Reproduction

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Reproduction Revision

Reproduction

Organisms reproduce in two different ways, sexually and asexually. Sexual reproduction involves the fusion of two gametes that are formed in meiosis and leads to genetic variation in the offspring. Asexual reproduction is when one parent cell divides by mitosis, producing genetically identical offspring. There are advantages and disadvantages to both methods of reproduction. Some organisms reproduce both ways, depending on certain circumstances.

Sexual Reproduction

Sexual reproduction is where genetic information from a mother and father is mixed together to form genetically different offspring. Each offspring will inherit different genes from their parents, giving them all different features and characteristics.

Genetic information from the mother and father is carried in sex cells called gametes which are formed in a process called meiosis. When the nuclei of the two gametes fuse together in fertilisation they form a zygote.

Gametes in animals are called sperm cells (from father) and egg cells (from mother).
Gametes in flowering plants are called pollen (from father) and egg cells (from mother).

Humans have $46$ chromosomes and their gametes (sperm and egg cells) have $23$ chromosomes. This is because when the nuclei of the gametes fuse, the genetic information combines together giving the offspring a full set of chromosomes.

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Asexual Reproduction

Asexual reproduction is where offspring are created from one parent only. As there is no fusion of gametes or mixing of genetic information, offspring will be genetically identical to each other and their parent (clones).

This usually happens through mitosis, where a cell makes a copy of its genetic information and then splits into two.

Bacteria and some plants and animals reproduce asexually.

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Sexual vs Asexual Reproduction

Advantages of Sexual Reproduction	Advantages of Asexual Reproduction
Offspring are genetically different from each other and parents, produces variation.	No mate is required for asexual reproduction so it is quicker and populations can increase rapidly.
Variation increases their chances of survival in a new environment as they are able to adapt to the different conditions. This is called natural selection.	It is also much more energy efficient as they do not need to find a mate to reproduce.
Variation also means diseases are less likely to affect whole populations.	Lots of identical offspring can be produced if conditions are favourable.

Humans often take advantage of natural selection and use selective breeding to increase food production. Animals with desirable traits (those who produce more milk or meat) are bred together to create offspring with these traits and produce more food.

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Organisms that Reproduce Sexually and Asexually

Some organisms reproduce sexually and asexually, which method they use at a given time depends on certain circumstances.

Some examples include:

Malarial parasites cause the disease malaria. Mosquitos act as a vector; they carry the parasite and spread the disease to humans by biting them. The parasite reproduces sexually when it is in the mosquito vector but reproduces asexually in the human host.
Fungi release spores that grow into new fungi if they land on a suitable surface. The spores are either produced sexually or asexually. The majority of the time they release asexual spores that produce offspring that are identical to the parent. However, sometimes they produce spores produced by sexual reproduction to introduce some variation to the population, particularly when conditions are unfavourable.
Strawberry Plants reproduce sexually, by producing gametes in their flowers but can also reproduce asexually by producing runners. Runners are stems that grow horizontally from the plant and new plants that are genetically identical to the parent grow at various points along them.
Daffodils reproduce sexually using gametes found in their flowers but can also reproduce asexually from their bulbs. Daffodil bulbs can form new bulbs that divide off and grow into new plants.