Geology Reference
In-Depth Information
Genetic nuts and bolts
In the early twentieth century, scientists determined that within their cells, all organ-
isms have molecules of DNA (deoxyribonucleic acid) organized as
chromosomes.
Each
cell in an organism's body has two sets of the chromosomes needed to create that or-
ganism, with one exception: the sex cells. The sex cells, sperm and egg cells, each have
only one set of chromosomes.
When an organism reproduces sexually (by combining sex cells from two sep-
arate individuals), the single set in the egg cell and the single set in the sperm cell
are combined to form a new, unique pair of chromosomes, resulting in a new (and
unique) combination of characteristics in the offspring.
Spontaneously mutating genes
The recombination and passing down of chromosomes from parent to offspring explains
how current traits are inherited, but it does not explain how new traits appear. So-
mething must change within the DNA to create a new trait, and how is this change pos-
sible? It's possible through genetic mutation.
Genetic mutation
occurs when the DNA of a chromosome changes in some
way. A mutation may be caused by a
mutagen
— a chemical that alters the chro-
mosome — or it may occur randomly and spontaneously.
A genetic mutation can be good, bad, or neutral for an organism's survival:
The good:
If the mutation provides the organism a way to better adapt to its envir-
onment, or a way to compete strongly with other organisms for resources (such as
a longer neck among giraffes), it is a
beneficial mutation.
A beneficial mutation im-
proves the organism's chance of survival and opportunity to reproduce, and
therefore the mutation is passed down to the next generation as part of the genet-
ic code in the chromosomes.
The bad:
If, however, the mutation prevents the organism from competing success-
fully or gets in the way of its survival, the chance of that organism surviving to re-
