1. State that continuous variation is influenced by genes and environment, resulting in a range of phenotypes between two extremes, e.g. height in humans.

Continuous variation is influenced by both genes and the environment, resulting in a range of phenotypes between two extremes, e.g. height in humans.

  1. State that discontinuous variation is caused by genes alone and results in a limited number of distinct phenotypes with no intermediates, e.g. A, B, AB and O blood groups in humans.

Discontinuous variation is caused by genes alone and results in a limited number of distinct phenotypes with no intermediates, e.g. A, B, AB and O blood groups in humans.

  1. Define mutation as a change in a gene or chromosome.

Learn the definition 😊

  1. Outline the effects of ionising radiation on the rate of mutation.

Ionising radiation increases the rate of mutation, as it causes unwanted reactions to occur in DNA, changing genes.

In some cases, this could result in uncontrolled cell division, leading to the formation of a tumour. These cells are cancer cells.

Ionising radiation can also kill cells.

If the testes or ovaries are exposed to ionising radiation, this may result in sterility. Or the genes in a sex gamete may change, resulting in the possibility of a faulty gene being passed on to a child.

It should be noted that mutations happen all the time in people, but in most cases, your body can fix the faulty genes before any serious consequences arise.

  1. Describe the role of artificial selection in the production of varieties of animals and plants with increased economic importance.

There are certain characteristics in plants or animals that are more desirable – such as friendliness when it comes to pets or higher milk yield when it comes to cows. Animals with these characteristics are more useful and hence hold increased economic importance.

Artificial selection is used to increase the number of animals with these desirable characteristics, to help boost the economy.

For example, take cows. Farmers tend to prefer cows that are more docile and have a higher yield.

Therefore, if two cows that exhibit these characteristics are crossed, it is more likely that their offspring will show these characteristics too. This is artificial selection (we are selecting which characteristics will be passed on to the next generation).

After several generations, these characteristics will become more prominent.

I’ll give you some examples of desirable characteristics in different species of plants and animals, to help you out.

  • Cows: docility and high milk yield
  • Pets: friendly and loyal
  • Guard dogs: fierce, alert, obedient, loyal
  • Guide dogs: Intelligent, alert and obedient
  • Cattle: docile
  • Crops: High yield, insect and pesticide resistant, high growth rate
  1. Describe variation, and state that competition leads to differential survival of, and reproduction by, those organisms best fitted to the environment.

Variation is the differences in a population. It is a term used at almost every level (to describe genetic variation, cellular variation, variation in the organisms as a whole, etc.)

Variation arises due to differences in genotype and environment.

E.g., A plant with the genotype Tt would be tall, but a plant with the genotype tt would be a dwarf. These two plants show both genotypic and phenotypic variation.

In another case, both plants may have the genotype TT, and so both are tall. However, one plant grows in better environmental conditions than the other, so one plant is taller. This is phenotypic variation caused by differences in the environment.

Organisms that arise from sexual reproduction will show much more variation than organisms that arise from asexual reproduction. This is because the random fusion of sex gametes allow offspring with different combinations of alleles to be born.

In asexual reproduction, however, offspring are genetically identical to parents so the only variation present is caused by mutation, and in some cases, the environment. (Although, in the exam, if they ask how variation in asexual organisms arises, only mention mutation.)

It should be noted that most organisms overproduce – they produce more young than necessary to maintain the size of their population.

There are, however, a limited number of resources (food, light, space, etc.), so the excess number of young leads to competition. This is the struggle for existence.

The better-adapted organisms – the ones that are more ‘fit’ – will survive to adulthood, and the rest die. This process is called survival of the fittest.

This means that only the genes carried by the ‘fittest’ organisms are passed on to the next generation, and the genes of the ‘weaker’ organisms slowly die out of the gene pool.

  1. Define natural selection as the greater chance of passing on of genes by the best-adapted organisms.

Learn yo definitions, kids. Honestly, I’m getting kinda bored of telling you guys to do this every time a definition comes up – but don’t brush it off! Learning them is essential for a good grade!

  1. Explain the importance of natural selection as a possible mechanism for evolution.

The theory of natural selection suggests that the best-adapted organism survive to pass on their genes to the next generation and that the poorly adapted organisms die out so their genes are eventually eradicated from the population.

This means that over time, the population will grow to be better adapted to the environment.

If the species survives long enough, it is possible that it will eventually change into a completely new species.

This gradual change in the species over time through natural selection, in response to changes in the environment, is a possible mechanism of natural selection.

  1. Describe the development of strains of antibiotic resistant bacteria as an example of natural selection.

Due to mutation, one or two bacteria in a population of bacteria may have developed genes that grant them immunity against a certain antibiotic, or sometimes, against multiple antibiotics. Therefore, if an antibiotic they are resistant to is used to wipe out their population, these resistant bacteria will survive and will be able to reproduce asexually to pass on their genes to the next generation. Bacteria multiply very quickly so entire colonies of antibiotic resistant bacteria can form in a couple of hours.

This is why it’s important to always complete the antibiotic course you’re on – whether you feel better before it’s over or not. Completing the course will ensure the death of all the bacteria, reducing the chance that some will survive and mutate to gain the resistance gene (Mutations in bacteria usually occur during reproduction). It is also a good idea to take a combination of antibiotics instead of relying on just one, in case you have bacteria that are resistant to one of them. However, you should never take antibiotics without consulting a doctor first.

Medicines are, after all, drugs, and if not administered properly, can be seriously harmful.



Notes submitted by Sarah

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