Asexual and sexual reproduction

Biology · Development of organisms and continuity of life. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Asexual reproduction produces genetically identical offspring from one parent. There are no gametes and no fertilisation. Examples are binary fission in bacteria, spores in fungi, and tubers, bulbs and runners in plants.

Sexual reproduction involves the fusion of two haploid nuclei, the nuclei of the gametes. This fusion is called fertilisation. It forms a diploid zygote. The offspring are genetically different from each other and from their parents.

Each method has a cost. Asexual reproduction is fast and needs only one parent, but there is no variation, so one disease or one change in the environment can kill the whole population. Sexual reproduction gives variation, so some offspring may survive a change, but it is slower and usually needs two parents.

Rules to remember

Asexual: one parent, no gametes, offspring genetically identical.
Sexual: fusion of haploid nuclei (fertilisation) → diploid zygote; offspring genetically different.
Asexual advantages: fast, only one parent needed, good characteristics are passed on unchanged.
Asexual disadvantages: no variation, so all may be killed by the same disease or change; overcrowding and competition near the parent.
Sexual advantages: variation, so the species can adapt to a changing environment and resist new diseases.
Sexual disadvantages: slower, two parents usually needed, good characteristics may not be passed on.

Common mistake

Do not say asexual offspring are similar to the parent. They are genetically identical, and that exact word is expected.

Worked example

A grower multiplies a potato variety by planting tubers, but breeders must grow potatoes from seed to produce a new variety that resists a disease. Explain both choices.

Tubers are asexual reproduction, so every new plant is genetically identical to the parent and has the same good yield.
Because they are identical, none has a new feature such as resistance to the disease.
Seeds come from sexual reproduction: gametes fuse, so the offspring are genetically different.
Among these varied offspring a few may resist the disease and can be selected.

Answer: Asexual reproduction keeps a good variety unchanged. Sexual reproduction gives the variation needed to find a new, resistant variety.

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