Genetics

• Temperature mutations:

  Siamese cat fur colour determined by a mutation in the melanin gene

  Tyrosinase (enzyme making melanin) works in cool parts of the body and is deactivated in warmer parts of the body
• Light mutations:

  Arctic hare has dark fur in summer and light fur in winter - short periods of light exposure turn off melanin action

  Plants loose leaves in autumn - cryptochrome molecules determine the length of day

  Mutations in circadian genes in humans alter mood dependent on day length (jet lag)
• Chemical mutations:

  • E. coli can grow on medium lacking the amino acid tryptophan but some mutants cannot (emergence of antibiotic resistance)
  • Phenylketonuria is amino acid sensitivity in humans, and homozygote recessive cannot break down phenylalanine which can be lethal, controlled by limiting diet
  • Porphyria - cannot break down haemin and build up porphin, controlled by avoiding light
• Obesity - risk of type II diabetes associated with weight, obese mutation in mice means they have an inability to know when to stop eating and lack leptin satiety hormone and humans can also have mutations in leptin

  homozygous for FTO mutations in humans makes you hungry and weigh more
• Age mutations:

  Huntington's disease shows in the phenotype over 50
• Quantitative genetics = the inheritance of characters that do not fall into distinct classes, e.g. height, weight
• 1 loci - 1:2:1

  2 loci - 1:4:5:4:1

  3 loci - 1:6:15:20:15:6:1

  n loci - continuous

  many ways to make the intermediate, few ways to make the extreme
• Heritability = the proportion of total variation in the population that is due to genetic variation

  H^2 = genetic variance (Vg) / phenotypic/total variance (Vp)
• Total phenotypic variance is a result of variance caused by the environment, genes and the interaction of the gene and the environment

  Vp = Ve + Vg + Vge
• H^2 = Vg / (Vg + Ve + Vge)

  this is the broad sense heritability
• Additive genetic variance (Va) = variance associated when the addition of an allele contributes a fixed value to the value quantitative trait

  i.e. -- = 0cm, +- = 1cm, ++ = 2cm
• Dominance genetic variance (Vd) = variance caused by the masking of recessive alleles by dominant alleles in the heterozygote

  i.e. -- = 0cm, +- = 2cm, ++ = 2cm
• Interaction genetic variance (Vi) = variance caused by the interaction between different loci

  i.e. ++ -- = 0cm, ++ +- = 1.2 cm, ++ ++ = 2cm
• Narrow sense heritability has not been measured for any human character and uses only additive variance

  h^2 = Va / Vp
• Concordance = extent to which twins share the same phenotype
• Both monozygotic and dizygotic twins share an environment, so variation will be genetic. BUT:

  • splitting of the embryo causes variation in MZ twins
  • MZ twins more likely to copy each other than DZ twins
  • Monochorionic twins share same blood supply but DZ twins do not so they are competing for nutrients and can differ at birth
• Human behaviours with a genetic component:

  • Fragile X (triple repeat CGG) frequently seen in males
  • Huntington's (CAG repeats) causing neurons to break down
  • Alzheimer's (locus on chromosome 21) is a common cause of dementia
  • Autism (deletions) associated with poor social behaviour
• Animal behaviour with a genetic component: (artificial selection)

  • diversity of dog pedigrees
  • silver foxes - as they become tamer, their coat colour changes
  • basset hound prone to paranoia
  • dachshund - narcolepsy (unable to regulate sleep patterns)
• Parthenogenesis = when a female can produce offspring either sexually or asexually (e.g. bananas, potatoes, stick insects, haplodiploids)
• Anisogamy = the union of two gametes that differ in size

  males produce many smaller gametes, females produce fewer larger gametes
• Y-chromosome structure:

  • Pseudoautosomal region - two regions at the tips of the chromosome
  • Male specific Y (MSY) makes up the remaining region, consisting of euchromatin and heterochromatin
  • Amplicon regions located in euchromatin containing palindromes where Y can recombine with itself
• Sex-determining region Y (SRY) protein is a transcription factor that turns on SOX-9

  SOX-9 is a transcription factor associated with turning on loci associated with maleness
• Mapping the Y-chromosome:

  it is passed on through the male lineage so any mutation which arises will only be found in males, used to study migration and estimate common ancestors
• Mitochondrial genome:

  It is inherited through the female line and represents one haplotype
• Inbreeding = the mating of relatives

  Alleles show identity by descent (IBD) if an individual is homozygous due to receiving two copies of the allele from the same ancestor
• Loop = closed structure within a pedigree in which two individuals trace descent from the same person
• When calculating the inbreeding coefficient, the child is not counted in the loop as it cannot be an ancestor to itself
• Inbreeding coefficient = the probability of homozygosity that results because an individual obtained two copies of the same ancestral allele
• Inbreeding coefficient calculation:

  f = the sum of the number of loops, times half to the power of the number of ancestors in a loop
• Inbreeding depression = reduction in fitness associated with inbreeding

  e.g. congenital myopathy in border collies

  decreased immune response in song sparrows
• Inbreeding in humans:

  Finland - 33 unique genetic diseases, e.g. VLINCL

  Thalassemia in Saudi Arabia
• Runs of homozygosity = continuous lengths of homozygous genotypes that are present in an individual due to parents passing on identical haplotypes to their offspring

  more common on islands than mainland, e.g. Croatia, Scotland