topic 7- genetics, inheritance

• 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 alleles for a single gene
• heterozygous 


  a pair of homologous chromosomes carrying two different alleles for a single gene
• recessive alleles 


  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 chromosomes
• autosomal linkage 


  genes that are located on the same chromosomes (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 characteristic determined by one gene
• dihybrid 


  genetic inheritance cross of a characteristic determined by two genes
• crossing over and autosomal linkage 


  this is only possible because crossing over must have occurred during meiosis to make new combinations of gametes
• how to know when to use spearman rho

  • continuous data - taking measurements
  • investigating an association between two measurements
• how to know when to use t-test

  • continuous data - taking measures
  • investigating a difference between two means
• how to know when to use chi-squared

  • frequencies - counting how many individuals are in a category
  • investigating a difference
• null hypothesis 


  there is no significant difference between the expected and observed frequency of the _______
• chi squared equation


  (O-E) SQUARED DIVIDED EXPECTED

  then calculate the degrees of freedom which is number of groups - 1
• probability

  • if the observed is smaller then the critical there is a more than 5% chance of probability that that the results are due to chance - accept the null
  • if the observed is larger then the critical there is a less then 5% chance of probability that the results are due to chance - reject the null
• hardy-weinberg principle

  • mathematical model which can be used to predict the allele frequencies within a population
  • p² + 2pq + q² = 1.
  • p = the frequency of the dominant allele
  • q = the frequency of the recessive allele
  • p² = the frequency of the homozygous dominant genotype
  • 2pq = the frequency of the heterozygous genotype
  • q² = the frequency of the homozygous recessive genotype
  • before using the long equation you need to find p and q = p+q=1
• 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
• variation

  • individuals within a population of a species may show a wide range of variation in the phenotype
  • this is due to genetic and environmental factors
  • the primary source of genetic variation is mutation
  • meiosis and the random fertilisation of gametes also introduces genetic variation
  • predation, disease and competition for the means of survival result in differential survival and reproduction e.g natural selection
• pure breeding

  • If pea plants with green pods are bred repeatedly with each other so they consistently give rise to plants with green pods, they are said to be pure-breeding for the characteristic for green pods.
  • Pure-breeding and strains can be bred for almost any characteristic.
  • This means that the organisms are homozygous
• Mendel's laws 


  This led to Mendel proposing his law of segregation

  which states:

  in diploid organisms, characteristics are determined

  by alleles that occur in pairs. Only one of each pair

  of

  alleles can be present in a single gamete.

  The other laws include:

  dominance

  independent assortment
• assumptions of the hardy-weinburg equation

  1. no mutations arise
  2. the population is isolated (ie. there is no flow of alleles into or out of the population)
  3. there is no selection (ie. all alleles have the same chance of being passes onto the next generation)
  4. the population is large
  5. mating within the population is random
• Directional Selection


  Favours phenotypes that vary in one direction from the mean of

  the population.

  • Occurs when the environment changes, slowly...

  
  

  ...therefore, there is ‘selective pressure’ for a species to

  
  

  change in response to the environment...

  
  

  (A selection pressure could be the introduction of a predator

  or a habitat change, for instance)
• Stabilising Selection


  Favours the mean phenotype

  • In a stable environment, selection operates against both

  
  

  extremes of a range...

  
  

  ...it operates to make the population more uniform...
• Disruptive Selection


  Favours the extreme phenotypes at the expense of the mean

  phenotype as they 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 becomes less frequent

  • continued disruptive selection can ultimately lead to speciation
• allopatric speciation

  • speciation is the process that results in the creation of new species due a physical barrier like a river
  • this occurs when one original population of the same species becomes reproductively isolated
  • this isolation means that there are two populations of the same species but they cannot bread
  • this can result in the accumulation of differences in their gene pool to the extent that the two populations would be unable to interbreed to make fertile offspring and therefore classed as two different species
• How does geographical isolation lead to speciation ?

  • A population of organisms has a gene pool and interbreeds.
  • There is variation due to mutation/meiosis
  • A geographical event occurs separating the population (gene pool separates)
  • The environmental conditions are different for the 2 populations (different selection pressures).
• genetic drift

  • change in the allele frequency within a population between generations
  • this is what causes evolution
  • the smaller a population is the bigger impact allele frequency changes have proportionally and this is why evolutions often occurs more rapidly in smaller populations
• population

  • group of organisms of the same species living in the same habitat
• habitat 


  part of an ecosystem in which particular organisms live
• community 


  all the populations of different species in the same area at the same time
• ecosystem 


  a community and the non-living components of an environment
• abiotic 


  non-living
• biotic 


  living