ratio likelihood in humans are 50:50, but how
many people do you know that have 2 sons or 2 daughters instead of the
"likely" one of each?
Especially if the genetic basis for the trait is not truly known,
Punnett squares may not tell you much, i.e. we may assume that an albino
is {aa} due to inheritance, but they may in fact be {AA},and have a
somatic mutation that makes them albino, but is not passed on. This
doesn't mean genetic principles are wrong, but rather that more study is
needed to determine the specific genetics of the situation.
Now that we're clear, those adorable little gliders could come from
genotypically {AA} parents, and have a NEW mutation in BOTH the genes
for deposition of pigment in the eyes and the skin/fur. Thus, they could
be "albino-burgundy" in their eyes, and have a related, NEW
mutation/change/reason for being creamy with a taupe stripe
(not-albino), which is not due to their parents genotypes. In this
scenario, the NEW mutation may or may not be heritable, as I said before
it depends on whether the babies have this mutation in their germ cells
or their somatic cells.
OR, it could be heritable, because of some recessive (non-functional)
OLD mutation/version of the pigment gene that the parents happen to both
carry {Aa} and passed onto the twins, such that each twin is {aa}. Only
their offspring will tell us...
And this brings me to my last point. I have a PhD in biology, I am a
molecular biologist by training, I work on the genetics of marine
animals for my research, and I am a biology instructor at a
university...but I would never attempt to obscure/hide the data about
genetics/biology from people based on the consideration that the truth
is "too hard for laymen to understand/accept". This is insulting to them
and our field, a good scientist/teacher can convey information to
everyone, that's our job. Here's an M.SC. in genetics making statements
that are obfuscating, untrue misconceptions about genetics, even sort of
slandering one's own field by saying the "science" of genetics in
quotes.
The fact that we can't yet know exactly what's up with these babies
doesn't mean genetics are somehow foggy, but that the practical
determinations are made with time and breeding crosses. A geneticist
would know that. We do everyone a disservice when we give them bad
data...
~Written by and posted here with permission from
Marla Davis
Robinson
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