We've all at one point or another wondered where we came from. Not as in
what country, but just how we were brought into this world. All living organisms
reproduce. That is, produce a living entity like itself. Reproduction is
the recreation of a living organism(s) creating the likeness of itself.
This process may be sexual which involves both male and female sex cells
or asexual which is by itself alone. In the human it is a sexual process.
You may wonder how we come out as a human like our mother and father and
not like a toad or another organism. All organisms have genes which hold
all their characteristics on them. Genes are composed of deoxyribonucleic
acid, also known as DNA. For example, the reason people have different color
eyes is due to that special strand of DNA contained within them.
Reproduction in humans starts with a male and a female. The basic organs
in the human's sexual reproduction are the gonads. Gonads in males are testes
and in females are ovaries. There are also hormones, or chemical secretions,
in the human body that activate these gonads. The primary hormone in males
is testosterone which is produced in the testes. The female hormones are
the estrogens and progesterones that come from the ovaries and placenta.
This activation occurs during sexual maturity which sparks the mating behavior
in males and females. Human gametes are produced by the gonads. In males
spermatozoa or sperm are produced in the oval testes. Sperm have tadpole-shaped
bodies and long tails which move in flip-like motion in order to propel
the sperm. In females ova or eggs are produced in the ovaries which look
like small almond-shaped organs. In both males and females, gonads come
After union of sperm and egg, a embryo begins to form. The embryo grows
into a fetus and develops for a period of nine months termed gestation.
You've probably wondered where you stay for those long nine months. Well,
it's your temporary 3X5 inch home also known as the uterus. After the fetus
is developed enough to survive on its own, the mother gives birth to a baby.
And, at last you are born.
Genes from both parents are carried within the gametes. Each gamete carries
23 chromosomes. So, when they meet both sets of chromosomes unite. This
adds up to the 46 chromosomes that each of us have. The disease of Down's
syndrome is brought about due to the excess number chromosomes in the offspring.
This is an example of an inherited genetic disorder.
You've probably heard the terms AIDS and HIV. AIDS is a sexually transmitted
disease (STD) brought about due to the HIV virus which attacks the human's
T-cells. T-cells are cells that aid in your immune system. As of now there
is no cure for this disease. Worse, AIDS is not the only STD humans can
achieve during copulation. Others such as herpes and even pubic lice can
also be transmitted from one to another.
Unlike a human mother who carries her young for 9 months,
an elephant mother carriers her children
for 20 to 22 months, the longest of any mammal. A new born
will weigh around 300 pounds and will
be 1 meter high.
Most bacteria reproduce by fission (splitting in two).
Depending on the type of bacteria,
division may take place every 15 to 40 minutes.
Theoretically, this means that bacteria
could multiply by the millions or billions in just
a few short hours.
Each earthworm is both a male and a female (hermaphrodites). This helps
when mates are not readily available, so
self-fertilization can occur. After the baby worms are born, one can
easily tell the difference between the baby
and the adult by counting their rings. Similar
to a tree, the younger the worm the fewer the rings it has.
Whether one has blue eyes and blond hair, or green eyes and red hair,
it all has to do with genetics and heredity. Children may look like their
mother or father, or may be a mixture of both of them. Perhaps Susie has
her mother's eyes and ears and her father's nose, while Johny, on the other
hand, does not look like either of them, and inherited his grandparent's
traits. Yet all these traits are controlled by genes inherited from their
ancestors. The study of genes is known as genetics. It includes heredity
also, how offspring resemble their parents internally and externally.
The first clue to the mysteries of heredity were discovered in the 1860's
by an Augustinian monk known as Gregor Mendal. He is now known as the father
of genetics because of the work he carried on plant breeding in his tiny
He crossed, or bred together, different kinds of pea plants and studied
the number, kind, and variety of pea plants produced. From this he was able
to unlock the key elements in the mystery of heredity.
Since then, scientists have learned that genes control inheritance. But,
what was it in the cell that behaved independently and was present in both
the egg and sperm cell? And, where were the genes located in the cell? The
answer was found that the genes were located in the nucleus, something the
sperm and egg both have. Genes are organized on chromosomes, long thin strands
that hold the genetic material. Every organism has a definite number and
order of genes that create the structures and functions within that organism.
The human has 46 chromosomes. Each sperm and egg have 23 chromosomes, half
the number in a regular cell.
On these chromosomes are different strands of DNA which tell our body to
construct a mouth, esophagus, stomach, small intestine, and large intestine
which are the basic structures in the human digestive system. DNA commands
the construction of all the other structures in our body too. It also gives
our body all its functions like chewing food. So, if it weren't for those
special genes that you got when you were born, you would never be able to
relish in the sweet taste of chocolate.
Twins are a pair of babies born to the same mother at the same time.
Identical twins are siblings that are alike in every way, even sex. After
an egg is fertilized, the mass of cells splits in two at an early time.
Each half continues to develop separately, so two embryos are formed. Since
the embryos and genes are alike, the babies formed are identical also.
Fraternal twins result form the fertilization of two different eggs. This
is when two mature eggs are both fertilized by different sperm. The babies
may have a few of the same genes, but most are different. These twins may
be of the same or opposite sex and may look as different as non-twin siblings.
Twins usually occur once in about 90 births. The two siblings are usually
said to be very close, and have an extremely tight bond. Some scientists
believe this is due to the fact that they shared very tight quarters for
9 months. It is also said that sometimes twins are able to read the others
thoughts, and feel their pain almost as if they were the same person.
Images copyright PhotoDisc, Inc., 1996.
Images copyright T/Maker Company, 1996.
New Book of Knowledge, 1987. Grolier Inc.