Patterns of Inheritance

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Patterns of Inheritance

• Chapter 9

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Genetics

• The science of heredity.
• A distinct genetic makeup results in a distinct
  set of physical and behavioral characteristics.
• The DNA you get from your parents determine your
  physical characteristics.

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History of Genetics

• Hippocrates (ancient Greek physician) proposed an
  explanation called pangenesis.
• Particles called pangenes travel from each part
  of the organisms body to the egg or sperm and
  are then passed to the next generation.
• Also thought that changes that occur during an
  organisms life are passed on in this way.
• Aristotle (384 B.C.) rejected this idea as
  simplistic, saying that what is inherited is the
  potential to produce body features rather than
  particles of the feature themselves.

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History of Genetics

• Biologists in the 19th century observed
  inheritance patterns in plants and concluded that
  offspring inherit traits from both parents.
• The favored explanation for inheritance then
  became the blending hypothesis.
• This is the idea that the hereditary materials
  contributed by the male and female parents mix in
  forming the offspring.

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History of Genetics Gregor Mendel

• Modern genetics began in the 1860s when a monk
  named Gregor Mendel experimented with breeding
  garden peas.
• With a history in mathematics, his research
  implemented a great deal of statistics.
• He stressed that the heritable factors (genes)
  retain their individuality generation after
  generation (no blending).

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History of Genetics Gregor Mendel

• He studied pea plants because they had short
  generation times, they produced large numbers of
  offspring, and they came in many varieties.
• Character flower color, height, seed shape, pod
  color, etc.
• Traits (each variant for a character)
  purple/white flower, tall/short height,
  round/wrinkled seed, green/yellow pod color.

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Characters
Traits
Mendel chose to study 7 characters, each of which
occurred in two distinct forms.
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History of Genetics Gregor Mendel

• He could strictly control mating of pea plants.
• Pea plants usually self-fertilize pollen grains
  (carrying sperm) released from the stamens land
  on the egg containing carpel of the same flower.
• He could also cross-fertilize
• fertilization of one plant by
• pollen from a different plant.

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Cross fertilization
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History of Genetics Gregor Mendel

• He worked with plants until he was sure they were
  a true-breed (one that produced offspring all
  identical to the parent if self-fertilized).
• Example parent plant had purple flowers, and if
  self fertilized it would only produce purple
  flowered plants generation after generation.

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History of Genetics Gregor Mendel

• Once he had a true-breed, he then investigated
  what would happen if he crossed true-breeding
  varieties with each other.
• Example what offspring would result from
  cross-fertilization of true-breeds?
• purple flowers x white flowers
• This offspring of two different true-breeds is
  called a hybrid.
• (the fertilization is called hybridization, or
  cross.)

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History of Genetics Gregor Mendel

• P generation the true-breeding parental plants.
• F1 generation the hybrid offspring.
• (F stands for filial, Latin for son)
• F2 generation result self-fertilization of the
  F1 plants.

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History of Genetics Gregor Mendel

• Mendel tracked and recorded the inheritance of
  characters, the results lead him to formulate
  several ideas about inheritance.
• Lets look at his monohybrid-cross (parent plants
  differ in only 1 character).

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History of Genetics Gregor Mendel

• He crossed a true-breed purple flower with a
  true-breed white flower.
• He observed that the F1 generation were all
  purple flowers.
• Self-fertilizing the F1 generation he found that
  the F2 generation had a ratio of 13. One white
  for every three purple.
• He concluded that the white trait did not
  disappear, and that they MUST carry two factors
  for the flower color character.
• He called these alleles alternative versions of
  a gene.

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Homologous chromosomes

• Alleles reside at the same locus on homologous
  chromosomes.

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History of Genetics Gregor Mendel

• Dominant allele is always expressed if present.
  (like a trump card)
• PP or Pp
• Recessive allele is only expressed if the
  dominant allele is NOT present.
• pp

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History of Genetics Gregor Mendel

• A homozygous genotype has identical alleles.
• PP or pp
• A heterozygous genotype has two different alleles
• Pp

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Law of Segregation

• A sperm or egg carries only one allele for each
  inherited character.
• because allele pairs separate from each other
  during the production of gametes.

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Genetic composition appearance

• Genotype organisms genetic makeup.
  (Represented by letters)
• Phenotype organisms expressed or physical
  traits.

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Genetic composition appearance

• Carriers organisms that are heterozygous, they
  carry the recessive allele for a trait but
  phenotypically only the dominant trait is
  expressed.

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Types of hybrid crosses

• Monohybrid cross the parents differ in only one
  character.
• Green or Yellow seeds.
• Dihybrid cross parents differ in two
  characters.
• (Round or Wrinkled) and (Green or Yellow) seeds.

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Law of Independent Assortment For Dihybrid
Crosses

• Each pair of alleles segregates independently of
  other pairs of alleles during gamete formation.
  (Each trait is inherited independently of one
  another.)
• Which means you could get your moms hair color
  and not get her eye color (or vice versa). The
  two traits are inherited INDEPENDENTLY of one
  another.

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Law of Independent Assortment
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Punnett Square

• Punnett squares are used to show the probability
  of what genotypes the offspring could have.

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Test Cross

• Used to determine the genotype of a unknown
  character.
• Used to verify if organism is in fact a
  true-breed.

B black. The dominant allele. But, is it BB
or Bb? This is unknown.
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Test Cross

• Mate organism with unknown genotype, with an
  organism that has a homozygous recessive
  genotype.
• The appearance of the offspring reveals the
  unknown genotype.