Heredity and Evolution

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Chapter 4

• Heredity and Evolution

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Genetic PrinciplesDiscovered by Mendel

• Gregor Mendel (1822-1884)
• basic principles of heredity.
• Crossed different strains of purebred plants and
  studied their progeny.
• Studied common garden peas (only one trait at a
  time).
• His work illustrates the basic rules of
  inheritance.

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Principle of Segregation

• Genes occur in pairs (like chromosomes).
• During gamete production, members of each gene
  pair separate.
• During fertilization, the full number of
  chromosomes is restored (allele pairs are
  reunited).
• Homozygous- same allele at same locus on both
  members of a chromosome pair. (i.e.TT, tt)
• Heterozygous- two different alleles at the same
  locus on a chromosome pair.

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Dominance And Recessiveness

• Recessive traits are not expressed in
  heterozygotes.
• For a recessive allele to be expressed, there
  must be two copies of the allele.
• Dominant traits are governed by an allele that
  can be expressed in the presence of another,
  different allele.
• Dominant alleles prevent the expression of
  recessive alleles in heterozygotes.

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Principle of Independent Assortment

• The distribution of one pair of alleles into
  gametes does not influence the distribution of
  another pair.
• The genes controlling different traits are
  inherited independently of one another.

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Mendelian Inheritance in Humans

• Mendelian principles apply to over 4,500 human
  traits.
• The human ABO blood system is an example of a
  simple Mendelian inheritance.
• The A and B alleles are dominant to the O allele.
• Neither the A or B allele are dominant to one
  another (codominant and both traits are
  expressed).

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ABO Genotypes and Associated Phenotypes
Genotype Antigens on Red Blood Cells ABO Blood Type(Phenotype)
AA, AO A A
BB, BO B B
AB A and B AB
OO None O
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Polygenic Inheritance

• Polygenic traits are continuous traits governed
  by alleles at more than one genetic locus.
• Continuous traits show gradations.
• Skin color is a common example of a polygenic
  trait it is governed by 6 loci and at least 12
  alleles.

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Mitochondrial Inheritance

• Convert energy into a form that can be used by
  the cell.
• Animals inherit their mtDNA (and mt traits) from
  their mothers.
• All the variation in mtDNA is caused by mutation,
  which makes it very useful for studying genetic
  change over time.

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Heredity and Evolution

• Evolution works at four levels
• Molecular
• Cellular
• Individual
• Population
• The levels reflect different aspects of evolution
  and are integrated in a way that produces
  evolutionary change.

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The Modern Synthesis

• Evolution is a two-stage process
• The production and redistribution of variation.
• Natural selection acting on this variation
  (affect ability to reproduce survive)
• Evolution Mutation or Natural Selection

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A Current Definition Of Evolution

• Modern Genetics evolution a change in allele
  frequency from one generation to the next.
• Allele frequencies are indicators of the genetic
  makeup of an interbreeding group of individuals
  known as a population.
• Small changes Time Natural Selection
  Evolution

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Mutation

• Mutation is a molecular alteration in genetic
  material
• For a mutation to have evolutionary significance
  it must occur in a gamete (sex cell).
• Such mutations will be carried on one of the
  individual's chromosomes.
• During meiosis the chromosome carrying the
  mutation will assort giving a 50 chance of
  passing the allele to an offspring.

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Gene Flow

• Gene flow is the exchange of genes between
  populations.
• If individuals move temporarily and mate in the
  new population (leaving a genetic contribution),
  they dont necessarily remain in the population.
• Example The offspring of U.S. soldiers and
  Vietnamese women.

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Genetic Drift

• Genetic drift is directly related to population
  size.
• Genetic drift occurs when some individuals
  contribute a disproportionate share of genes to
  succeeding generations.
• Drift may also occur solely because the
  population is small
• Alleles with low frequencies may simply not be
  passed on to offspring, so they eventually
  disappear from the population.

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Founder Effect

• Genetic drift in which allele frequencies are
  altered in small populations from larger
  populations.
• A new population will be established.
• (all members will be descended from the
  founders)
• An allele that was rare in the founders parent
  population but is carried by even one of the
  founders can eventually become common.
• Example French Canadians in Quebec

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Recombination

• In sexually reproducing species both parents
  contribute genes to offspring.
• The genetic information is reshuffled every
  generation.
• Recombination doesnt change allele frequencies,
  however, it does produce different combinations
  of genes that natural selection may be able to
  act on.

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Natural Selection

• Natural selection provides directional change in
  allele frequency relative to specific
  environmental factors.
• If the environment changes, selection pressures
  also change.
• If there are long-term environmental changes in a
  consistent direction, then allele frequencies
  should also shift gradually each generation.

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Levels of Organization in the Evolutionary Process
Evolutionary Factor Level Evolutionary Process
Mutation DNA Storage of genetic information ability to replicate influences phenotype by production of proteins
Mutation Chromosomes A vehicle for packaging and transmitting DNA
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Levels of Organization in the Evolutionary Process
Evolutionary Factor Level Evolutionary Process
Recombination (sex cells only) Cell Basic unit of life, contains chromosomes, divides for growth and production of sex cells
Natural selection Organism The unit that reproduces and which we observe for phenotypic traits
Drift, gene flow Population Changes in allele frequencies between generations
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Quick Quiz
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• 1. Mendel used the term dominant for
• plants that were larger than others of the same
  variety.
• a trait that prevented another trait from
  appearing.
• a variety of pea plants that eliminated a weaker
  variety.
• a trait that "skipped" a generation.

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Answer b

• Mendel used the term dominant for a trait that
  prevented another trait from appearing.

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• 2. Genes exist in pairs in individuals during
  the production of gametes, the pairs are
  separated so that a gamete has only one of each
  kind. This is known as the
• principle of segregation.
• principle of independent assortment.
• mitosis.
• unification theory.

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correct a

• Genes exist in pairs in individuals during the
  production of gametes, the pairs are separated so
  that a gamete has only one of each kind. This is
  known as the principle of segregation.

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• 3. Traits that have a range of phenotypic
  expressions and show a continuum of variation are
  termed
• co-dominant.
• polygenic.
• polymorphic.
• sex-linked.

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Answer b

• Traits that have a range of phenotypic
  expressions and show a continuum of variation are
  termed polygenic.

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• 4. When alleles are introduced into a population
  from another population, this is known as
• genetic drift.
• gene flow/migration.
• founder effect.
• bottleneck effect.

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Answer b

• When alleles are introduced into a population
  from another population, this is known as gene
  flow/migration.

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• 5. The most complete definition of biological
  evolution is
• change.
• mutation.
• survival of the fittest.
• a change in allele frequency from one generation
  to the next.

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Answer d

• The most complete definition of biological
  evolution is a change in allele frequency from
  one generation to the next.