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Meiosis and variation Lesson 9 Continuous and discontinuous variation Learning objectives You should be able to: • • • • Describe the differences between continuous and discontinuous variation Explain the basis of continuous and discontinuous variation by reference to the number of genes which influence the variation Explain that both genotype and environment contribute to phenotypic variation. (No calculations of heritability will be expected) Explain why variation is essential in selection Variation All organisms display variation This can be either continuous or discontinuous What’s the difference? Discontinuous variation describes qualitative differences between phenotypes. This means phenotypes fall in to clearly distinguishable categories Human blood group is a good example. The four possible categories are O, A, B and AB Continuous variation describes quantitative differences between phenotypes. This means that there are no distinct categories of phenotype. Height and mass in humans are good examples The genetic basis of discontinuous and continuous variation Discontinuous variation Discontinuous variation is usually the result of just one gene controlling the phenotype. This is described as being monogenic and cystic fibrosis is a good example The only way we can get discontinuous variation of a phenotype that is controlled by two or more genes is if the genes interact with each other through a mechanism such as epistasis In discontinuous variation, each allele will have a large effect on the phenotype Different gene loci may have very different effects on the phenotype Dominant, codominant and recessive alleles may be involved Continuous variation Continuous variation is the result of two or more genes contributing to the phenotype. It’s genetic basis is usually polygenic, in other words the characteristic is controlled by many unlinked genes (called polygenes) Each gene provides an additive component to the phenotype Each allele has only a small effect on the phenotype Made up example of how continuous variation might be inherited Imagine banana plants in which the length of the banana is controlled by 3 genes, A/a, B/b and C/c The presence of a dominant allele at each locus will add 8cm to the length of the bananas on the plant Two recessive alleles as a locus means that only 4cm is added to the length of the bananas on the plant What length will the bananas growing on plants with the following genotypes be? AABBCC AabbCc aabbCc aabbcc Genotype and environment interact to contribute to phenotype In the example we have just looked at a plant with alleles AABBCC should produce bananas that are 24cm long But suppose the plant didn’t get enough light, water or nitrate? The plant and the bananas on it would be smaller In other words, environmental factors may limit the expression of some genes Polygenic traits tend to be more affected by environmental factors than those under monogenic control Determination of human intelligence This is an immensely controversial topic, not least because intelligence is very hard to define and therefore measure Some scientists propose that both genes and environmental factors, such as good nutrition, contribute to human intelligence It’s probably best to avoid making any definitive statements about this subject though I Variation and selection Selection occurs where individuals with particular characteristics are more likely to breed and so pass on those characteristics Over several generations this can lead to significant genotypic and phenotypic changes within a population Selection always requires genetic variation to exist within a population if it is to be heritable There are two kinds of selection Natural selection This occurs where individuals within a population are at a selective advantage and so more likely to breed and pass on their advantageous alleles Artificial selection (there’s no such thing as unnatural selection) This is where humans select which plants or animals to breed from so that desired characteristics are passed on The domestication of animals and plants for agricultural purposes is the most common reason for this How continuous and discontinuous variation relate to Mendel’s work By showing that pea plants inherited discontinuous traits, such as green or yellow pods, Mendel showed that inheritance was particulate (it came in particles). The trouble was that Mendel had no idea what these particles of inheritance might be as noone knew about DNA or chromosomes Worse still for Mendel, his ideas clashed with prevailing thought at the time which was that the characteristics of the two parents somehow blended to produce the phenotype of the offspring. Even Charles Darwin believed this This meant that Mendel’s work was ignored and then forgotten What changed this? The discovery of chromosomes in the late nineteenth century and the observation of their behaviour during meiosis during the early twentieth century supported Mendel’s idea of inheritance being particulate This coincided with scientists rereading Mendel’s work after doing similar experiments themselves and realising that he had been right all along The only question that now remains about Mendel’s work is his recording of the results he obtained. There are those that say they are a little too perfect…………………..