CIE A Level Biology复习笔记17.1.1 Variation: Phenotype

• The observable characteristics of an organism are its phenotype
• Phenotypic variation is the difference in phenotypes between organisms of the same species
• In some cases, phenotypic variation is explained by genetic factors
  • For example, the four different blood groups observed in human populations are due to different individuals within the population having two of three possible alleles for the single ABO gene
• In other cases, phenotypic variation is explained by environmental factors
  • For example, clones of plants with exactly the same genetic information (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
  • For example, the 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
• The complete phenotype of an organism is determined by the expression of its genotype and the interaction of the environment on this:

Phenotype = Genotype + Environment

Genetic variation

• • Organisms of the same species will have very similar genotypes, but two individuals (even twins) will have differences between their DNA base sequences
  • Considering the size of 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
  • Genetic variation is caused by the following processes as they result in a new combination of alleles in a gamete or individual:
    • Independent assortment of homologous chromosomes during metaphase I
    • Crossing over of non-sister chromatids during prophase I
    • Random fusion of gametes during fertilization
  • Mutation results in the generation of new alleles
    • The new allele may be advantageous, disadvantageous or have no apparent effect on phenotype (due to the fact that the genetic code is degenerate
    • New alleles are not always seen in the individual that they first occur in
    • They can remain hidden (not expressed) within a population for several generations before they contribute to phenotypic variation
  • Genes can have varying effects on an organism's phenotype
    • The phenotype may be affected by a single gene or by several
    • The effect that the gene has on the phenotype may be large, small and/or additive

Sources of genetic variation table

Environmental factors

• The environment that an organism lives in can also have an impact on its phenotype
• Different environments around the globe experience very different conditions in terms of the:
  • Length of sunlight hours (which may be seasonal)
  • Supply of nutrients (food)
  • Availability of water
  • Temperature range
  • Oxygen levels
• Changes in the factors above can affect how organisms grow and develop
  • For example, 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 caused 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