Continuous and discontinuous variation Genes in population

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Continuous and discontinuous variationGenes in
population
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Discontinuous Variation

• The phenotypes we have consider so far have been
  due to two segregating alleles of single genes.
• This means that alternative phenotypes are
  clearly different.
• This variation or difference between phenotypes
  is known as discontinuous variation.

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Continuous Variation

• Many phenotypes show continuous variation there
  are many intermediate forms and the distribution
  of the phenotypes in a population is a bell
  curve.
• Traits that show continuous variation include
  height and weight in humans, milk production in
  cows and the size of flowers.
• Continuous traits are usually polygenic, with
  alleles of many different genes contributing to
  produce the final phenotype.
• This results in considerable phenotypic
  variation, and further variation may also occur
  due to interactions with the environment.

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Variation in Phenotype

• Variation in phenotype in populations is
  influenced by the following factors
• One gene can have many alleles
• Meiosis involves independent assortment and
  genetic recombination of alleles
• Dominance may not be complete
• Several genes can affect the one trait
• One gene can determine more than one trait
• The expression of genes can be affected by other
  genes and by the environment

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Genes in populations

• Different people in same community have different
  risks for disease.
• The reasons for this are a complex mix of
  genetic, environmental and social risk factors.
• Epidemiology is the description and analysis of
  the pattern of diseases in the population, the
  causes of these different patterns, and the use
  of this information to improve public health.
• We know that some diseases, both rare and common,
  seem to run in families.
• Genetic epidemiology attempts to determine the
  size of genetic influences on disease.

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Heterozygote Advantage

• Some extremely serious and often fatal genetic
  diseases continue to be present in the population
  but why????
• If individuals who are homozygous for such a
  disease normally die before reproducing, we would
  expect natural selection to lead to low
  frequencies for the disease allele in the
  population.
• This is not always the case take the sickle
  cell anaemia gene and the cystic fibrosis gene as
  examples.

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Heterozygote AdvantageSickle Cell Anaemia

• High frequency of the mutant allele in people of
  African descent
• Homozygotes suffer from life-threatening anaemia
• The explanation for the unexpected frequency is
  that being heterozygous for this allele is
  advantageous in areas where malaria is present.
• The mutant allele results in defective
  haemoglobin molecules alter the red blood cells
  so that they are less susceptible to infection by
  the malarial parasite.

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Heterozygote AdvantageCystic Fibrosis

• High frequency of the mutant allele in people of
  Caucasian background (1 in 28 Caucasians are
  heterozygotes)
• Mutant copy of CFTR gene leads to defective
  channel proteins being produced in the cell
  membrane
• The bacteria Salmonella typhi requires the
  channel protein in order to get across the
  intestine wall
• This means that heterozygotes are less
  susceptible to typhoid fever
• The gene frequencies currently observed are a
  result of previous heterozygote advantage when
  typhoid fever was widespread in Europe

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What is the gene pool

• The genetic information in an individual is a
  genotype.
• The genetic information in a population is a gene
  pool.
• A gene pool is described in terms of the allele
  frequencies (proportions) of each gene.
• When genotypes of all members of a population are
  known, allele frequencies may be calculated
  directly from genotype frequencies.
• When allele frequencies are not known, they may
  be estimated starting from the frequency of the
  homozygous recessive phenotype.