Mendel

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Mendels Laws of Heredity

• - Monohybrid Crosses
• - Phenotypes and Genotypes
• - Dominant vs. Recessive Traits

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Gregor Mendel

• Gregor Mendel is the Austrian monk turned
  scientist who did pioneering work in the study of
  how physical characteristics are inherited.
• He is often referred to as the Father of
  Genetics
• He used garden pea plants for his experiments
  because they reproduce sexually so they have
  gametes.

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More on Peas..

• In peas, both male and female gametes are in the
  same flower. The male is in the pollen grain,
  and the female is in the ovule.
• Fertilization occurs when the male gamete in the
  pollen grain meets and fuses with the female
  gamete in the ovule. This transfer is called
  pollination and a seed results.

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Mendel, a Careful Researcher!

• Mendel controlled his experiments and peas he
  used by only studying one trait at a time to
  control variables, and analyzed data
  mathematically.
• If he was looking at tallness he only chose
  plants that were tall for many generations and
  always produced tall offspring. These are said
  to be true breeding or pure for tallness.

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Monohybrid Crosses

• Mendel took these tall and short pea plants and
  crossed them (cross pollination) to produce new
  plants.
• This is a
• Monohybrid cross because they only differ by one
  trait - height.
• So what did he find.?????

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Offspring

• He found that all of the offspring in the first
  generation were as tall as the taller parent. It
  was as if the shorter parent never existed!
• Then he crossed two of the offspring (look tall)
  and 3/4 of their offspring were as tall as the
  tall plants in the parent generation, and 1/4
  were as short as the ones in the parent
  generation.

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Some Terms to Know

• The original parents are known as the P1
  generation. (P stands for parent)
• The offspring of the parent are known as the F1
  generation. (F stands for filial or son or
  daughter)
• When two F1 offspring are crossed their offspring
  are called the F2 generation or the second filial
  generation.

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Multiple Alleles

• Mendel did this for seven different pairs of
  traits. In every cross he found that one trait
  seemed to disappear in the F1 generation only to
  reappear unchanged in one-fourth of the F2
  plants.
• Each of these traits has two factors which are
  genes located on chromosomes. Genes exist in
  alternative forms called alleles.

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Dominance

• Remember how in the F1 generation offspring were
  all tall? That means only one trait was
  observed. Mendel called this the dominant trait
  and the one that disapeared he called recessive.
• So in this experiment the allele for short plants
  is recessive and tall is dominant.

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The Law of Segregation

• To explain why the shortness reappeared Mendel
  concluded that the two alleles for each trait
  must separate when gametes are formed.
• So a parent passes on at random only one allele
  for each trait to each offspring. This is called
  the Law of Segregation

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Genotype and Phenotype

• Two organisms can look alike but have different
  gene combinations. The way an organism looks is
  its phenotype (tall, blonde, etc.)
• The gene combination an organism contains is its
  genotype. The genotype of a tall plant that has
  two alleles for tallness is TT.

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Homozygous and Heterozygous

• An organism is homozygous for a trait if its two
  alleles for the trait are the same like TT
  (homozygous dominant) or tt (homozygous
  recessive) PURE
• An organism is heterozygous for a trait if its
  two alleles for the trait are different. So Tt is
  heterozygous for the height trait. HYBRID

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Law of Independent Assortment

• Mendel performed dihybrid crosses in plants that
  were true-breeding for two traits. For example, a
  plant that had green pod color and yellow seed
  color was cross-pollinated with a plant that had
  yellow pod color and green seeds. In this cross,
  the traits for green pod color (GG) and yellow
  seed color (YY) are dominant. Yellow pod color
  (gg) and green seed color (yy) are recessive.
• What did he find out?

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Independent Assortment Contd
The resulting offspring or F1 generation were all
heterozygous for green pod color and yellow seeds
(GgYy).
After observing the results of the dihybrid
cross, Mendel allowed all of the F1 plants to
self-pollinate.
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Independent Assortment
He referred to these offspring as the F2
generation. Mendel noticed a 9331 ratio. About
9 of the F2 plants had green pods and yellow
seeds, 3 had green pods and green seeds, 3 had
yellow pods and yellow seeds and 1 had a yellow
pod and green seeds.
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Independent Assortment

• From these experiments Mendel formulated what is
  now known as Mendel's law of independent
  assortment.
• This law states that allele pairs separate
  independently during the formation of gametes.
• Therefore, traits are transmitted to offspring
  independently of one another.