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• Before the 20th century, 2 concepts were the basis for ideas about heredity: Heredity occurs within species, and traits are transmitted directly from parent to offspring
• It was believed traits were borne (carried) through fluid and blended in offspring
• Paradox - if blending occurs why don't all individuals look alike?
• Early plant biologists produced hybrids


  • Differed from both parents
  • Additional variation observed in 2nd generation offspring contradicts direct transmission

  
• Crosses of the garden pea

  1. Crossed 2 true-breeding strains

  2. 1st generation resembled only 1 parent strain

  3. 2nd generation resembled both

  
• Gregor Mendel


  • Used pea plants because: Other research showed that pea hybrids could be produced, Many pea varieties were available, Peas are small plants and easy to grow, Peas can self-fertilize or be cross-fertilized
• Mendel's experimental method


  1. Produce true-breeding strains for each trait

  2. Cross-fertilize true-breeding strains having alternate forms of a trait

  3. Allow the hybrid offspring to self-fertilize for several generations and count the number of offspring showing each form of the trait

  
• Mendel's quantification of results and mathematical analysis of experimental results led to the inheritance model that we still use today
• How Mendel conducted his cross-fertilization

  1. The male anther contains pollen grains, which give rise to haploid sperm

  2. The female carpel contains ovules, which give rise to haploid eggs

  3. Cross-fertilization yields all hybrid seeds that give rise to purple flowers

  4. Using pollen from a purple flower to fertilize a white flower gives the same result

  5. Hybrids strain self-fertilizes

  
• Monohybrid crosses


  Cross to study only 2 variations of a single trait
• F1 generation


  • Offspring produced by crossing 2 true-breeding strains
  • For every trait Mendel studied, all F1 plants resembled only 1 parent
  • Referred to this trait as dominant
  • Alternative trait was recessive
  • No plants with characteristics intermediate between the 2 parents were produced
• F2 generation


  • Offspring resulting from the self-fertilization of F1 plants
  • The recessive trait had reappeared among some F2 individuals
  • Counted proportions of traits, always found about 3:1 ratio
• The 3:1 ratio is actually 1:2:1
• The F2 generation is a disguised 1:2:1 ratio
• Mendel's five-element model


  1. Parents transmit discrete factors (genes)

  2. Each individual receives one copy of a gene from each parent

  3. Not all copies of a gene are identical (Allele - alternative form of a gene, Homozygous - 2 of the same allele, Heterozygous - different alleles)

  4. Alleles remain discrete - no blending

  5. Presence of allele does not guarantee expression (Dominant allele - expressed, Recessive allele - can be hidden by dominant allele)

  
• Principle of Segregation


  Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization

  
• Mendel had no knowledge of chromosomes or meiosis - had not yet been described
• Using a Punnett square to analyze Mendel's cross


  Each true-breeding parent makes only one type of gamete

  F1 are all purple heterozygotes, make two types of gametes and produce three kinds of F2 offspring: PP homozygous dominant (purple), Pp heterozygous (also purple), pp homozygous recessive (white)
• Some human traits exhibit dominant/recessive inheritance


  • Some human traits are controlled by a single gene

  Pedigree analysis is used to track inheritance patterns in families

  
• Dominant pedigree for hereditary juvenile glaucoma

  • Disease causes degeneration of optic nerve leading to blindness
• Recessive pedigree for albinism

  • Condition in which the pigment melanin is not produced

  Form of albinism due to a nonfunctional allele of the enzyme tyrosinase

  Males and females affected equally

  Most affected individuals have non-albino parents
• Dihybrid crosses


  Examination of 2 separate traits in a single cross

  Produced true-breeding lines for 2 traits

  F1 generation shows only the dominant phenotypes for each trait

  F2 generation shows all four possible phenotypes in a 9:3:3:1 ratio

  
• Principle of Independent Assortment


  In a dihybrid cross, the alleles of each gene assort independently

  Independent alignment of different homologous chromosome pairs during metaphase I leads to the independent segregation of the different allele pairs

  
• Probability and the rule of multiplication

  Probability of 2 independent events both occurring is the product of their individual probabilities
• Probability and the rule of addition


  Probability of either of 2 mutually exclusive events occurring is the sum of their individual probabilities
• Testcross


  Cross used to determine the unknown genotype of an individual with dominant phenotype

  Cross the individual with unknown genotype with an individual expressing the recessive phenotype

  Phenotypic ratios among offspring are different, depending on the genotype of the unknown parent

  
• Extensions to Mendel


  • Mendel's model assumes: Each trait is controlled by a single gene, Each gene has only 2 alleles, There is a clear dominant-recessive relationship between the alleles

  Most genes and phenotypic characteristics do not meet these criteria

  
• Polygenic inheritance


  Occurs when multiple genes are involved in controlling the phenotype of a trait

  The phenotype is an accumulation of contributions by multiple genes

  These traits show continuous variation and are referred to as quantitative traits

  
• Pleiotropy


  Refers to an allele which has more than one effect on the phenotype

  Pleiotropic effects are difficult to predict, because a gene that affects one trait often performs other, unknown functions

  Multiple symptoms can be traced back to one defective allele

  
• Mendel's investigation


  • Used true-breeding pea plants
  • Quantified results
  • Examined many different traits
  • Examined the segregation of traits

  
• Monohybrid cross


  Cross between two true-breeding parents that differ in one trait
• F1 generation of a monohybrid cross between purple (PP) and white (pp) flower pea plants


  All have purple flowers

  
• F2 generation of the previous monohybrid cross

  Phenotypic ratio is 3 purple:1 white
• Mendel's five-element model


  • Traits have alternative forms (alleles)
  • Parents transmit discrete traits to offspring
  • If an allele is present it will be expressed
  • Traits do not blend

  
• Genotype


  An organism's genetic makeup
• Phenotype


  An organism's observable characteristics
• Phenotypes that show continuous distribution

  • Usually result from the action of multiple genes on a single phenotype

  
• Monohybrid cross between red tall and white short Japanese four o'clocks


  1. F1 is pink and tall

  2. F2 phenotypic ratio is 3 red tall:6 pink tall:3 white tall:1 red short:2 pink short:1 white short
• Monohybrid cross between red tall and white short Japanese four o'clocks with complete linkage


  F2 phenotypic ratio is 1 red tall:2 pink tall:1 white short
• Probability of obtaining bb genotype from Bb parents

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