Do animals always have two parents?
No, not all animals have two parents. When necessary, some animals can be produced from just one parent. Some reptiles, such as this Komodo dragon, have only one parent. The process of creating offspring from just one individual is called asexual reproduction.
Animals and other organisms cannot live forever. They must reproduce if their species is to survive. But what does it mean to reproduce? Reproduction is the ability to make the next generation, and it is one of the basic characteristics of life. Two methods of reproduction are:
- Asexual reproduction, the process of forming a new individual from a single parent.
- Sexual reproduction, the process of forming a new individual from two parents.
There are advantages and disadvantages to each method, but the result is always the same: a new life begins.
When humans reproduce, there are two parents involved. DNA must be passed from both the mother and father to the child. Humans cannot reproduce with just one parent; humans can only reproduce sexually. But having just one parent is possible in other eukaryotic organisms, including some insects, fish, and reptiles. These organisms can reproduce asexually, meaning the offspring ("children") have a single parent and share the exact same genetic material as the parent. This is very different from reproduction in humans. Bacteria, being a prokaryotic, single-celled organism, must reproduce asexually.
The advantage of asexual reproduction is that it can be very quick and does not require the meeting of a male and female organism. The disadvantage of asexual reproduction is that organisms do not receive a mix of traits from both parents. An organism that is born through asexual reproduction only has the DNA from the one parent. In fact, the offspring is genetically an exact copy of the parent. This can cause problems for the individual. For example, if the parent has a gene that causes a particular disease, the offspring will also have the gene that causes that disease. Organisms produced sexually may or may not inherit the disease gene because they receive a mix of their parents' genes.
Types of organisms that reproduce asexually include:
- Prokaryotic organisms, like bacteria. Bacteria reproduce through binary fission, where they grow and divide in half (Figure below). First, their chromosome replicates and the cell enlarges. The cell then divides into two cells as new membranes form to separate the two cells. After cell division, the two new cells each have one identical chromosome. This simple process allows bacteria to reproduce very rapidly.
- Starish can repair lost or damaged body parts identical to the original, in a process called regeneration.
- Organisms, such as the eukaryotic Hydra, can develop a tiny bud that pinches off and grows to full size. This is called budding. Yeast and the kalanchoe plant are more examples of budding organisms.
Bacteria reproduce by binary fission. Shown is one bacterium reproducing and becoming two bacteria.
During sexual reproduction, two parents are involved. Most animals have male and females, with the male producing sperm and the female producing eggs. When a sperm and egg meet during fertilization, a zygote, the first cell of a new organism, is formed (Figure below). This process combines the genetic material from both parents. The resulting organism will be genetically unique. The zygote will divide by mitosis and grow into the embryo.
During sexual reproduction, a sperm fertilizes an egg.
Let's explore how animals, plants, and fungi reproduce sexually:
- Animals often have gonads, organs that produce eggs or sperm. The male gonads are the testes, and the female gonads are the ovaries. Testes produce sperm; ovaries produce eggs. Sperm and egg, the two sex cells, are known as gametes, and can combine two different ways, both of which combine the genetic material from the two parents. Gametes have half the amount of the genetic material of a regular body cell. In humans, gametes have one set of 23 chromosomes. Gametes are produced through a special type of cell division known as meiosis.
- Fish and other aquatic animals release their gametes in the water, which is called external fertilization (Figure below). These gametes will combine by chance.
- Animals that live on land reproduce by internal fertilization. Typically males deposit sperm into the female, which combines with an egg to form a zygote.
This fish guards her eggs, which will be fertilized externally.
- Plants can also reproduce sexually, but their reproductive organs are different from animals’ gonads. Plants that have flowers have their reproductive parts in the flower. The sperm is contained in the pollen, while the egg is contained in the ovary, deep within the flower. The sperm can reach the egg two different ways:
Fungi can also reproduce sexually, but instead of female and male sexes, they have (+) and (-) strains. When the filaments of a (+) and (-) fungi meet, the zygote is formed. Just like in plants and animals, each zygote receives DNA from two parent strains.
- asexual reproduction: Process of forming a new individual from a single parent.
- binary fission: Reproduction through growth and division of the cell, as in bacteria.
- external fertilization: Process during which egg and sperm meet after being released into water.
- fertilization: Union of a sperm and egg.
- gamete: Sex cell, such as sperm or egg.
- internal fertilization: Process during which egg and sperm meet inside the female.
- meiosis: Process of cell division during which the chromosome number is halved in order to produce gametes.
- reproduction: Process of forming a new individual.
- sexual reproduction: Process of forming a new individual from two parents.
- zygote: Cell that forms when a sperm and egg unite; the first cell of a new organism.
- Types of asexual reproduction, when a new individual is formed from a single parent.
- During sexual reproduction in animals, fertilization can be internal or external.
Use the resource below to answer the questions that follow.
- Plant reproduction: Asexual Reproduction at http://www.youtube.com/watch?v=drcnTg7ZCoc (2:57)
- How does the production of "bulbs" benefit plants?
- How can an organism benefit from asexual reproduction? Be specific and thorough in your answer.
- What kinds of environmental conditions favor asexual reproduction?
- What can be a negative effect of asexual reproduction? Is this more applicable to the individual or the population?
- What is the advantage of sexual reproduction?
- How does internal fertilization differ from external fertilization?
- What are the types of asexual reproduction discussed?