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GENETICS IN STORYTELLING
by
ALLISON STARKWEATHER
Genetics can play an important part in a story. It affects how everyone
and everything looks; your characters, their parents, their children,
their cats, dogs, horses, and the potted flower sitting on their front
porch. It can help your story, by giving it an extra touch of realism,
or hurt it, by being completely unbelievable.
Organisms can reproduce one of two ways—sexually or asexually. When
they reproduce asexually, there is only one parent, and the offspring
are genetically identical to the parent. However, most organisms
reproduce sexually, where two parents are needed and they each give the
offspring half of their chromosomes.
Genes are the sections of DNA that tell what a specific trait, such as
hair color, will be like. For most traits, there are two different
alleles, or variants of that trait, which are either dominant or
recessive. If an allele is dominant, then it will always be the one
expressed, even if the organism also has a recessive allele for that
trait. The only way a recessive trait can be expressed is if both
alleles are recessive. Because of this, it is possible for two people
who show dominant alleles to have a child who expresses the recessive
allele, if they both
carry the recessive allele and the child receives one copy of it from
each parent.
Though you can never be completely sure what a child will look like,
there are laws of inheritance that you can use to find the probability
of the child's expressing or carrying a certain trait. One of the tools
you can use to help you is called a Punnett square.
__ __ (parent # 1)
_____
__ |__|__|
__ |__|__|
^
|(parent # 2)
To use the Punnett square, you put the alleles of one parent along one
side, and the alleles of the other parent along the other side, with
one allele in each blank space. Dominant alleles are written with
capital letters, and recessive alleles are written with lowercase
letters. Next, you combine the alleles in the boxes, so you can see all
of the possible combinations. For example, if you wanted to know if
your character’s child could have a widow’s peak (a dominant trait),
and what the chances of that would be, you could the Punnett square. If
the mother did not have a widow’s peak (which means her alleles would
have to be rr), but the father did, and his alleles were Rr, then we
could stick them into the Punnett square:
r r (mother)
_____
R__ |Rr|Rr|
r__ |rr |rr |
(father)
So, you can see that the child has a fifty percent chance of having a
widow’s peak, and a fifty percent chance of not having one. A Punnett
square shows you the possible genotype of the offspring; their genetic
make-up. From this you can determine the phenotype; what physical
characteristics the child has.
Up until now, we’ve been assuming that all alleles are either
dominant or recessive, but that is not always true. There are several
types of intermediate phenotypes, two of them being incomplete
dominance, and polygenic traits. Incomplete dominance is where, rather
than one allele completely blocking out the other, the two alleles form
a combined phenotype. One example of this is the snapdragon; if a
snapdragon receives a red allele and a white allele, rather than being
only one of them, the alleles blend, and the flowers will be pink. This
does not mean that the alleles
blend, however. The flower still has one white and one red allele, not
two pink alleles. An example of this in humans is hair type; curly hair
and straight hair combine to form wavy hair.
Another form of incomplete dominance is polygenic traits. That’s where
more than one gene controls a single trait. An example of this is eye
color. This trait is controlled by three genes; one for the color of
the pigmentation in the iris, one for the darkness of the color, and
one for the distribution of color that creates the patterns in our
irises. Polygenic traits are much harder to predict than monogenic
traits.
Other factors in inheritance are multiple alleles. Some genes have more
than two alleles. They could have three, four, or more. Pleiotropy is
also a factor. Pleiotropy is the opposite of polygenic traits. It is
where more than one trait is controlled by one gene. This is the only
instance where traits are linked, and inheriting one means you must inherit the other.
Environmental effects can also be important. Climate can play a major
part in determining an organism’s phenotype. In some organisms, such as
the Siamese cat, the fur color is determined by temperature, which is
why the cat’s ears, nose, paws, and tail are darker than it’s body:
because they are farther from the body, their temperature is cooler.
Social environment can also stimulate changes. Some fish can, if all of
the males die, change their sex from female to male. If a new male
shows up, they can then return to female form.
All of this information can help when writing your story. If a
character gets pregnant, is it possible for the child to look a certain
way, even if the parents don’t show any of those traits? Do your
characters’ parents have
to look a certain way, in order for the character to have turned out
the way he or she did? Also, if you’re writing fantasy or science
fiction and create your own beasts, it can be incredibly useful in
helping you decide how traits are inherited. Perhaps your stories
include dragons. Are certain colors linked to the dragon's size? If it
has horns or a neck frill, can they be different sizes, or are they
either there or not? These are all questions to think about, not only
when creating a new species or deciding to get a character pregnant,
but also when creating any type of family.
Originally published in Holly Lisle's VISION, issue #2.
Copyright © Allison
Starkweather, 2001. All rights reserved.
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