Patterns of Inheritance

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Patterns of Inheritance

• Chapter 11

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Early Ideas of Heredity

• Before the 20th century, 2 concepts were the
  basis for ideas about heredity
• 1.) heredity occurs within species
• 2.) traits are transmitted directly from parent
  to offspring (The homunculus myth)
• This led to the belief that inheritance is a
  matter of blending traits from the parents.

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Early Ideas of Heredity

• Botanists in the 18th and 19th centuries produced
  hybrid plants.
• When the hybrids were crossed with each other,
  some of the offspring resembled the original
  strains, rather than the hybrid strains.
• This evidence contradicted the idea that traits
  are directly passed from parent to offspring.

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Gregor Mendel

• Born in 1822
• Education University of Vienna
• Failed exit examinations
• Returned to monastery
• Mendel published his work in 1865.
• That work was lost until ca. 1900.
• With the rediscovery of Mendels conceptual
  work the hunt was on for the physical nature of
  the gene.
• What was it and how did it function?
• These questions were largely answered from 1940s
  through the 1960s and lead to the biotech
  revolution beginning of the 1970s.

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The Garden Pea

• Pisum sativum
• Easy to grow
• Produces many varieties
• Male and female organs in the same flower
• Self-fertilization
• Cross-fertilization
• What if Mendel choose to work with sheep instead?

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Early Ideas of Heredity

• Gregor Mendel
• chose to study pea plants because
• 1. other research showed that pea hybrids could
  be produced
• 2. many pea varieties were available
• 3. peas are small plants and easy to grow
• 4. peas can self-fertilize or be cross-fertilized

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Early Ideas of Heredity

• Mendels experimental method
• 1. produce true-breeding strains for each trait
  he was studying
• 2. cross-fertilize true-breeding strains having
  alternate forms of a trait
• -perform reciprocal crosses as well
• 3. allow the hybrid offspring to self-fertilize
  and count the number of offspring showing each
  form of the trait

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Monohybrid Crosses

• Monohybrid cross a cross to study only 2
  variations of a single trait
• Mendel produced true-breeding pea strains for 7
  different traits
• each trait had 2 alternate forms (variations)
• Mendel cross-fertilized the 2 true-breeding
  strains for each trait

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Monohybrid Crosses

• F1 generation (1st filial generation) offspring
  produced by crossing 2 true-breeding strains
• For every trait Mendel studied, all F1 plants
  resembled only 1 parent
• no plants with characteristics intermediate
  between the 2 parents were produced

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Monohybrid Crosses

• F1 generation offspring resulting from a cross
  of true-breeding parents
• F2 generation offspring resulting from the
  self-fertilization of F1 plants
• dominant the form of each trait expressed in the
  F1 plants
• recessive the form of the trait not seen in the
  F1 plants

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Monohybrid Crosses

• F2 plants exhibited both forms of the trait in a
  very specific pattern
• ¾ plants with the dominant form
• ¼ plant with the recessive form
• The dominant to recessive ratio was 3 1.
• 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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Monohybrid Crosses

• gene information for a trait passed from parent
  to offspring
• alleles alternate forms of a gene
• homozygous having 2 of the same allele
• heterozygous having 2 different alleles

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Monohybrid Crosses

• genotype total set of alleles of an individual
• PP homozygous dominant
• Pp heterozygous
• pp homozygous recessive
• phenotype outward appearance of an individual

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Monohybrid Crosses

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

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Monohybrid Crosses

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

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Dihybrid Crosses

• Dihybrid cross examination of 2 separate traits
  in a single cross
• for example RR YY x rryy
• The F1 generation of a dihybrid cross (RrYy)
  shows only the dominant phenotypes for each trait.

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Dihybrid Crosses

• The F2 generation is produced by crossing members
  of the F1 generation with each other or allowing
  self-fertilization of the F1.
• for example RrYy x RrYy
• The F2 generation shows all four possible
  phenotypes in a set ratio
• 9 3 3 1

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Dihybrid Crosses

• Principle of Independent Assortment
• In a dihybrid cross, the alleles of each gene
  assort independently.

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Probability Predicting Results

• Rule of addition the 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 Predicting Results

• Rule of multiplication the probability of 2
  independent events occurring simultaneously is
  the PRODUCT of their individual probabilities.
• When crossing Rr Yy x RrYy, the probability of
  obtaining rr yy offspring is
• probability of obtaiing rr ¼
• probability of obtaining yy ¼
• probability of rr yy ¼ x ¼ 1/16

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Testcross

• Testcross a 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)
• the 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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Extensions to Mendel

• 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

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Extensions to Mendel

• Pleiotropy refers to an allele which has more
  than one effect on the phenotype.
• This can be seen in human diseases such as cystic
  fibrosis or sickle cell anemia.
• In these diseases, multiple symptoms can be
  traced back to one defective allele.

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Extensions to Mendel

• Incomplete dominance the heterozygote is
  intermediate in phenotype between the 2
  homozygotes.
• Codominance the heterozygote shows some aspect
  of the phenotypes of both homozygotes.

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Extensions to Mendel

• The human ABO blood group system demonstrates
• multiple alleles there are 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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Extensions to Mendel

• The expression of some genes can be influenced by
  the environment.
• for example coat color in Himalayan rabbits and
  Siamese cats
• an allele produces an enzyme that allows pigment
  production only at temperatures below 30oC

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Extensions to Mendel
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Extensions to Mendel

• The products of some genes interact with each
  other and influence the phenotype of the
  individual.
• Epistasis one gene can interfere with the
  expression of another gene

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4 general themes emerge from Mendels work

• Variation, as expressed in alternative forms of a
  traits (a green pea or a yellow one or a
  high-pitched voice or a low one) is widespread in
  nature
• Observable variation is essential for following
  genes (If all the traits of all offspring
  resembled their parents, Mendel would have had
  no basis for discerning and analyzing patterns of
  transmission.
• Variation is not distributed solely by chance
  rather. It is inherited according to genetic laws
  that explain why like begets both like and
  unlike. Dogs beget other dogs, pea plants beget
  pea plants, and people beget people, but there
  are 100s of breeds of dogs and even within a
  breed- Labrador retrievers- 2 black dogs could
  have a litter of black, chocolate, and yellow
  puppies
• The laws Mendel discovered about heredity apply
  equally to all sexually reproducing organisms,
  from protists to peas to people.