Introduction%20to%20Mendelian%20Genetics%20Monohybrid%20Crosses

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Introduction to Mendelian GeneticsMonohybrid
Crosses

• SBI3U

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Learning goals

• Understand the basic terminology of mendelian
  genetics, not limited to (phenotype, genotype,
  homozygous, heterozygous, P,F1,F2 generation(s),
  dominant, recessive, alleles, gene
• Solve monohybrid problems using Punnett square
  method

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Hippocrates (460 377 B.P.)

• every part of the body was involved in the
  production of seeds
• The seeds would fuse together to make a new person

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ARISTOTLE (384- 322 B.C.E)

• Theory of Pangenesis
• Egg and sperm consist of particles ? pangenes
  from all parts of body
• upon fertilization the pangenes developed into
  the parts of the body from which they were
  formed.

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WILLIAM HARVEY(1578 1657)

• New individual arose through the process of
  EPIGENESIS
• ? embryo grew in stages and was affected by
  factors both inside and outside of the mother

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ANTON VAN LEEUWENHOEK (1632
1723)

• Used a microscope to examine gametes
• found animalcules in semen and decided they
  were preformed embryos
• Female contribution was only the influence of the
  uterus during embryo development

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BLENDING THEORY

• Characteristics of parents blended in the
  offspring
• Blending was irreversible, thus original
  characteristics would not appear in future
  generations

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FRANCIS GALTON (1870s)

• Disproved the theory of pangenesis (still
  influenced biology 100 years ago)
• Genes formed in every organ of both male and
  female body and moved to genitals

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CHARLES DARWIN(1809 1882)

• Offspring have variations of their parents
  characteristics
• Couldnt explain why suggested current science
  had not yet discovered the puzzle pieces yet

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GREGOR MENDEL (1822 1884)

• Developed the fundamental principles of modern
  genetics
• Experimented on pea plants and proposed
• Mendels law of heredity

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1677 Anton van Leewenhoek

• First discovered living sperm (also designed
  first microscope)
• Believed he saw a complete miniature person in
  the head of a sperm homunculi

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19th Century British

• From breeding ornamental plants, scientists
  believed in a blending theory of inheritance.

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Gregor Mendel (1822-1884)?

• Austrian monk
• Considered the father of genetics (now aka
  Mendelian Genetics)?
• Why the pea plant?
• grows easily and quickly
• has seven traits that are easily identified
• it is easy to manipulate

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Gregor Mendel (1822-1884)

• Used pea plants to study genetics, heredity and
  variation.
• Looked at 7 different traits that only had two
  variations.
• From his meticulous work he came up with many
  key terms and, more importantly, two
  generalizations that later became known as
  Mendels Laws.

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7 traits Mendel looked at
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Mendels true breeding experiments

• Pea plants reproduce sexually, but usually self
  fertilize
• True breeding organisms that exhibit the same
  traits generation after generation

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Mendel's Pea Plant Experiment

• Mendel selected true bred plants for different
  traits and cross-fertilized them to see what
  would happen.
• True breeding Individuals that only contain one
  variation of a trait and therefore can only pass
  this one variation on to future generations. We
  now call these individuals homozygous, or having
  two alleles that are the same.
• For example
• Tall x dwarf
• What do you think the offspring looked like?

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• 100 of the offspring plants were tall! (Instead
  of a blending where all plants would be medium
  sized...)

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• Mendel then let the F1 generation of offspring
  self-pollinate...
• F1 generation (filial (son or daughter)
  generation) offspring from the cross
  (reproduction) of the Parent or P generation.
• this time most were tall, but some were short and
  ALWAYS 31 ratio

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Why?

• Each plant in the F1 generation carried an allele
  from the P generation a tall allele and a dwarf
  allele. They are what are called hybrids.
• Hybrids Individuals that contain more than one
  variation of a trait and therefore can pass on
  more than one variation to future generations.
  We now call these individuals heterozygous, or
  having two alleles that are different from each
  other.
• Traits are dominant or recessive. If a dominant
  trait is inherited, it will be expressed.
• What is the dominant trait for height in pea
  plants?

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Mendels Key Terms

• Phenotype the appearance of traits in an
  organism (eg. short or tall)?
• Genotype the specific genes that an organism has
  (which alleles, dominant or recessive)?
• Every individual has a phenotype and genotype for
  every trait.
• The phenotype is easy to figure out, but
  occasionally the genotype is hard to determine,
  especially for dominant traits.

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Alleles Alternate forms of a gene
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Genotype vs. Phenotype
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Which Allele is expressed?
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Can you tell the genotype of an individual by
their phenotype?
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(No Transcript)
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Mendel's Laws and Principles

• Law of Segregation when any individual produces
  gametes, the copies of a gene separate so that
  each gamete receives only one copy of a gene and
  therefore only one allele for all possible
  traits.

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Law of Segregation
Principle of Segregation Demystified
The principle of segregation is explained by the
behavior of homologous chromosomes at meiosis.
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• Law of Independent Assortment alleles of
  different genes assort independently of one
  another during gamete formation, so different
  traits are inherited independently of each other
• In plain English, this means that if you are able
  to be dominant for one trait, it will have no
  influence on whether you will be dominant for
  another trait.

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• http//www.youtube.com/watch?vmx6maAOOfxQ

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Punnett Square
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Punnett Square
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Dominant traits

• This phenomenon can be explained by the
• fact that some ________ are dominant. A
• dominant allele is an allele that __________
• its phenotypic effect when present in the
• individual.

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Recessive traits

• 9. A recessive allele is expressed only when
  _____ alleles are of the recessive form. The
  genotype of an individual showing a recessive
  phenotype must be ____________ for the recessive
  allele.

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Punnett Square
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Punnett Square
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Genotypic proportions

• 11. The genotypic proportions of the offspring
  from the cross of two heterozygous parents (Bb)
  are
• BB __/4
• Bb __/4
• bb __/4

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Phenotypic proportions

• 12. The phenotypic proportions of the offspring
  are
• Purple __/4 or __
• White __/4 or __

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Monohybrid cross problem

• Curly hair is dominant over straight hair. If a
• mother is homozygous dominant for curly
• hair and the father has straight hair, what
• type of hair will the children have?

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Test Crosses

• When geneticists want to know if an individual is
  heterozygous or homozygous
• Why? For breeding purposes
• Cross between unknown phenotype and homozygous
  recessive individual
• (Pp or PP) x pp
• Pp x pp 50 purple, 50 white
• PP x pp 100 purple