Reproduction!!!!
Reproduction in Animals:
Sexual Reproduction
By Regina Bailey, About.com Guide
Individual organisms come and go,
but, to a certain extent, organisms
transcend time through producing
offspring. Reproduction in animals
occurs in two primary ways: through
sexual reproduction and through
asexual reproduction.
Now let's take a look at sexual
reproduction.
Sexual Reproduction
In sexual reproduction, two
individuals produce offspring that
have genetic characteristics from
both parents. Sexual reproduction
introduces new gene combinations
in a population.
Gametes
In animals, sexual reproduction
encompasses the fusion of two
distinct gametes to form a zygote.
Gametes are produced by a type of
cell division called meiosis.
The gametes are haploid (containing
only one set of chromosomes) while
the zygote is diploid (containing two
sets of chromosomes).
In most cases, the male gamete,
called the spermatozoan, is relatively
motile and usually has a flagellum.
On the other hand, the female
gamete, called the ovum, is
nonmotile and relatively large in
comparison to the male gamete.
Types of Fertilization
There are two mechanisms by which
fertilization can take place.
The first is external (the eggs are
fertilized outside of the body); the
second is internal (the eggs are
fertilized within the female
reproductive tract).
Patterns and Cycles
Reproduction is not a continuous
activity and is subject to certain
patterns and cycles. Oftentimes
these patterns and cycles may be
linked to environmental conditions
which allow organisms to reproduce
effectively.
For example, many animals have
estrous cycles that occur during
certain parts of the year so that
offspring can typically be born under
favorable conditions. Humans
however, do not undergo estrous
cycles but menstrual cycles.
Likewise, these cycles and patterns
are controlled by hormonal cues.
Estrous can also be controlled by
other seasonal cues such as rainfall.
All of these cycles and patterns allow
organisms to manage the relative
expenditure of energy for
reproduction and maximize the
chances of survival for the resulting
offspring.
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