Fundamentals of Genetics

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Fundamentals of Genetics

• Genetics the scientific study of heredity

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Some early ideas

• Trait- any characteristic that can be passed from
  parent to child
• Dogs and corn were domesticated long ago. One
  trait was chosen over another.
• Aristotle thought that pangenes in blood carried
  a memory of each body structure the blending
  hypothesis proven untrue

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Gregor Mendel

• The Father of Genetics he recognized that
  traits are inherited as discrete units from the
  parents and that offspring inherit two copies of
  those units that we now call genes
• Experimented with pea plants
• Well suited to genetic study
• grown easily in large numbers
• their reproduction can be manipulated
• He chose traits based on either/or
• They have both male and female reproductive
  organs perfect flower
• For more on this topic read
• the Basic Principles of Genetics
• DNA from the Beginning

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Seven traits of Peas

• flower color is purple or white
• seed color is yellow or green
• flower position is axil or terminal
• pod shape is inflated or constricted
• stem length is long or short
• pod color is yellow or green
• seed shape is round or wrinkled

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Mendel's work

• Matings are called crosses
• P parent generation
• F1 filial 1 generation results from cross of
  parents
• F2 Filial 2 generation results when F1
  generation cross breeds.
• True breeding pure bred after self
  fertilization, offspring have the same traits as
  parents

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Mendels work ..

• Hybrid cross between two parents. Produces a
  hybrid with genetic information from both parents
• Dominant trait expressed trait in a hybrid
• Recessive trait unexpressed trait in a hybrid
• Mendel hypothesized each trait was controlled by
  a factor. We know these factors today to be
  genes- A genetic factor that controls a trait.
  Each parent gives one of these factors to the
  offspring. These are alleles - a different
  version of a gene for each trait

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Mendel's Laws and Modern Genetics

• Law of segregation- each pair of alleles
  segregates during meiosis.
• Organisms inherit two copies of each gene, one
  from each parent
• Organisms donate only one copy of each gene in
  their gametes. The two copes of each gene
  segregate during gamete formation
• Law of Dominance when two different alleles are
  present one can dominate over the other. The
  expressed gene is the dominant allele. The gene
  that is not expressed is the recessive allele.
• Law of independent assortment gene (homologous)
  pairs segregate into gametes randomly and
  independently of each other. Different traits
  often appear to be inherited separately.

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Use of letters to show traits

• Genotype the actual genetic make up of the
  organism.
• B brown hair dominant trait is shown with
  upper case
• b blond hair recessive trait is shown with
  lower case
• Phenotype the form of the trait that is
  observed.
• Homozygous an organism in which two alleles for
  a trait are identical
• BB brown hair
• bb blond hair
• Heterozygous an organism in which two alleles
  for a trait are different
• Bb Brown hair

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Probability and Genetics

• Probability is the likelihood that an event will
  happen.
• Probability is used to predict the results of a
  genetic cross the phenotypes and genotypes in
  breeding experiments
• Probability number of ways an event can occur
  divided by the number of total outcomes

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Use of Punnett Square

• Punnett Square a grid for organizing genetic
  information. Predicts expected outcome of a
  genetic cross
• need the alleles of parents
• put alleles of one parent along top of grid, the
  other along the side
• combine alleles by multiplying
• determine genotypes and phenotypes

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Types of crosses

• Monohybrid crosses - The study of only one trait.
• Dihybrid cross - the study of two traits at a
  time.
• Test cross used to distinguish heterozygous
  dominant individual from the recessive. A cross
  between an unknown and a homozygous recessive
  individual.

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Monohybrid cross
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Construct a Punnett Square for one trait

• .

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The principle of segregation

• Pictures from Introduction To Genetics.

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The Dihybrid Cross
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The laws of probability govern Mendelian
inheritance

• Mendels laws of segregation and independent
  assortment reflect the rules of probability
• When tossing a coin, the outcome of one toss has
  no impact on the outcome of the next toss
• In the same way, the alleles of one gene
  segregate into gametes independently of another
  genes alleles

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The Multiplication and Addition Rules Applied to
Monohybrid Crosses

• The multiplication rule states that the
  probability that two or more independent events
  will occur together is the product of their
  individual probabilities
• Probability in an F1 monohybrid cross can be
  determined using the multiplication rule
• Segregation in a heterozygous plant is like
  flipping a coin Each gamete has a 1/2 chance of
  carrying the dominant allele and a 1/2 chance of
  carrying the recessive allele

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• The rule of addition states that the probability
  that any one of two or more exclusive events will
  occur is calculated by adding together their
  individual probabilities
• The rule of addition can be used to figure out
  the probability that an F2 plant from a
  monohybrid cross will be heterozygous rather than
  homozygous

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LE 14-9
Rr
Rr
Segregation of alleles into eggs
Segregation of alleles into sperm
Sperm
R
r
1
1
2
2
R
R
r
R
R
1
2
1
1
4
4
Eggs
r
r
R
r
r
1
2
1
1
4
4
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Crossing over

• Crossing over is the exchange of chromosome
  segments during prophase I of meiosis.
• This process can happen in many places along a
  length of non sister chromatids of the homologous
  pair.
• Recombination or sexual recombination, is any
  mixing of parent alleles including crossing over
  or independent assortment

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Gene linkage

• Sometimes genes located close together on the
  same chromosome are inherited together and are
  said to be linked.
• The inheritance of these genes do not obey
  Mendels laws
• The farther genes are apart from each other on
  the same chromosome the more likely it is for
  them to be exchanged with a non sister chromatid
  during a crossing over event.
• The inheritance of genes far apart on the same
  chromosome approach expected probability values.