MENDELIAN INHERITANCE

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MENDELIAN INHERITANCE

• Chapter 2

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INTRODUCTION

• The concept of heredity is ancient
• Dates back to at least 400 b.c.a.
• Our understanding of genetics is rather recent
• Gregor Mendels work began only 150 years ago
• Many inaccurate views of heredity were held prior
  to Mendels time

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INTRODUCTION

• Hippocrates
• ca. 400 b.c.a.
• Greek physician
• First to attempt to explain transmission of
  hereditary traits
• Pangenesis
• Seeds are produced by all parts of the body
• Collected and transmitted at time of conception
• These seeds caused certain traits of offspring
  to resemble their parents

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INTRODUCTION

• Spermists
• Microscopes invented in the late 1600s
• Sperm viewed through microscopes
• Some imagined a tiny creature inside each sperm
• Homunculus
• Hypothesized to be a miniature human waiting to
  develop within the womb of its mother
• Only the father was responsible for creating
  future generations
• Any resemblance to mother was due to influences
  within the womb

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INTRODUCTION

• Ovists
• Views in opposition to those of the spermists
• Considered the egg to be solely responsible for
  human characteristics
• Role of sperm was simply to stimulate the egg on
  its path of development

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INTRODUCTION

• Joseph Kolreuter
• First to conduct systematic studies of genetic
  crosses
• 1761 1766
• Crossed different strains of tobacco plants
• Found that offspring were generally intermediate
  in appearance between the two parents
• Both parents make equal genetic contributions to
  the offspring
• Consistent with the blending theory of
  inheritance

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INTRODUCTION

• Blending theory of inheritance
• Factors that dictate genetic traits can blend
  together from generation to generation
• Blended traits are passed to the next generation

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INTRODUCTION

• Popular views before the 1960s
• Combined notions of pangenesis and the blending
  theory of inheritance
• Hereditary traits were rather malleable
• Could change and blend over one or two
  generations
• Mendels work was crucial in refuting these
  archaic views of heredity

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GREGOR JOHANN MENDEL

• 1822 1884
• Father of genetics
• Augustinian monk
• Austria, now Czech Republic
• Training in
• Agriculture
• Scientific method
• Mathematics
• Statistical analysis
• Studied inheritance in garden peas
• Pisum sativum

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GARDEN PEAS

• Several properties made Pisum sativum a superb
  model organism for genetic analysis
• Available in many varieties
• Easy to maintain
• Control over mating
• Short generation time
• Numerous offspring
• No major ethical issues
• Findings applicable to other organisms

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FLOWER STRUCTURE

• Most flowers are simultaneously both male and
  female
• Carpels are female structures (?)
• Produce eggs
• Stamen are male structures (?)
• Produce sperm

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FLOWER STRUCTURE

• Pollination involves the transmission of
  sperm-containing pollen to a (female) stigma
• Fertilization follows successful pollination
• Self-pollination and cross-pollination are both
  possible

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FLOWER STRUCTURE

• Structure of a pea flower
• Produces both pollen and egg cells
• Reproductive structures enclosed by a modified
  petal
• Keel
• Self-pollination is the rule, not the exception

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FLOWER STRUCTURE

• Pollination
• Pollen grain lands on stigma
• Pollen grain sends out long pollen tube
• Sperm travel toward ovules (and eggs)
• One sperm fertilizes an egg to form zygote
• One sperm fuses with two polar nuclei to form the
  nutrient-rich endosperm
• Storage material for developing embryo

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MENDELS EXPERIMENTS

• Mendel obtained several varieties of peas
• Differences between varieties were confined to
  one (or more) of seven different traits
• e.g., Purple or white flowers
• e.g., Yellow or green seeds

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MENDELS EXPERIMENTS
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MENDELS EXPERIMENTS

• Mendel began his experiments with true-breeding
  lines
• Traits did not vary in appearance between
  generations
• e.g., White-flowered lines that produced only
  white-flowered offspring for many generations

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MENDELS EXPERIMENTS

• Mendels first crosses involved a single trait
• Two variants existed for each trait
• e.g., Purple and white are two forms of the
  flower color trait
• Purple and white are two phenotypes for flower
  color
• Single-factor cross

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MENDELS EXPERIMENTS

• True-breeding purple x true-breeding white
• Parental generation
• Stamen (?) removed from female
• Pollen transfer
• Cross-fertilization
• Seeds are offspring
• F1 generation
• Single-trait hybrids
• Monohybrids

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MENDELS EXPERIMENTS

• True-breeding purple x true-breeding white
• Parental generation
• Stamen (?) removed from female
• Pollen transfer
• Cross-fertilization
• Seeds are offspring
• F1 generation
• Single-trait hybrids
• Monohybrids

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MENDELS EXPERIMENTS

• True-breeding purple x true-breeding white
• F1 monohybrids are produced
• F1 monohybrids all possess purple flowers
• White flowers are absent

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MENDELS EXPERIMENTS

• F1 monohybrids are allowed to self-pollinate
• Monohybrid cross
• How did Mendel facilitate this pollination?
• Explain why this is sexual reproduction
• F2 generation is produced
• Both phenotypes are present in the F2 generation

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MENDELS EXPERIMENTS

• Mendel obtained similar results for each of the
  seven traits he studied
• One phenotype disappeared in the F1 generation
• This recessive phenotype reappeared in
  approximately ¼ of the F2 individuals
• 31 phenotypic ratio

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MENDELS CONCLUSIONS

• Mendels results argued strongly against a
  blending mechanism of heredity
• F1 individuals have the characteristics of one
  parent, not intermediate characteristics
• Units of heredity are discrete units
• Now called genes
• Mendels Law of Segregation explained these
  results
• Described the particulate nature of inheritance

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LAW OF SEGREGATION

• Alternative versions of genes account for
  variations in inherited characteristics
• These alternative versions of genes are termed
  alleles
• The flower color gene exists in two forms
• Purple allele
• White allele

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LAW OF SEGREGATION

• For each character, an organism inherits two
  alleles, one from each parent
• Diploid organisms possess two copies of each
  chromosome
• Genes reside upon chromosomes
• A genes position on a chromosome is called its
  locus
• Diploid organisms possess two copies of each gene
• Paired chromosomes ? paired alleles
• Each parent donates one copy of each chromosome
• Each parent donates one copy of each gene

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LAW OF SEGREGATION

• For each character, an organism inherits two
  alleles, one from each parent
• These alleles may be either identical or
  non-identical

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LAW OF SEGREGATION

• If these two alleles differ
• The allele that is visibly apparent is termed
  dominant
• The allele that is masked is termed recessive
• P purple allele (dominant)
• p white allele (recessive)
• PP ? purple flowers
• Pp ? purple flowers
• pp ? white flowers

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LAW OF SEGREGATION

• The two alleles for each character segregate
  during gamete production
• Gametes are formed by meiosis
• Meiosis separates homologous chromosomes
• Meiosis separates pairs of alleles
• Each gamete receives only one allele of each gene

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MENDELS EXPERIMENTS

• Parental generation
• True-breeding
• Possess identical alleles
• Homozygous for the relevant gene
• e.g., Genotype AA or aa
• F1 generation
• Hybrids
• Possess non-identical alleles
• Heterozygous for the relevant gene
• e.g., Aa

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MENDELS EXPERIMENTS

• F2 generation
• Both phenotypes present
• What genotypes are present?

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PUNNETT SQUARES

• A Punnett square can be used to determine the
  genotypes of potential offspring from a given
  mating
• Genotypes of female gametes listed on one axis
• Genotypes of male gametes listed across other
  axis
• Offspring inside boxes
• Products of fertilizations

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MENDELS EXPERIMENTS

• The F2 generation in a monohybrid cross displays
  specific ratios
• 31 phenotypic ratio
• 121 genotypic ratio

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MENDELS EXPERIMENTS
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MENDELS EXPERIMENTS

• Individuals with the dominant phenotype may be
  either homozygotes or heterozygotes
• 1/3 of the purple-flowered plants are expected
  to be homozygous dominant
• 2/3 of the purple-flowered plants are expected
  to be heterozygous

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TEST CROSS

• A test cross can be used to determine the
  genotype of an individual with the dominant
  phenotype
• This individual is crossed to a recessive
  homozygote
• Phenotypes of offspring will uncover the
  genotype of the parent

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MULTIPLE CHARACTERISTICS

• Mendel also analyzed crosses involving two
  different traits
• Simultaneously investigated the pattern of
  inheritance for two different traits
• e.g., Seed color and seed texture
• Two-factor crosses
• Dihybrid crosses

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MULTIPLE CHARACTERISTICS

• One of Mendels two factor crosses
• Seed color
• Yellow seeds are dominant to green seeds
• Seed texture
• Smooth seeds are dominant to wrinkled seeds

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DIHYBRID CROSS

• YYRR (yellow, smooth) x yyrr (green, wrinkled)
• The F1 generation was all YyRr (yellow, smooth)
• Dihybrids
• What would the F2 generation look like?

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DIHYBRID CROSS

• Will the segregation of one allele pair influence
  the segregation of a second allele pair?
• Perhaps the genes for these two traits are
  physically linked and inherited as a single unit
• Perhaps the genes for these two traits can assort
  themselves independently during meiosis

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DIHYBRID CROSS

• YYRR x yyrr ? YyRr F1 generation
• YyRr x YyRr ? more complex F2 generation
• The F2 generation consisted of individuals with
  four different phenotypes
• Yellow, round
• Yellow, smooth
• Green, round
• Green, smooth

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DIHYBRID CROSS

• What is the expected frequency of each of the
  following phenotypes in this dihybrid cross?
• Y_R_ (yellow, round)
• Y_rr (yellow, wrinkled)
• yyR_ (green, round)
• yyrr (green, wrinkled)

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DIHYBRID CROSS

• The ratio Mendel observed was 31510810132
• Similar results were obtained for different
  combinations of traits
• How close is this ratio to the expected ratio?
• (First, reduce the ratio to ???1 by dividing
  all numbers by the smallest)
• 9.83.43.21 is obtained
• Within experimental error of expected 9331

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DIHYBRID CROSS

• The ratio Mendel observed was 31510810132
• Similar results were obtained for different
  combinations of traits
• How close is this ratio to the expected ratio?
• (Reduce the ratio to ???1 by dividing all
  numbers by the smallest)
• 9.83.43.21 is obtained
• Within experimental error of expected 9331

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DIHYBRID CROSS

• Mendel obtained similar results with other pairs
  of traits
• e.g., TtYy x TtYy dihybrid cross
• 9331 ratio in F2 generation

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DIHYBRID CROSS

• For a moment, ignore one characteristic and focus
  only on the other
• What is the yellowgreen ratio?
• What is the smoothwrinkled ratio?

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DIHYBRID CROSS

• Viewing only one characteristic while ignoring
  the other generates a ratio we have seen before
• The more complex ratio (9331) seen in a
  dihybrid cross results from the superimposition
  of two simpler ratios (31)

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INDEPENDENT ASSORTMENT

• Mendels Law of Independent Assortment
• Two pairs of alleles segregate independent of
  each other during gamete formation
• The segregation of alleles for seed color has no
  effect on the segregation of alleles for seed
  shape
• etc.

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PUNNETT SQUARES

• A Punnett square can be used to predict the
  outcomes of various crosses
• One allele pair 2 x 2 4 boxes
• Two allele pairs 4 x 4 16 boxes
• Three allele pairs 8 x 8 64 boxes
• etc.
• Larger Punnett squares become unmanageable
• Do you really want to draw 64 boxes? You do??
  Are you insane???

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NO PUNNETT SQUARES

• Predicted outcomes can be determined
  mathematically without drawing unnecessarily
  large Punnett squares
• The segregation of one allele pair is independent
  of other allele pairs
• e.g., YyRr x YyRr
• ¾ of the offspring are expected to be yellow
• ¾ of the offspring are expected to be smooth
• (¾)(¾) 9/16 are expected to be yellow and
  smooth
• What fraction are expected to be yellow and
  wrinkled?

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LOSS-OF-FUNCTION ALLELES

• Most genes encode proteins
• Mendel studied seven protein-encoding genes
• The recessive alleles of these genes were
  defective
• Rendered inactive by mutation
• Did not encode a functional protein
• Loss-of-function alleles
• Provide critical clues concerning the function of
  the encoded protein

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LOSS-OF-FUNCTION ALLELES

• Flower color gene in Mendels peas
• Encodes an enzyme required for the synthesis of
  purple pigment
• White allele does not encode functional enzyme
• Homozygous recessive individuals are white
  because they cannot make purple pigment

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PEDIGREE ANALYSIS

• Peas are very convenient model organisms for
  genetic analysis
• Humans are much less convenient
• Could you please reproduce with that person over
  there so I can see what your litters of offspring
  look like?
• Are there any ethical issues with this?
• etc.

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PEDIGREE ANALYSIS

• Inheritance of traits in humans can be followed
  using pedigree analysis
• Often less definitive than Mendels crosses
• Commonly used to determine the inheritance
  patterns of human genetic diseases
• e.g., Dominant or recessive
• Frequently able to provide important clues
  concerning inheritance patterns of various traits
• e.g., Cancer

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PEDIGREE ANALYSIS

• Reading human pedigrees
• Shape denotes gender
• Circle female
• Square male
• Lines denote relationships
• Parents connected by horizontal line
• Vertical line from parents denotes offspring
• Oldest on left, youngest on right

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PEDIGREE ANALYSIS

• Reading human pedigrees
• Shading denotes condition
• Filled in disorder
• Half filled known heterozygote
• Heterozygotes cannot always be identified

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PROBABILITY

• Laws of inheritance can b used to predict the
  outcome of genetic crosses
• Useful in many ways
• Agriculture
• Inherited diseases

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PROBABILITY

• Mendels laws of segregation and independent
  assortment reflect the rules of probability
• So do the results of flipping coins, rolling
  dice, etc.
• What is the chance of flipping two coins and
  getting heads on both?
• one of each?

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PROBABILITY

• A probability calculations allows us to predict
  the likelihood that an event will occur in the
  future
• Probability number of times an event occurs
  total number of events
• Deviation between expected and observed is due to
  random sampling error
• Difference between predicted and expected
  percentages can be large for small sample sizes
• Random sampling error should be much smaller for
  large samples

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SUM RULE

• Sum rule
• The probability that one of two or more mutually
  exclusive events will occur is equal to the sum
  of the individual probabilities of the events

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SUM RULE

• In the cross AaBb x AaBb, what is the probability
  that an offspring will have either the dominant
  phenotype for both traits or the recessive
  phenotype for both traits?
• Calculate the individual probabilities of each
  phenotype
• Dominant for both 9/16
• Recessive for both 1/16
• Add together the individual probabilities
• 9/16 1/16 10/16 5/8

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PRODUCT RULE

• Product rule
• The probability that two or more independent
  events will occur is equal to the product of
  their individual probabilities

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PRODUCT RULE

• In the cross Aa x Aa, what is the probability
  that none of the first three offspring will be
  albino?
• Calculate the individual probability of this
  phenotype
• Accomplished using a Punnett square
• Probability of unaffected 3/4
• Multiply the individual probabilities
• ¾ ¾ ¾ 27/64

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PRODUCT RULE

• Product rule
• Can also be used to predict the outcome of a
  cross involving two or more genes

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PRODUCT RULE

• In the cross AaBbCC x AabbCc, what is the
  probability that the first offspring will have
  the dominant phenotype for all three traits?
• Calculate the individual probabilities of this
  phenotype
• Accomplished using a Punnett square
• PA ¾
• PB ½
• PC 1
• Multiply the individual probabilities
• ¾ ½ 1 3/8