Reproduction

• Sexual reproduction produces genetically different cells
• Sexual reproduction is where genetic information from two organisms (mother and father) is combined to produce offspring which are genetically different to either parent
• In sexual reproduction, the mother and father produce gametes by meiosis - e.g. egg and sperm cells in animals
• In humans, each gamete contains 23 chromosomes - half the number of chromosomes in a normal cell. (Instead of having 2 chromosomes each, gamete has just one of each)
• The egg (from the mother) and the sperm (from the father) then fuse together (fertilisation) to form a cell with the full number of chromosomes
• Sexual reproduction involves the fusion of male and female gametes. Because there are 2 parents, the offspring contain a mixture of their parents' genes
• Offspring inherits features from both parents - it's received a mixture of chromosomes from its mum and dad
• The mixture of genetic information produces variation in the offspring
• Flowering plants can reproduce sexually too. They also have egg cells, but their version of sperm is known as pollen
• Asexual reproduction produces genetically identical cells
• In asexual reproduction there's only one parent so the offspring are genetically identical to the parent
• Asexual reproduction happens by mitosis - an ordinary cell makes a new cell by dividing in two
• In asexual reproduction, the new cell has exactly the same genetic information as the parent cell - it's called a clone
• In asexual reproduction there's only one parent. There's no fusion of gametes, no mixing of chromosomes and no genetic variation between parent and offspring. The offspring are genetically identical to the parent - they're clones
• Bacteria, some plants and some animals reproduce asexually
• Sexual reproduction has advantages over asexual reproduction
• Offspring from sexual reproduction have a mixture of two sets of chromosomes. The organism inherits genes (and therefore features) from both parents, which produces variation in the offspring
• Variation increases the chance of a species surviving a change in the environment. While a change in environment could kill some individuals, it's likely that variation will have led to some of the offspring being able to survive in the new environment. They have a survival advantage
• Because individuals with characteristics that make them better adapted to the environment have a better chance of survival, they're more likely to breed successfully and pass the genes for the characteristics on. This is known as natural selection
• We can use selective breeding to speed up natural selection. This allows us to produce animals with desirable characteristics
• Selective breeding is where individuals with a desirable characteristic are bred to produce offspring that have the desirable characteristic too. This means we can increase food production, e.g. by breeding animals that produce a lot of meat
• Asexual production has advantaged over sexual reproduction

  • There only needs to be one parent
  • This means that asexual reproduction uses less energy than sexual reproduction, because organisms don't have to find a mate
  • This also means that asexual reproduction is faster than sexual reproduction
  • Many identical offspring can be produced in favourable conditions
• Some organisms can reproduce by both methods
• Some organisms can reproduce sexually or asexually depending on their circumstances
• Malaria is caused by a parasite that's spread by mosquitoes. When a mosquito carrying the parasite bites a human, the parasite can be transferred to the human. The parasite reproduces sexually when it's in the mosquito and asexually when it's in the human host
• Many species of fungus can reproduce both sexually and asexually. These species release spores, which can become new fungi when they land in a suitable place
• Spores can be produced sexually and asexually
• Asexually-produced spores from fungi that are genetically identical to the parent fungus and sexually-produced spores introduce variation and are often produced in response to an unfavourable change in the environment, increasing the chance that the population will survive the change
• Loads of species of plant produce seeds sexually, but can also reproduce asexually
• Asexual reproduction in plants can take place in different ways. E.g. strawberry plants produce 'runners'. These are stems that grow horizontally on the surface of the soil away from a plant. At various points along the runner, a new strawberry plant forms that is identical to the original plant
• Plants that grow from bulbs (e.g. daffodils) also reproduce asexually. New bulbs can form from the main bulb and divide off. Each new bulb can grow into a new identical plant