Mendelian Genetics and the Forces of Evolution

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Mendelian Genetics and the Forces of Evolution

• Heredity and Evolution

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Genetic PrinciplesDiscovered by Mendel

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

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• Mendel conducted 8 years of extensive breeding
  experiments.
• He crossed plants that exhibited different
  expressions of a trait and then crossed hybrids
  with each other.
• He used only traits that were monogenica trait
  coded for by a single gene.
• He reach the conclusion that each organism
  possess two genes from each traitone from each
  parent.
• Not only does each organism posses two of each
  gene, but genes may come in different versions.
• These are called alleles.
• Alleles variants of a gene.

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Principle of Segregation

• Genes occur in pairs because chromosomes occur in
  pairs.
• During gamete production, members of each gene
  pair separate so each gamete contains one member
  of a pair.
• During fertilization, the full number of
  chromosomes is restored and members of a gene or
  allele pairs are reunited.

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Results of Crosses When One Trait at a Time is
Considered
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Independent Assortment

• Mendel made crosses with two traits
  simultaneously, such as plant height and seed
  color.
• The results indicated that the proportion of F2
  traits did not affect each other.
• Mendel stated this relationship as the principle
  of independent assortment.
• The loci (location) coding for height and seed
  color happened to be on different chromosomes
  that assort independently of each other during
  meiosis and were therefore not linked.

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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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Punnett Square
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Results of Crosses When 2 Traits Are Considered
Simultaneously
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Dominance and Recessiveness

• Some alleles are dominate and some are recessive
• Dominate the allele of a pair that is expressed
  in the phenotype.
• Recessive the allele of a pair that is only
  expressed if homozygous.
• Homozygous having two of the same allele in a
  gene pair.
• Heterozygous having tow different alleles in a
  gene pair.
• Dominate alleles are not necessarily better or
  more common.
• They simply mean that if two alleles in the
  relationship are in a heterozygous genotype, the
  action of the dominate will be expressed and the
  action of the recessive will be hidden.

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Dominant Mendelian Traits in Humans
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Dominant Mendelian Traits in Humans
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Recessive Mendelian Traits in Humans
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Recessive Mendelian Traits in Humans
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Mendelian Inheritance in Humans

• Over 4,500 human trains are known to be inherited
  according to Mendelian principles.
• 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 They are codominant and both traits are
  expressed.

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ABO Genotypes and Associated Phenotypes
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Patterns of Inheritance

• Six different modes of Mendelian inheritance have
  been identified in humans through the use of
  pedigree analysis autosomal dominant, autosomal
  recessive, X-linked recessive, X-linked dominant,
  Y-linked, and mitochondrial.
• Autosomal dominant traits are governed by loci on
  the autosomes.
• Autosomes are all chromosomes except the sex
  chromosomes.
• All affected family members have at least one
  affected parent.
• Males and females are equally affected.

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• Autosomal recessive traits are also influenced by
  loci on autosomes.
• Pedigrees for autosomal recessive traits differ
  from those for autosomal dominant traits.
• Recessive traits may appear to skip generations
  if both parents are carriers.
• Most affected individuals have unaffected
  parents.
• The frequency of affected offspring from most
  matings is less than 50.
• As in autosomal dominant traits, males and
  females are equally affected.
• Sex-linked traits are affected by loci on either
  the X or Y chromosome.
• Most of the approximately 250 known sex-linked
  traits have loci on the X chromosome.
• Because females have two X chromosomes, they
  have an autosomal-like pattern of expression.
• Males, having only one X chromosome, are
  hemizygous, and cannot express dominance or
  recessiveness for X-linked traits.

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Pattern of Inheritance of Autosomal Dominant
Traits
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Inheritance of anAutosomal Dominant Trait
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Partial Pedigree for Albinism
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Discontinuous Distribution of a Mendelian Trait
(ABO Blood Type)
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Inherited Genetic Disorders

• Genetic disorders can be inherited as dominant or
  recessive traits.
• Dominant disorders are inherited when one copy of
  a dominant allele is present.
• Recessive disorders require the presence of two
  copies of the recessive allele.
• Recessive conditions that affect humans cystic
  fibrosis, Tay-Sachs disease, sickle cell anemia,
  and albinism.

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Polygenic Inheritance

• Polygenic traits are continuous traits governed
  by alleles at more than one genetic locus.
• Continuous traits show gradations, there is a
  series of measurable intermediate forms between
  two extremes.
• 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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Mendelian Traits Comparedwith Polygenic Traits
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Frequency of the Sickle-cell Allele Distribution
in the Old World
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Malaria Distribution in the Old World
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Genetic and Environmental Factors

• The genotype sets limits and potentials for
  development and interacts with the environment.
• Aspects of the phenotype are influenced by this
  genetic-environmental interaction.
• The environment influences many polygenic traits,
  such as height.
• Mendelian traits are less likely to be influenced
  by the environment.

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Levels of Evolution

• These levels are integrated in a way that
  eventually produces evolutionary change
• Molecular
• Cellular
• Individual
• Population

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Mutation and Evolution

• 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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The Modern Synthesis

• Evolution is defined as a two-stage process
• The production and redistribution of variation
  (inherited differences between individuals).
• Natural selection acting on this variation
  (whereby inherited differences, or variation,
  among individuals differentially affect their
  ability to reproduce successfully).

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Factors That Lead to Increases in Allele
Frequencies

• Genetic drift occurs in small populations where
  random factors cause significant changes.
• Gene flow occurs when individuals migrate and
  mate outside their original population.
• Differential reproduction occurs when individuals
  with particular alleles have more offspring than
  others, leading to changes in allele frequency
  and evolution.

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New Technologies

• Polymerase chain reaction (PCR) makes it possible
  to analyze and identify DNA as small as one
  molecule and produce multiple copies of the
  original DNA.
• Recombinant DNA techniques allow scientists to
  transfer genes from the cells of one species into
  the cells of another.
• Genetic manipulation is controversial due to
  safety and environmental concerns.

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