From Simple English Wikipedia, the free encyclopedia
Sexual reproduction
is how most
animals
and
plants
reproduce.
[1]
Some
protists
and
fungi
also reproduce this way.
Organisms
that reproduce sexually have two different
sexes
:
male
and
female
.
In sexual reproduction, offspring are produced when
sperms
fertilise
eggs
from the female. Various steps are involved in this process.
[2]
The
cells
of an animal or
higher plant
have two sets of
chromosomes
: they are
diploid
. When
gametes
(sex cells) are produced, they have only one set of chromosomes: they are
haploid
. They have undergone a process of
cell division
called "reduction division" or
meiosis
. Two things happen during meiosis, each of which makes the offspring more variable. That means they are different from their parents and from each other.
Assortment is when the double set of chromosomes becomes a single set in each gamete. Of each pair of chromosomes,
which one goes into a single gamete is
random
. Because the gene
alleles
on each chromosome are not always the same, this means that there is genetic variation between gametes. This process was
Mendel
's 'first law', the law of segregation.
Because
crossing over
occurs during meiosis, this increases the variety of the chromosomes. This makes
recombination
possible.
Assortment and crossing over make it certain that normally no two offspring of the same mother and father are identical.
Identical twins
are the exception. They are identical genetically because they developed from the same
fertilised egg
.
There are advantages and disadvantages of sexual reproduction, compared to
asexual reproduction
. The main issues are:
- Advantages:
More variation assists with survival. It increases the chance that at least some offspring of a parent survive. To give an example, suppose a deadly infection occurs in the population. Greater variety increases the chance that some of the population will survive.
- Disadvantages:
Requires two parents. So, supposing the total number of eggs laid by a female is the same, a population reproducing sexually would produce only half as many offspring as a population reproducing asexually. The actual search for a partner, and mating, often takes huge chunks of time out of the lifetime of sexual animals.
- Gonads
are specialized sex organs where gametes are formed. In the male, the gonad is the
testes
; in the female, the gonad is the
ovaries
.
- Cleavage: early stage of
embryo
development. Cell number increases by
cell division
.
[3]
- 1.
Morula
: a solid ball of cells
- 2.
Blastula
: a hollow ball of cells filled with fluid
- 3.
Gastrulation
: blastula continues to grow, and cells multiply by
mitosis
. Several hundred cells on one side begin to move in and form a two-layered embryo. It develops into a three-layered embryo with endoderm, mesoderm, and ectoderm.
The origin of sexual reproduction is an advanced topic that cannot be dealt with here. A source that may help readers who are
graduates
in biological science may be helpful.
[4]
- ↑
Pang K. 2004.
Certificate Biology: new mastering basic concepts
. Hong Kong.
- ↑
Wolpert, Lewis & Tickle, Cheryll 2011.
Principles of development
. Oxford: Oxford University Press.
ISBN 978-0-19-955428-7
- ↑
Wolpert, Lewis 1991.
The triumph of the embryo
. Oxford: Oxford University Press.
ISBN 0-19-854243-7
- ↑
Maynard Smith J. & Szathmary, Eors. 1995.
The major transitions in evolution
. Oxford University Press, Chapter IX.
ISBN 0-19-850294-X