Topic 7 AQA alevel biology

• Genotype 


  The genetic constitution of an organism (the alleles it has for a gene)
• Phenotype 


  The expression of the genes and its interaction with the environment
• Homozygous 


  A pair of homologous chromosomes carrying the same allele for a single gene
• Heterozygous 


  A pair of homologous chromosomes carrying two different alleles for a single gene
• Recessive allele 


  An allele only expressed if no dominant allele is present
• Dominant allele 


  An allele that will always be expressed in the phenotype
• Codominant 


  Both alleles are equally dominant and expressed in the phenotype
• Multiple alleles 


  More than two alleles for a single gene
• Sex-linkage 


  A gene whose locus is on the X chromosome
• Autosomal linkage 


  Genes that are located on the same chromosome (not the sex chromosome)
• Epistasis 


  When one gene modifies or masks the expression of a different gene at a different locus
• Monohybrid 


  Genetic inheritance cross of a characteristics determined by one gene
• Dihybrid 


  Genetic inheritance cross for a characteristic determined by two genes
• How does autosomal linkage affect predicted gametes

  alleles for each gene linked on the same chromosome will be inherited together
• Dihybrid 


  9:3:3:1 ratio
• Autosomal linkage

  3:1
• How crossing over during meiosis makes new combinations of gametes 


  Homologous pair of chromosomes after DNA replication (interphase), chromatids cross over during meiosis, the four possible chromosomes in each gamete (new combinations of alleles have been made)
• Chi squared is the statistic used to 


  Investigate differences between frequencies, to determine if there is a significant difference between the frequency you expect and the frequency you observe
• Gene pool 


  All the alleles of all the genes within a population at one time
• Population 


  All the individuals of one species in one area at one time
• Allele frequency 


  The proportion of an allele within the gene pool
• When is hardy weinberg principle used

  To predict the allele frequencies within a population
• Hardy-Weinburg equation 


  p^2 + 2pq + q^2 = 1

  p = frequency of the dominant allele

  q = frequency of the recessive allele

  p^2 = frequency of the homozygous dominant genotype

  2pq = frequency of the heterozygous genotype

  q^2 = frequency of the homozygous recessive genotype
• Variation 


  Individuals within a population of a species may show a wide range of variation in phenotype, this is due to genetic and environmental factors, the primary source of genetic variation is mutation but meiosis and random fertilisation of gametes also introduces genetic variation, predation, disease and competition for the means of survival result in differential survival and reproduction ie natural selection, those organisms with phenotypes providing selective advantages are likely to produce more offspring and pass on their advantageous alleles to the next generation
• Variation continued 


  This then changes the allele frequencies within a gene pool
• Disruptive selection is when individuals which contain the alleles

  Coding for either extreme trait are more likely to survive and pass on their alleles as a result the allele frequency changes and more individuals possess the allele for the extreme trait and the middling trait allele becomes less frequent, continued disruptive selection can lead to speciation
• Speciation is the process which results in the

  Creation of a new species, it occurs when one original population of the same species becomes reproductively isolated meaning there are now two populations of the same species but they cannot breed together, this can result in the accumulation of differences in their gene pools to the extent that the two populations would be unable to interbreed to make fertile offspring and are therefore classed as two different species
• Allopatric speciation 


  Two populations become reproductively isolated due to being geographically separated results in the formation of two new species, could be due to new bodies of water separating land masses, the populations will continue to accumulate different beneficial mutations that would lead to them becoming so genetically different that they would be unable to interbreed to create fertile offspring
• Sympatric Speciation

  Two populations become reproductively isolated whilst in the same location

  e.g. due to changes in behaviour results in the formation of two new species, a random mutation could impact reproductive behaviour by causing individuals to perform a different courtship ritual meaning individuals will not reproduce together and there will be no gene flow between the two groups, overtime the populations will accumulate different mutations causing their DNA to be so different that they cannot interbreed to create fertile offspring
• What is genetic drift

  The change in the allele frequency within a population between generations, continual substantial genetic drift results in evolution, the smaller a population is the bigger the impact allele frequency changes have proportionally and this is why evolution occurs more rapidly in smaller populations
• Population 


  All the individuals of one species in the same area at the same time
• Habitat 


  The range of physical, biological and environmental factors in which a species can live
• Community 


  all the species in a particular area at a particular time
• Ecosystem 


  a community and the non-living components of an area

  they can range from very small to very large in size
• Niche 


  An organism's role within an ecosystem

  their position in the food web and their habitat
• Carrying capacity 


  The maximum population size an ecosystem can support
• Abiotic factors 


  Non-living conditions of an ecosystem eg temperature, oxygen + carbon dioxide concentration, plants and animals are adapted to the abiotic factors within their ecosystem, these adaptations develop through the process of natural selection over many generations, the less harsh the abiotic factors such as plenty of water and light, the larger the range of species and the larger the population sizes
• Biotic factors 


  impact of the interactions between organisms eg competition and predation
• Interspecific competition 


  Competition between members of different species, for resources eg habitat, food, water
• Intraspecific competition 


  Competition between members of the same species, will also be in competition for a mate