Asexual Reproduction

Essential Question

Why are offspring of asexual reproduction genetic copies of the parent?

Some organisms reproduce with only one parent, so their offspring do not get a mix of genes from two parents. In this lesson, you’ll learn the main types of asexual reproduction, see why copying DNA before division makes offspring genetically identical to the parent, and compare asexual and sexual reproduction.

One Parent, Identical Offspring

• Asexual reproduction needs only one parent
• The parent’s DNA is copied before the cell divides
• Each offspring receives a complete copy of that DNA
• Offspring are usually clones: genetically identical to the parent

In asexual reproduction, one parent produces offspring without fertilization. Before a cell divides, it copies its DNA, so each new cell receives a complete set of the parent’s DNA. In the diagram, a parent with genotype Aa produces two offspring that are also Aa. Offspring made this way are clones: genetically identical to the parent and to each other. The only differences come from mutations, which are rare changes in DNA. Eukaryotes copy their DNA and divide by mitosis (Unit 7), and bacteria divide by a similar process called binary fission.

Types of Asexual Reproduction

• Binary fission: one cell splits into two similar cells
• Budding: a small outgrowth grows and breaks off
• Fragmentation: a piece of the parent grows into a new organism
• Parthenogenesis: an unfertilized egg develops into a new individual
• The first three copy the parent’s DNA, so offspring are clones; parthenogenesis offspring are not always exact copies

In binary fission, a single-celled organism, such as a bacterium, copies its DNA and splits into two cells of about equal size. In budding, a new individual grows as a small bud on the parent and later separates. Yeast and hydra reproduce this way. In fragmentation, a piece that breaks off the parent grows into a whole new organism. Some sea stars, flatworms called planarians, and many plants can do this. In parthenogenesis, an unfertilized egg develops into a new individual. It occurs in some insects, such as honeybee drones, and in some reptiles and sharks, including some lizards. In many cases the egg is made by meiosis, so the offspring are not always exact copies of the mother. A honeybee drone, for example, has only one set of chromosomes, which is half the number its mother has. Under good conditions, asexual reproduction can be fast: one bacterium that divides every 20 minutes becomes 64 cells in 2 hours.

Asexual and Sexual Reproduction

• Sexual reproduction mixes genes from two parents
• Asexual reproduction usually does not, so offspring have few new combinations
• Clones can do well in a stable environment but share the same weaknesses
• Variation helps a population survive when conditions change

Feature	Asexual reproduction	Sexual reproduction
Number of parents	One	Usually two
Offspring genes	Usually identical to the parent (clones)	A new mix from both parents
Variation in the population	Low (mostly from mutations)	High
Speed	Usually fast, and no mate is needed	Usually slower, and a mate is needed
Examples	Bacteria, yeast, hydra	Most animals and flowering plants

Because clones are identical, one change in the environment can affect them all in the same way. A disease that kills one clone may kill many others. In a sexually reproducing population, variation means that some individuals may be able to survive. This connects to natural selection in Unit 4: selection can only act on variation that already exists. Some species use both kinds of reproduction, such as strawberry plants and aphids.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Binary Fission

A bacterium divides every 20 minutes under ideal conditions. (a) How many cells are there after 2 hours, starting with 1 cell? (b) How many after 3 hours? (c) The starting bacterium has the genotype Aa and no mutations occur. What is the genotype of each of the cells after 3 hours?

Show the solution

1. (a) Divisions in 2 hours: 2 hours = 120 minutes, and 120 min / 20 min per division = 6 divisions. Each division doubles the number of cells, so the number of cells = 2⁶ = 64.
2. (b) Divisions in 3 hours: 180 min / 20 min = 9 divisions. Cells = 2⁹ = 512.
3. (c) Genotype: Each division copies the DNA first, so all the cells are clones of the starting bacterium. Every cell is Aa.
4. Answer: (a) 64 cells. (b) 512 cells. (c) All cells are Aa.

Example 2: Clones and Disease

Sample data (made up for practice): a farmer grows 200 strawberry plants from runners, which are clones of one parent, and 200 plants from seeds made by sexual reproduction. A fungus infects the field. (a) What percent of each group died? (b) Write a claim, evidence, reasoning statement that explains the difference.

Group	Plants	Plants that died	Plants that survived
Clones (runners)	200	196	4
From seeds	200	38	162

Show the solution

1. (a) Percent that died: Clones: 196 / 200 = 0.98 = 98%. Seed plants: 38 / 200 = 0.19 = 19%.
2. (b) Claim: Plants grown from seeds were much more likely to survive the fungus.
3. Evidence: 98% of the clones died, but only 19% of the seed-grown plants died.
4. Reasoning: Clones have identical genes, so if they lack an allele that protects against the fungus, almost all of them are affected. Sexual reproduction makes offspring with different gene combinations, so some seed-grown plants had alleles that helped them survive.
5. Answer: (a) Clones 98%, seed plants 19%. (b) Variation from sexual reproduction protected some plants, but identical clones shared the same weakness.

Example 3: Classify the Reproduction

Classify each scenario as binary fission, budding, or fragmentation. Then answer part (d). (d) A parent plant has the genotype Rr and reproduces asexually. What genotype do the offspring have, assuming no mutation?

Scenario	Description
(a)	A hydra grows a small bump that develops tentacles and then detaches.
(b)	A bacterium copies its DNA, grows longer, and splits into two cells.
(c)	A piece of a planarian flatworm breaks off and regrows into a whole worm.

Show the solution

1. (a) Budding: a new individual grows on the parent and then separates.
2. (b) Binary fission: one cell divides into two cells of similar size.
3. (c) Fragmentation: a broken-off piece grows into a new organism.
4. (d) Genotype: The parent copies its DNA before dividing, so offspring are Rr.
5. Answer: (a) Budding. (b) Binary fission. (c) Fragmentation. (d) Rr.

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Practice

Practice calculating cell numbers after binary fission, classifying types of asexual reproduction, predicting offspring genotypes, and explaining why clones share the same weaknesses. Try it on your own first, then check your work with the answer key.

• Asexual Reproduction Practice Worksheet (PDF)
• Asexual Reproduction Practice Answer Key (PDF)

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Lesson Slides

• 10.1 Asexual Reproduction Slides (PDF)

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Big Ideas

• Asexual reproduction needs only one parent
• DNA is copied before the cell divides
• Offspring are usually clones of the parent
• Binary fission, budding, fragmentation, and parthenogenesis are types of asexual reproduction
• In parthenogenesis, an unfertilized egg develops into a new individual
• Clones share the same weaknesses, so variation matters

Back to the essential question: Offspring made by binary fission, budding, and fragmentation are genetic copies because the parent copies its DNA before dividing, and each offspring receives that complete copy. New alleles appear only by mutation. Parthenogenesis is a related case in which an egg develops without fertilization, and its offspring are not always exact copies. Next, you’ll see how sexual reproduction mixes genes by making gametes with meiosis.

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