phenotypes and variation

• continuous variation occurs when there are quantitative differences in the phenotypes of individuals within a population for particular characteristics
• quantitative differences do not fall into discrete categories

  a range of values exist between 2 extremes within which the phenotype will fall (e.g mass or height)
• causes of continuous variation

  some phenotypes are affected by multiple different genes or by multiple alleles for the same gene at many different loci (polygenic inheritance) as well as the environment

  this often gives rise to phenotypes that show continuous variation
• causes of continuous variation

  different alleles at a single locus have a small effect on the phenotype

  different genes can have the same effect on the phenotype and these add together to have an additive effect

  if a large number of genes have a combined effect on the phenotype they are known as polygenes
• phenotypic variation is the difference in phenotypes between organisms of the same species
• in some cases, phenotypic variation is explained by genetic factors

  e.g 4 different blood groups observed in human populations are due to different individuals within the population having 2 of 3 possible alleles for the single ABO gene
• in other cases, phenotypic variation is explained by environmental factors e.g clones of plants with exactly the same genetic info (DNA) will grow to different heights when grown in different environmental conditions
• phenotypic variation can also be explained by a combination of genetic and environmental factors

  e.g recessive allele that causes sickle cell anaemia has a high frequency in populations where malaria is prevalent due to heterozygous individuals being resistant to malaria

  phenotype = genotype + environment
• organisms of the same species will have very similar genotypes but 2 individuals (even twins) will have differences between their DNA base sequences
• considering the size of the genomes, these differences are small between individuals of the same species

  the small differences in DNA base sequences between individual organisms within a species population is called genetic variation
• genetic variation is transferred from one generation to the next and it generates phenotypic variation within a species population
• some characteristics (phenotype) are controlled by a single gene - these characteristics are known as monogenic

  these characteristics usually show discontinuous variation (blood group)
• other characteristics are controlled by several genes - these characteristics known as polygenic

  these characteristics usually show continuous variation (height, mass, skin colour)
• the different alleles an organism has at a single gene locus can determine the phenotype

  diploid organisms will inherit 2 alleles of each gene, these alleles can be the same or different

  e.g F8 gene codes for blood clotting protein factor VIII. the different alleles at the F8 gene locus dictate whether or not normal factor VIII is produced and whether the individual has the condition haemophilia
• different environments around the globe experience very different conditions, including:

  length of sunlight hours (may be seasonal)

  supply of nutrients (food)

  availability of water

  temp range

  oxygen levels
• changes in environment conditions can affect how organisms grow and develop e.g plants with a tall genotype growing in an environment that is depleted in minerals, sunlight and water will not be able to grow to their full potential size determined by genetics
• variation in phenotype causes solely by environmental pressures or factors cannot be inherited by an organism's offspring (only alterations to the genetic component of gametes will ever be inherited)
• example of how the environment can affect phenotypic variation

  fruit fly is normally grey but there is a genetic mutant that is yellow (genotypic characteristic that is expressed regardless of the environment)

  if larvae of normal grey flies are given a diet of silver salts, they develop the yellow colour regardless of their genotype

  this means that flies that should be grey (according to genes) can become yellow due to an environmental factor (their diet)
• example of how the environment can affect phenotypic variation

  plants that are grown in the dark or that cannot access enough magnesium become yellow eventhough genetically they should be green (chlorosis: occurs because synthesis of chlorophyll is slowed down or stopped completely)

  plants that are grown in the dark may also develop long stems with small, curled leaves eventhough genetically they should develop normally (etiolation)
• qualitative differences in the phenotypes of individuals within a population give rise to discontinuous variation, they fall into discrete, distinguishable categories
• continuous variation occurs when there are quantitative differences in phenotypes of individuals within a population for particular characteristics, they don't fall into discrete categories

  a range of values exist between 2 extremes within which the phenotype will fall
• causes of continous variation

  at the genetic level;

  diff alleles at a single locus have a small effect on the phenotype

  diff genes can have the same effect on the phenotype and these add together to have an additive effect

  if a large number of genes have a combined effect on the phenotype they are known as polygenes
• the additive effect of genes

  the height of a plant is controlled by 2 unlinked genes H / h and T /t

  the 2 genes have an additive effect

  the recessive alleles h and t contribute x cm to the height of the plant

  the dominant alleles H and T contribute 2x cm to the height of the plant

  e.g genotype H h T t : 2x + x + 2x + x = 6x cm