Mendelian Genetics

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Mendelian Genetics

• Chapter 2

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• Phenotype and Genotype

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Genotype and Phenotype

• Genotype genetic constitution of an organism
• Phenotype observable characteristic
• Genotype and environment
• Contribution of environment varies between genes
• Can be controlled by many genes
• Random developmental events

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• Mendels Experimental Design

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Mendelian Genetics

• Modern genetics began with Gregor Mendels
  quantitative genetic experiments
• Austrian monk
• Mathematician
• Numerical and observational data
• Several generations

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Mendelian Genetics

• Heritable, obvious traits
• Simple crosses at first
• Used peas because
• Easy to grow and available
• Many distinguishable characteristics
• Self-fertilization
• True breeding peas

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Pea Traits
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• Monohybrid Crosses and Mendels Principle of
  Segregation

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Breeding Crosses

• Initial cross is the P generation
• Parents
• Progeny of parents is first filial generation
• F1 generation
• Inbreeding of first generation creates second
  filial generation
• F2 generation

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Monohybrid Crosses

• Cross between true-breeding individuals with one
  different trait
• Mendels first crosses
• Resembled only one of the parents
• Planted progeny and allowed self-fertilization
• Revealed both phenotypes

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Monohybrid Cross

• Mendel determined that
• Particulate factors for genes, each contains a
  set of two
• Transmitted by both parents
• Alternate forms called alleles
• True breeding forms contains identical set

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Monohybrid Cross

• F1 generation had both alleles
• Only one expresses
• One allele masks
• Dominant
• Recessive
• Identical alleles homozygous
• Different alleles - heterozygous

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Monohybrid Cross
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Principle of Segregation

• Recessive characteristics are masked
• Reappear in F2
• Members of a gene pair (alleles) segregated
  during gamete formation

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How cells carry characteristics

• Genes on chromosomes
• At a specific loci
• Homologous pairs carry the same genes at the same
  locus
• Different versions
• Separation of homologous chromosomes yields
  separation of alleles

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Branch Diagrams

• Punnett squares can become messy with more than
  one gene
• Use branch diagram to figure out genotype and
  phenotype expected frequency

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Test Cross

• Mendel did several crosses
• Followed over several generations
• Selfing also very important
• Allowed plants to reveal their genotype and not
  just their phenotye

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Test Cross
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Recessive Alleles

• Wild-type allele functional allele
• Predominates in population
• Dominant allele
• Loss-of-function mutations causes protein
  product to be absent, partially functional, or
  nonfunctional
• Recessive
• Function of other in heterozygote is sufficient

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Wrinkled Peas

• SS type contains more starch and lower sucrose
• Also more water
• SBEI - starch-branching enzyme
• Extra 800 bp piece in mutation

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• Dihybrid and Trihybrid Crosses and Mendels
  Principle of Independent Assortment

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The Principle of Independent Assortment

• Factors for different traits assort independently
  of one another
• Genes are inherited independently of each other
• Segregate randomly in gametes
• Dihybrid Cross

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Branch Diagram of Dihybrid Cross
Phenotype
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Genotype vs. Phenotype
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Test Cross With Dihybrid
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Trihybrid Cross
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Tribble Traits Activity
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• Statistical Analysis of Genetic Data The
  Chi-Square Test

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Statistical Analysis

• Data from genetics is quantitative
• Use statistics to show deviation of observed
  results from predicted results
• Chance factors cause deviations
• Null-hypothesis no difference between the
  predicted and observed
• If not accepted then have to come up with a new
  hypothesis for deviation

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Chi-Square Test

• Goodness of fit test
• How much observed number deviates from the
  expected number

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• Mendelian Genetics in Humans

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Pedigree Analysis

• Inheritance patterns are studied using family
  trees
• Pedigree analysis
• Phenotypic records
• Proband is where gene was discovered

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Examples of Human Genetic Traits

• Most genetic disorders are recessive
• Due to lack of function
• Homozygous recessive expression
• Dominant usually selected out
• Albinism

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Characteristics of Recessive Inheritance Traits

• Most have normal heterozygous parents
• Heterozygotes have 31 ratio
• When both parents have the trait then all progeny
  have the trait
• Cystic Fibrosis, Sickle Cell Anemia, Tay Sachs

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Characteristics of Dominant Inheritance Traits

• Gain of function mutations
• New property of the mutant gene
• No loss of function
• Must have one parent with disease
• Does not skip generations
• Will transmit to half its progeny
• Huntingtons disease, Marfan syndrome,
  achondroplasia