Views of Dominant and Codominant
Posted by Randy Remington on January 4, 2004 On King
Snake and edited by him the same day. Posted with his permission.
Dominant vs. Co-dominant
Randy Remington at
Sun Jan 4
06:46:21 2004 [
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The short answer is that
even though spider may be a completely dominant mutation,
heterozygous spider X heterozygous spider does not produce 100%
spiders - each egg has a 25% chance of producing a normal.
The following is the long answer on why and how I find it easier to
understand inheritance by ignoring mutation types and looking at
genotypes and then use the mutation type later to figure out the
There seems to be a lot of confusion over the term "dominant".
For one thing, people
use "dominant" to describe both a proven completely dominant
mutation and any type of non-recessive mutation before we know yet
if it's co-dominant.
To avoid this
confusion I prefer to use "some type of dominant" for mutations such
as spider where we know it's not recessive but we really aren't
completely sure yet if it's co-dominant or what I like to call
"completely dominant". I don't think "completely dominant" is really
a text book term but then in the scientific community they probably
would wait until they knew the inheritance for sure before they
described it and we need a way to talk about it for the years that
it's sold before being completely figured out.
The next point of confusion is between the mutation types
(recessive, co-dominant, and completely dominant) and the
inheritance. The mutation type tells you what the mutant genotypes
(heterozygous mutant, homozygous mutant) look like relative to each
other and normal. It doesn't change the basic rules of how the genes
are inherited which apply to all non sex linked mutations and are
pretty simple until you start throwing in mutant alleles (different
mutations of the same gene).
With the recessive mutation type, the heterozygous mutant (one
mutant copy of the gene and one normal copy of the gene) should look
normal. Heterozygous refers to the genotype (describing it's genes)
of the animal and it's appearance is it's phenotype.
With co-dominant (or perhaps better referred to as incompletely
dominant) the heterozygous genotype does not look normal and the
homozygous mutant genotype looks different yet. Usually the
phenotype of the homozygous mutant genotype is more extreme than the
heterozygous genotypes (i.e. super pastel vs. pastel).
With the completely dominant mutation type we would expect the
homozygous mutant to look just like the heterozygous mutant but
different than normal. If it turns out that spider is completely
dominant then homozygous spiders will look just like heterozygous
spiders. Even just one copy of the mutant gene is completely
dominant over one normal copy of the gene so the heterozygous spider
is just as spider as the homozygous one with two copies.
As far as inheritance of the genotypes, you can forget for a minute
what type of mutation you are talking about. If we are talking about
single non-sex linked mutant genes without multiple mutant alleles
then the same simple rules apply regardless of the mutation type.
Each egg has the following odds:
1. Heterozygous X Normal = 50% chance heterozygous, 50% chance
2. Heterozygous X Heterozygous = 25% chance homozygous, 50% chance
heterozygous, 25% chance normal
3. Heterozygous X Homozygous = 50% chance homozygous, 50% chance
4. Homozygous X Homozygous = 100% chance homozygous
5. Homozygous X Normal = 100% chance heterozygous
If you can remember these 5 rules then all you have to do is figure
out what you are breeding.
The vast majority if not all spiders so far have been heterozygous
for the spider gene. From #1 above you can see that heterozygous
spider X normal should produce babies that each have a 50% chance of
being heterozygous spider and a 50% chance of being normal for the
spider gene. From #2 you see that heterozygous spider X heterozygous
spider produces babies that still have a 25% chance of being normal.
However, here is where it gets a little interesting, will 1/3 of the
spiders produced be homozygous super spiders and the other 2/3
heterozygous spiders like the parents? If the mutation is completely
dominant the homozygous spiders will look just like the heterozygous
spiders and you will not know for sure that they have been produced
until you grow them up and produce a large number of only spiders
breeding one of the homozygous ones to normals as in #5. This takes
time and is why we can't be sure yet if spider is completely
Maybe spider will turn out to be co-dominant after all and NERD just
hasn't been lucky enough to produce a homozygous spider from
heterozygous spider X heterozygous spider breedings so far. Maybe
two copies of the gene are lethal and there will never be a
homozygous spider (this will take even longer to reasonably prove).
Saturday, May 14, 2016 01:29 PM