Mystery of Heredity

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Mystery of Heredity

• Before the 20th century, 2 concepts were the
  basis for ideas about heredity
• heredity occurs within species
• traits are transmitted directly from parent to
  offspring
• Thought traits were borne through fluid and
  blended in offspring
• Paradox if blending occurs why dont all
  individuals look alike?

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Early Work

• Josef Kolreuter 1760 crossed tobacco strains
  to produce hybrids that differed from both
  parents
• additional variation observed in 2nd generation
  offspring contradicts direct transmission
• T.A. Knight 1823 crossed 2 varieties of
  garden pea, Pisum sativa
• crossed 2 true-breeding strains
• 1st generation resembled only 1 parent strain
• 2nd generation resembled both

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Gregor Mendel

• Chose to study 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

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Mendels Experimental Method

• Usually 3 stages
• Produce true-breeding strains for each trait he
  was studying
• Cross-fertilize true-breeding strains having
  alternate forms of a trait
• also perform reciprocal crosses
• Allow the hybrid offspring to self-fertilize for
  several generations and count the number of
  offspring showing each form of the trait

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Monohybrid Crosses

• Cross to study only 2 variations of a single
  trait
• Mendel produced true-breeding pea strains for 7
  different traits
• each trait had 2 variants

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F1 Generation

• First filial 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

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F2 Generation

• Second filial generation
• Offspring resulting from the self-fertilization
  of F1 plants
• Although hidden in the F1 generation, the
  recessive trait had reappeared among some F2
  individuals
• Counted proportions of traits
• always found about 31 ratio

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31 is actually 121

• F2 plants
• ¾ plants with the dominant form
• ¼ plants with the recessive form
• the dominant to recessive ratio was 31
• Mendel discovered the ratio is actually
• 1 true-breeding dominant plant
• 2 not-true-breeding dominant plants
• 1 true-breeding recessive plant

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Conclusions

• His plants did not show intermediate traits
• each trait is intact, discrete
• For each pair, one trait was dominant, the other
  recessive
• Pairs of alternative traits examined were
  segregated among the progeny of a particular
  cross
• Alternative traits were expressed in the F2
  generation in the ratio of ¾ dominant to ¼
  recessive

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Five-Element Model

• Parents transmit discrete factors (genes)
• Each individual receives one copy of a gene from
  each parent
• Not all copies of a gene are identical
• Allele alternative form of a gene
• Homozygous 2 of the same allele
• Heterozygous different alleles
• Alleles remain discrete no blending
• Presence of allele does not guarantee expression
• Dominant allele expressed
• Recessive allele hidden by dominant allele
• Genotype total set of alleles an individual
  contains
• Phenotype physical appearance

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Principle of Segregation

• Two alleles for a gene segregate during gamete
  formation and are rejoined at random, one from
  each parent, during fertilization
• Physical basis for allele segregation is the
  behavior of chromosomes during meiosis
• Mendel had no knowledge of chromosomes or meiosis
  had not yet been described

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Punnett Square

• Cross purple-flowered plant with white-flowered
  plant
• P is dominant allele purple flowers
• p is recessive allele white flowers
• True-breeding white-flowered plant is pp
• homozygous recessive
• True-breeding purple-flowered plant is PP
• homozygous dominant
• Pp is heterozygote purple-flowered plant

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Human Traits

• Some human traits are controlled by a single gene
• some of these exhibit dominant and recessive
  inheritance
• Pedigree analysis is used to track inheritance
  patterns in families

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• Dominant pedigree
• dominant trait appears in every generation

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• Recessive pedigree
• most affected individuals have unaffected parents

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Dihybrid crosses

• Examination of 2 separate traits in a single
  cross
• Produced true-breeding lines for 2 traits
• RRYY x rryy
• The F1 generation of a dihybrid cross (RrYy)
  shows only the dominant phenotypes for each trait
• Allow F1 to self-fertilize to produce F2

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• F1 self-fertilizes
• RrYy x RrYy
• The F2 generation shows all four possible
  phenotypes in a set ratio
• 9331
• R_Y_R_yyrrY_rryy
• round yellowround greenwrinkled yellowwrinkled
  green

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Principle of Independent Assortment

• In a dihybrid cross, the alleles of each gene
  assort independently
• The segregation of different allele pairs is
  independent
• Independent alignment of different homologous
  chromosome pairs during metaphase I leads to the
  independent segregation of the different allele
  pairs

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Probability

• Rule of addition
• probability of 2 mutually exclusive events
  occurring simultaneously is the sum of their
  individual probabilities
• When crossing Pp x Pp, the probability of
  producing Pp offspring is
• probability of obtaining Pp (1/4), PLUS
  probability of obtaining pP (1/4)
• ¼ ¼ ½

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Probability

• Rule of multiplication
• probability of 2 independent events occurring
  simultaneously is the product of their individual
  probabilities
• When crossing Pp x Pp, the probability of
  obtaining pp offspring is
• probability of obtaining p from father ½
• probability of obtaining p from mother ½
• probability of pp ½ x ½ ¼

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Testcross

• Cross used to determine the genotype of an
  individual with dominant phenotype
• Cross the individual with unknown genotype (e.g.
  P_) with a homozygous recessive (pp)
• Phenotypic ratios among offspring are different,
  depending on the genotype of the unknown parent

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Extensions to Mendel

• Mendels model of inheritance assumes that
• 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 do not meet these criteria

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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
• for example human height
• histogram shows normal distribution

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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
• This can be seen in human diseases such as cystic
  fibrosis or sickle cell anemia
• multiple symptoms can be traced back to one
  defective allele

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Multiple Alleles

• May be more than 2 alleles for a gene in a
  population
• ABO blood types in humans
• 3 alleles
• Each individual can only have 2 alleles
• Number of alleles possible for any gene is
  constrained, but usually more than two alleles
  exist for any gene in an outbreeding population

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• Incomplete dominance
• heterozygote is intermediate in phenotype between
  the 2 homozygotes
• red flowers x white flowers pink flowers
• Codominance
• heterozygote shows some aspect of the phenotypes
  of both homozygotes
• Type AB blood

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Human ABO Blood Group

• The system demonstrates both
• multiple alleles
• 3 alleles of the I gene (IA, IB, and i)
• Codominance
• IA and IB are dominant to i but codominant to
  each other

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Environmental Influence

• Coat color in Himalayan rabbits and Siamese cats
• allele produces an enzyme that allows pigment
  production only at temperatures below 33oC