Asexual vs. Sexual
Reproduction
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
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Printed: February 20, 2015
AUTHORS
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Asexual vs. Sexual Reproduction
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Asexual vs. Sexual
Reproduction
• Compare and contrast asexual and sexual reproduction.
One parent or two?
That is the main difference between sexual and asexual reproduction. Sexual reproduction just means combining
genetic material from two parents. Asexual reproduction produces offspring genetically identical to the one parent.
Did You Know??? Some organisms can reproduce both asexually and sexually. The paramecium for example,
can reproduce asexually by splitting in two during the process of binary fission. However, under certain conditions,
the paramecium seek out others of their own kind and exchange genetic material to produce offspring that are not
identical to either parent.
Reproduction: Asexual vs. Sexual
Cell division is how organisms grow and repair themselves. It is also how many organisms produce offspring.
For many single-celled organisms, reproduction is a similar process. The parent cell simply divides to form two
daughter cells that are identical to the parent. In many other organisms, two parents are involved, and the offspring
are not identical to the parents. In fact, each offspring is unique. Look at the family in Figure 1.2. The children
resemble their parents, but they are not identical to them. Instead, each has a unique combination of characteristics
inherited from both parents.
Reproduction is the process by which organisms give rise to offspring. It is one of the defining characteristics of
living things. There are two basic types of reproduction: asexual reproduction and sexual reproduction.
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FIGURE 1.1
Paramecium can produce genetically
identical offspring by asexual reproduction, or may use sexual reproduction with
another paramecium to produce offspring
that are genetically different from the parents.
FIGURE 1.2
Family Portrait: Mother, Daughter, Father,
and Son. Children resemble their parents,
but they are never identical to them. Do
you know why this is the case?
Asexual Reproduction
Asexual reproduction involves a single parent. It results in offspring that are genetically identical to each other
and to the parent. All prokaryotes and some eukaryotes reproduce this way. There are several different methods of
asexual reproduction. They include binary fission, fragmentation, and budding.
• Binary fission occurs when a parent cell splits into two identical daughter cells of the same size.
• Fragmentation occurs when a parent organism breaks into fragments, or pieces, and each fragment
develops into a new organism. Starfish can reproduce this way. A new starfish can develop from a single
ray, or arm. Starfish, however, are also capable of sexual reproduction.
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Chapter 1. Asexual vs. Sexual Reproduction
• Budding occurs when a parent cell forms a bubble-like bud. The bud stays attached to the parent cell while
it grows and develops. When the bud is fully developed, it breaks away from the parent cell and forms a new
organism. Yeast commonly reproduce by budding.
FIGURE 1.3
There are a number of ways that living
organisms can reproduce exact genetic
copies of themselves. This is called asexual reproduction.
FIGURE 1.4
Starfish reproduce by fragmentation and yeasts reproduce by budding. Both are types of asexual reproduction.
Asexual reproduction can be very rapid. This is an advantage for many organisms. It allows them to crowd out
other organisms that reproduce more slowly. Bacteria, for example, may divide several times per hour. Under ideal
conditions, 100 bacteria can divide to produce millions of bacterial cells in just a few hours! However, most bacteria
do not live under ideal conditions. If they did, the entire surface of the planet would soon be covered with them.
Instead, their reproduction is kept in check by limited resources, predators, and their own wastes. This is true of
most other organisms as well.
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Sexual Reproduction
Sexual reproduction involves two parents. As you can see from Figure 1.6, in sexual reproduction, parents produce
reproductive cells—called gametes—that unite to form an offspring. Gametes are a specialized type of cell known
as haploid cells. This means they contain only half the number of chromosomes found in all of the other cells
of the organism. Gametes are produced by a special type of cell division called meiosis, which is described in detail
in a concept to follow. The process in which two gametes unite is called fertilization. The fertilized cell that results
is referred to as a zygote. A zygote is diploid cell, which means that it has twice the number of chromosomes as a
gamete.
In humans, almost all cells in the body are diploid cells with 23 pairs of chromosomes, for a total of 46 chromosomes
in each cell. The only cells in the body that do not have 46 chromosomes are the haploid reproductive cells (sperm
cells and eggs) called gametes.
Sexual reproduction produces offspring with genetic variation. Without genetic variation, species cannot adapt
and evolve to meet changing conditions. The production of offspring with a wide variety of characteristics allows
the process of natural selection to "pick" which traits are best suited for survival. In times where the habitat changes,
this may be crucial to the survival of a species.
FIGURE 1.5
sperm cells and egg cells are the only
cells in the human body that are haploid (contain only 23 chromosomes rather
than 46).
Summary
• Asexual reproduction involves one parent and produces offspring that are genetically identical to each other
and to the parent.
• Sexual reproduction involves two parents and produces offspring that are genetically unique.
• During sexual reproduction, two haploid gametes join in the process of fertilization to produce a diploid
zygote.
• Meiosis is the type of cell division that produces gametes.
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Chapter 1. Asexual vs. Sexual Reproduction
FIGURE 1.6
Cycle of Sexual Reproduction.
Sexual
reproduction involves the production of
haploid gametes by meiosis. This is followed by fertilization and the formation of
a diploid zygote. The number of chromosomes in a gamete is represented by the
letter n. Why does the zygote have 2n, or
twice as many, chromosomes?
Review
1. What are three types of asexual reproduction?
2. Define gamete and zygote. What number of chromosomes does each have?
3. What happens during fertilization?
4. How do the offspring of asexual reproduction differ from offspring produced by sexual reproduction?
References
1. Image copyright Juan Carlos Tinjaca, 2014. A family portrait shows children resemble their parents but are
not identical to them. Used under license from Shutterstock.com
2. Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation. Illustrates binary fission in single-celled organisms and multiple fission in multinucleated cells.
3. Starfish: Flickr:amanderson2; Yeast: Zappy’s. Starfish and yeasts are examples of organisms that reproduce
asexually. Starfish: CC BY 2.0; Yeast: CC BY-NC 3.0
4. Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation. An illustrative overview of the sexual reproduction process. CC BY-NC 3.0
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