FUNDAMENTALS OF GENETICS

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FUNDAMENTALS OF GENETICS

• CHAPTER 8

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GENETICS

• Genetics is a field of Biology that is devoted to
  understanding HOW characteristics are passed on
  from parents to offspring.
• Gregor Johann Mendel the father of genetics

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GREGOR MENDAL

• 1843 Gregor Mendal joined the monastery at the
  age of 21. His task was to tend to the garden,
  which allowed him to observe and think about the
  growth of many generations of plants.
• 1851 He entered the U. of Vienna to study math
  and science, where he studied statistics
• Statistics helped him in the field of Heredity
  the transmission (passing on) of characteristics
  from parents to offspring.

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GARDEN PEAS

• Mendel is remembered most for his work with Pisum
  sativum, Garden Peas.
• Seven Characteristics of garden peas were
  observed.
• For each characteristic, two contrasting traits
  were observed
• A trait is a genetically determined variant of a
  characteristic.

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GARDEN PEAS

• The Seven Characteristics
• Plant Height (traits Long Short)
• Flower Position along stem (traits axial
  terminal)
• Pod Color (traits green yellow)
• Pod appearance (traits inflated and constricted)
• Seed Texture (traits round and wrinkled)
• Seed Color (traits yellow green)
• Flower Color (traits purple white)

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GARDEN PEAS

• Mendel collected the seeds of his pea plants and
  recorded each plants traits and seeds
• He planted the seeds
• FLOWER COLOR He observed that purple flowering
  plants came from most of the seeds that he had
  collected from purple plants
• But there where some white flowering plants that
  came from purple plant seeds.

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GARDEN PEAS

• PLANT HEIGHT
• Mendel observed that while tall plants grew from
  most of the seeds that were obtained from tall
  plants
• But Mendel also observed some short plants grow
  from seeds that were also obtained from tall
  plants.
• Mendel wanted to find an explanation for such
  variation.

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MENDELS METHODS

• Mendel observed how traits were passed from one
  generation to the next by controlling how the
  plants were pollenated.
• Pollination when the pollen grains from the
  male reproductive part of the flower (anthers) is
  transferred to the female reproduction part of
  the flower (stigma).
• Self pollination occurs when pollen is
  transferred from anthers to stigma of the same
  plant
• Cross-pollination occurs between two different
  plants

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MENDELS METHODS

• To Control his Results
• Mendel removed the anthers of a plant and cross
  pollinated the plants by manually transferring
  pollen from the flower of a second plant to the
  stigma of the antherless plant.
• This way he prevented self pollination and
  controlled the specific traits of the parents.

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MENDELS EXPERIMENTS

• Mendels Experiments he first studied each
  characteristic and its contrasting traits
  individually.
• He began growing plants that were true-breeding,
  or pure for each trait. True-breeding plants
  always produce offspring with that trait when
  they self pollinate.
• Mendel produced true-breeding plants, by self
  pollination, for each of the characteristics and
  for each trait. So in the end he had 14 true
  breeding plants.

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MENDELS EXPERIMENTS
He began to cross-pollinate pairs of plants w/
opposite traits of a characteristic. He called
the first true-breeding parents the P generation.
He cross-pollinated a plant true-breeding for
yellow pods with another plant true-breeding for
green pods
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MENDELS EXPERIMENTS

• Mendel recorded the number of each type of
  offspring that resulted from the
  cross-pollination of the P generation. He called
  the offspring of the P generation the F1
  generation F for Filial which has its roots in
  latin for son or daughter.
• He then allowed the flowers of the F1 generation
  to self- pollinate and then collected the seeds.
• He called the offspring of the F1 generation the
  F2 generation.
• Mendel performed hundreds of crossings and
  documented the results of each generation.

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Mendels Results Conclusions

• The resulting F1 generation from the cross of the
  green pod plant and the yellow pod plant resulted
  in only green pod plants
• Mendel saw that no yellow pod plants developed
  even though one parent had been true-breeding for
  yellow pods.
• Mendel then allowed the F1 generation to self
  pollinate Three Fourths (75) of the F2
  generation produced green pods and One-Fourth
  (25) produced yellow pods
• He concluded and hypothesized that something
  inside the pea plants controlled the
  characteristics and traits that were observed.
  He reasoned that a pair of control factors must
  control each trait.

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Mendels Results Conclusions

• Recessive and Dominant Traits Whenever Mendel
  crossed traits one of the P traits failed to
  appear in the F1 plants. An every case, the
  trait reappeared in the F2 generation at a ratio
  of 31. This pattern lead Mendel to hypothesize
  that one factor in the pair may prevent the other
  from having an effect.
• Dominant Trait masks or dominates the appearing
  characteristic
• Recessive Traits is only expressed when paired
  with another plant or animal displaying that
  recessive trait

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Results and Conclusions

• The Law of Segregation
• Mendel concluded that each reproductive cell or
  gamete, receives one factor from each parent.
  When the gametes combine during fertilization,
  the offspring have two factors for each
  characteristic. The Law of Segregation states
  that a pair of factors is segregated, or
  separated, during the formation of gametes.

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Results and Conclusions

• The Law of Independent Assortment
• Mendel also crossed plants that differed in two
  characteristics, such as flower color and seed
  color. The results from these crosses showed
  that the traits produced by dominate factors do
  not necessarily appear together. A white
  flowering plant can produce green pods. Mendel
  hypothesized that the factors for individual
  characteristics are not connected.

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RESULTS CONCLUSIONS

• REMEMBER that in meiosis, the random separation
  of homologous chromosomes is called independent
  assortment.
• The Law of Independent Assortment states that
  characteristics separate independently of one
  another during the formation of gametes.

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SUPPORT FOR CONCLUSIONS

• Support for Mendels Conclusions Most of what
  Mendel found is supported by what biologists now
  know about molecular genetics. Molecular
  genetics is the study of the structure and
  function of chromosomes and genes.
• A gene is the segment of DNA on a chromosome that
  controls a certain hereditary trait.
• Remember that chromosomes occur in pairs so genes
  also occur in pairs. An allele is the name given
  to one of the alternate forms of a gene that
  governs a characteristic.

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Section 2 Genetic Crosses

• Differentiate between the genotype and phenotype
  of an organism
• Explain how probability is used to predict the
  results of genetic crosses
• Use a Punnett square to predict results of
  monohybrid and dihybrid genetic crosses
• Explain how a testcross is used to show the
  genotype of an individual whose phenotype
  expresses the dominant trait
• Differentiate a monohybrid cross from a dihybrid
  cross

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Genotype and Phenotype

• Genotype and Phenotype
• Genotype is the organisms genetic make up. It
  is the alleles that the organism inherits from
  the parents. When you think of Genotype Think
  GENES
• Example The genotype for a white-flowering plant
  would be pp. The genotype of a purple flowering
  plant would be Pp or PP.

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Genotype Phenotype

• Phenotype is the organisms appearance. When you
  hear phenotype think PHYSICAL.
• Example The phenotype of PP or Pp would be
  purple-flowers. The phenotype of pp would be
  white flowers.

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GENOTYPE PHENOTYPE

• Genotype doesnt necessarily mean that phenotype
  and visa versa. A plant with the genotype to be
  tall can be short because of environmental
  factors.
• When both alleles of a pair are alike, the
  organism is said to be homozygous. This means
  that they can be homozygous dominant (PP) or
  homozygous recessive (pp).
• When the alleles are different, they are said to
  be heterozygous. An example of heterozygous is
  Pp.

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Probability

• Probability is the likelihood that a specific
  event will occur. (the chances that something
  will happen). Probability may be expressed as a
  decimal number, a percentage, or a fraction.
• Probability is determined by the following
  equation
• Probability the of times an event is
  expected to happen
• The of times an
  event could happen

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Predicting Results..

• A monohybrid cross is a cross in which only one
  characteristic is tracked.
• Monohybrids are the offspring of the monohybrid
  cross.
• A Punnett square is an aid in the prediction of
  the probable distribution of inherited traits in
  the offspring.
• Homozygous x Homozygous
• We are going to cross a Pea Plant that is
  homozygous for Purple flowers (the alleles are
  PP) and a pea plant that is homozygous for White
  flowers (the alleles are pp). The alleles that
  are carried by each parents gamete are
  represented by the letters on the outside of the
  boxes

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Predicting Results

• The combinations within the four boxes represent
  the possible genotypes that can result from the
  cross of the homozygous pea plants. The outcome
  was Pp in every case, so the probability of the
  offspring having Pp is 100. The probability of
  the flower being purple is 100 as well

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PREDICTING RESULTS..

• We are going to cross a Vegasaurous Rex that is
  homozygous dominant (allele is CC) for crazy
  curly hair and another Vegasaurous Rex (allele is
  Cc) that is heterozygous for crazy curly hair.

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Predicting Results

• The two possible genotypes from this cross are CC
  and Cc. The probability of an offspring having
  the genotype CC is 2/4 or 50.
• You could expect about 2/4 or 50 of the
  offspring to have Cc. The probable phenotype of
  this cross is crazy curly hair. Thus, there is a
  100 probability that the offspring will have
  crazy curly hair.

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Predicting Results

• If it were the other way around cc x Cc , then
  the probability of an offspring having the
  genotype cc is 2/4 or 50. You can expect the
  other half, 50 or 2/4 to have Cc. So the
  phenotype results would change the genotype Cc
  would exhibit crazy curly hair and the cc would
  not.

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Predicting results

• Heterozygous x Heterozygous
• In rabbits the allele for brown coat color (B) is
  dominant over white coat color (b). Now, we are
  going to cross to rabbits that are heterozygous
  Bb. They are both brown rabbits.

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Predicting results

• As you can see, ¼ or 25 of the offspring
  predicted will have the genotype BB
• Another 25 or ¼ of the offspring predicted will
  have the genotype bb and the rest, 50 or ½ will
  have the genotype Bb.
• The phenotype results in 75 or ¾ being brown
  rabbits while the rest, 25 or ¼ will have white
  fur.

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Predicting Ratios

• Ratio
• Genotype Ratio the ratio of the genotype that
  appear in the offspring
• Example 1 BB 2Bb 1bb
• Phenotype Ratio the ratio of the phenotype that
  appear in the offspring
• Example 3 brown 1 white

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TESTCROSS

• Is performed when the genotype of an individual
  is unknown.
• In a test cross the individual with the unknown
  genotype is crossed with a homozygous recessive
  individual.
• Example Brown Bunnies. Is it BB or Bb? Cross it
  with Homozygous Recessive Bunny
• If all the offspring appear Brown then the bunny
  is Homozygous Dominant (BB)
• If half the bunnies are white, then the bunny is
  Heterozygous Dominant (Bb)

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INCOMPLETE DOMINANCE

• When the dominant allele completely masks
  recessive allele is called complete dominance.
• But there are some traits, where the recessive
  allele comes through a little bit. (Incomplete
  dominance)
• For Example When crossing a certain type of
  Flower Red Flowers (RR) and White Flowers (rr)
  The F1 generation are all (Rr) but the phenotype
  is Pink.
• In humans, skin color, eye color and hair shades
  are an example of incomplete dominance

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Codominance

• Occurs when BOTH alleles for a gene are expressed
  in a heterozygous offspring
• In codominance NIETHER allele is dominant or
  recessive, and they dont blend as in incomplete
  dominance.
• Example is in Blood Types

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Predicting Results of Dihybrid Crosses

• A dihybrid cross is a cross in which two
  characteristics are tracked.
• Dihybrids are the offspring of these crosses
• Naturally, with the addition of a trait, more
  combinations are possible.

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Homozygous x Homozygous

• By using a Punnett square, we are going to
  predict the possible offspring of a cross between
  two Pea plants with are Homozygous for
• Round, Yellow Seeds (RRYY)
• Wrinkled, Green Seeds (rryy)

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Homozygous x Homozygous
Predicting Results

• Assort the alleles
• RY, RY, RY, RY
• ry, ry, ry, ry
• Each box gets filled by the letters above it and
  to the left
• In this case each box has RrYy, so ach plant
  would have round yellow seeds.

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Heterozygous x Heterozygous
Predicting Results

• So, now lets take two pea plants from the F1
  generation that is heterozygous yellow round
  (RrYy)
• This cross would result in NINE different
  genotypes
• This cross would result in FOUR different
  phenotypes

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Heterozygous x Heterozygous

• 9/16 would have the phenotype of round, yellow
  seeds (genotypes RRYY, RRYy, RrYY, RrYy)
• 3/16 would have the phenotype of round, green
  seeds (genotypes RRyy Rryy)
• 3/16 would have the phenotype of wrinkled, yellow
  seeds (genotypes rrYY, rrYy)
• 1/16 would have the phenotype of wrinkled, green
  seeds (genotype rryy)