AQA A Level Biology复习笔记7.3.1 Genetic Variation

Phenotype variation

• The observable characteristics of an organism are its phenotype
• Phenotypic variation is the difference in phenotypes between organisms of the same species
• This variation means that the individuals within a population of a species may show a wide range of variation in phenotype
• 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 phenotypic variation of the individuals in a population is determined by the genetic variation within the population and the interaction of the environment on the individuals:

Phenotypic variation = Genetic variation + 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
• The primary source of genetic variation is mutation (changes in the DNA base sequence)
• Mutation results in the generation of new alleles
  • The new allele may be advantageous, disadvantageous or have no apparent effect on phenotype
  • 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

   

• Genetic variation is also caused by the following processes as they result in a new combination of alleles in a gamete or individual:
  • Crossing over of non-sister chromatids during prophase I of meiosis
  • Independent assortment of homologous chromosomes during metaphase I of meiosis
  • Random fusion of gametes during fertilization

   

Crossing over

• Crossing over is the process by which non-sister chromatids exchange alleles
• Process:
  • During meiosis I homologous chromosomes pair up and are in very close proximity to each other
  • The non-sister chromatids can cross over and get entangled
  • These crossing points are called chiasmata (singular = chiasma)
  • The entanglement places stress on the DNA molecules
  • As a result of this a section of chromatid from one chromosome may break and rejoin with the chromatid from the other chromosome

   

• This swapping of alleles is significant as it can result in a new combination of alleles on the two chromosomes
• There is usually at least one chiasma present in each bivalent during meiosis but often there are multiple chiasmata
• Crossing over is more likely to occur further down the chromosome away from the centromere

Crossing over

Independent assortment

• Independent assortment is the production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I
• The different combinations of chromosomes in daughter cells increases genetic variation between gametes
• In prophase I homologous chromosomes pair up and in metaphase I they are pulled towards the equator of the spindle
  • Each pair can be arranged with either chromosome on top, this is completely random
  • The orientation of one homologous pair is independent / unaffected by the orientation of any other pair

   

• The homologous chromosomes are then separated and pulled apart to different poles
• The combination of alleles that end up in each daughter cell depends on how the pairs of homologous chromosomes were lined up
• To work out the number of different possible chromosome combinations the formula 2n can be used, where n corresponds to the number of chromosomes in a haploid cell
• For humans this is 223 which calculates as 8,324,608 different combinations

Independent assortment

Random fertilisation of gametes

• Meiosis creates genetic variation between the gametes produced by an individual through crossing over and independent assortment
• This means each gamete carries substantially different alleles
• During fertilization any male gamete can fuse with any female gamete to form a zygote
• This random fusion of gametes at fertilization creates genetic variation between zygotes as each will have a unique combination of alleles
• There is an almost zero chance of individual organisms resulting from successive sexual reproduction being genetically identical

Sources of genetic variation table