Variation: Phenotype (CIE A Level Biology)

• 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 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 are called genetic variation
  • Genetic variation is transferred from one generation to the next and it generates phenotypic variation within a species population
  • The following processes cause genetic variation 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 fertilisation

  • Mutation results in the generation of new alleles
    • The new allele may be advantageous, disadvantageous or have no apparent effect on phenotype (because 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 impact that the gene has on the phenotype may be large, small and/or additive

Sources of genetic variation table

Process	Mechanism	Consequences
Independent assortment of homologous chromosomes during metaphase I	Random alignment of chromosomes results in different combinations of chromosomes and different allele combinations in each gamete	Genetic variation between gametes
Crossing over of non-sister chromatids during prophase I	The exchange of genetic material between non-sister chromatids leads to new combinations of alleles on chromosomes. It can also break the linkage between genes	Genetic variation between gametes
Random fusion of gametes during fertilisation	Any male gamete can fuse with any female gamete (Random mating in a species population)	Genetic variation between zygotes (and resulting individuals)
Mutation	Random change in the DNA base sequence results in the generation of a new allele. Mutation must exist within gametes for it to be passed on to future generations	Genetic variation between individuals within a species population

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 following:
  • 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