|Figure 7-2 A sexual life cycle. The life cycle begins with haploid germ cells,
formed by meiosis, combining to form a diploid zygote, which grows by
mitosis to an adult.
Most of the life cycle is spent as a diploid organism.
Bisexual reproduction is the production
of offspring formed by the union of
gametes from two genetically different
(Figures 7-1C and D, and 7-2).
The offspring will thus have a new
genotype different from either of the
parents. The individuals sharing parenthood
are characteristically of different sexes
, male and female (there are
exceptions among sexually reproducing
organisms, such as bacteria and
some protozoa in which sexes are
lacking). Each has its own reproductive
system and produces only one
kind of germ cell, spermatozoon or
ovum, but never both. Nearly all vertebrates
and many invertebrates have
separate sexes, and such a condition is
, two, + oikos
, house). An exception to this is
found in individual animals that have
both male and female reproductive
organs, a condition which is called monoecious
, single, + oikos
, house). These animals are called hermaphrodites
(from a combination
of the names of the Greek god Hermes
and goddess Aphrodite) and this form
of reproduction will be described in
the next section.
The distinction between male and
female is based, not on any differences
in parental size or appearance, but on
the size and mobility of the gametes
they produce. The ovum (egg) is produced
by the female. Ova are large
(because of stored yolk to sustain early
development), nonmotile, and produced
in relatively small numbers. The spermatozoon
(sperm) is produced
by the male. Sperm are small, motile,
and produced in enormous numbers.
Each is a stripped-down package of
highly condensed genetic material
designed for the single purpose of
reaching and fertilizing an egg.
There is another crucial event that
distinguishes sexual from asexual
reproduction: meiosis, a distinctive
type of gamete-producing nuclear division).
Meiosis differs from ordinary cell division
(mitosis) in being a double division.
The chromosomes split once, but
the cell divides twice
, producing four
cells, each with half the original number
of chromosomes (the haploid
number). Meiosis is followed by fertilization
in which two haploid gametes
are combined to restore the normal
(diploid) chromosomal number of the
The new cell (zygote), which now
begins to divide by mitosis, has equal
numbers of chromosomes from each
parent and accordingly is different from
each. It is a unique individual bearing a
recombination of parental characteristics.
Genetic recombination is the great
strength of sexual reproduction that
keeps feeding new genetic combinations
into the population.
Many unicellular organisms reproduce
both sexually and asexually.
When sexual reproduction does occur,
it may or may not involve male and
female gametes. Sometimes two
mature sexual parents merely join
together to exchange nuclear material
or merge cytoplasm (conjugation, Protozoan Groups
). Distinct sexes do
not exist in these cases.
The male-female distinction is
more clearly evident in most animals.
Organs that produce germ cells are
called gonads. The gonad that produces
sperm is a testis
and that which forms eggs is an ovary
. Gonads represent
the primary sex organs, the
only sex organs found in certain
groups of animals. Most metazoa, however,
have various accessory sex
(such as penis, vagina, uterine
tubes, and uterus) that transfer and
receive germ cells. In the primary sex
organs the germ cells undergo many
complicated changes during their
development, the details of which are