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					Genetics:
Mendel & Beyond

Genetics: the foundations
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most early studies were done with plants & animals of economic importance
– evidence has been found that crossbreeding of palm trees and horses began as early as 5,000 years ago

Genetics: the foundations
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notion of blending:
– Josef Gottlieb Kolreuter (late 18th century) did experiments of reciprocal crossing in plants; found that if a plant had one form of a characteristic (i.e. red flowers) was crossed with a plant with another form (i.e. blue flowers), the offspring would show a blending of the two forms (i.e. purple flowers)
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once forms of a characteristic were blended, they could no longer be seen in their separate forms in subsequent offspring

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Mendel came along a century later

Genetics: the foundations
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advantage of using of plants in genetic studies:
– easily grown in large quantities – produce large numbers of offspring (from seeds) – have short generation times – breeding be easily controlled and manipulated

Gregor Mendel: father of genetics
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was an Austrian monk, but intensively studied physics, chemistry, mathematics, and biology at the University of Vienna failed the test for teacher certification Biology was his weakest subject he used knowledge of physics & mathematics to get quantitative results and apply experimental methods to genetics

Gregor Mendel: father of genetics
researched basic inheritance patterns in plants for about 9 years  1865: public lecture of findings  1866: detailed, written publication
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– printed in 120 library journals & sent 40 copies to scholars
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theories were not accepted (for approx. 30 years)

Why was Mendel ignored?


most biologists of his time did not think in mathematical terms, including Charles Darwin
– Darwin did almost the exact same experiments as Mendel, using snapdragon plants; believed in concept of blending

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Mendel had little credibility as a biologist

Mendel’s theories accepted
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in 1900: meiosis & chromosomes had been observed and described
– Mendel’s work was cited as a source & became prominently accepted – Mendel’s theories provided an explanation of how chromosomes & meiosis work, and vice versa

Mendel’s methods
  

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used garden peas controlled pollination, thus fertilization chose characteristics to observe that had well-defined, contrasting alternates (purple vs. white flowers) began with plants that were true-breeding
– observed character was present for many generations – used self-pollination & inbreeding to achieve this

Mendel’s methods


Mendel’s terms:
– character: observed feature, such as flower color – trait: particular form of a character, such as white flowers – heritable character: trait that is passed to offspring

Mendel’s methods
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cross-pollinated true-breeding plants
– collected pollen from one strain and placed it on the stigma of another

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concentrated on seven pairs of contrasting traits

Mendel’s methods
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P generation (parental): true-breeding plants that provided and received pollen F1 generation (filial): seeds and resulting new plants from the cross of the P generation F2 generation (filial): seeds and resulting plants from the self-pollination of the F1 plants

Mendel’s methods


Characteristics of all F1 & F2 plants were counted and recorded
Quantitative Data described relative proportions of the different kinds of progeny

Mendel’s 1st Experiment
monohybrid cross: one trait  P generation: true-breeding spherical seeds x true-breeding wrinkled seeds
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F1 : all had spherical seeds

Mendel’s 1st Experiment
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Mendel allowed F1 plants to selfpollinate spherical seeds x spherical seeds
F2: 7, 324 spherical seeds 1, 850 wrinkled seeds

Mendel’s conclusions from 1st experiment
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disproved belief of blending
– see reappearance of both traits in F2 generation

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concept of dominance and recessiveness
– spherical (S) & wrinkled (s)

spherical seed characteristic was dominant over the wrinkled seed characteristic

Mendel’s conclusions from 1st experiment
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particulate theory: “units” responsible for inheritance of specific traits are present as discrete particles that occur in pairs and segregate during the formation of gametes
– also negates blending concept

Mendel’s conclusions from 1st experiment
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through math, Mendel concluded that each pea plant had two “units of inheritance;” one from each parent
– only one unit is give to gametes – when gametes fuse, result has two sets of “units”

Mendel’s conclusions from 1st experiment
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law of segregation: when any individual produces gametes, the two “units of inheritance” separate so that each gamete one receives one

Mendel’s 2nd set of experiments
Do “units of inheritance” that come from mother (or father) stick together when gametes are made?  Mendel looked at two characteristics simultaneously (seed shape & seed color)
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Mendel’s 2nd set of experiments
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P generation:
spherical, yellow seeds (SSYY) x wrinkled, green seeds (ssyy)

Mendel’s 2nd set of experiments
F1 generation: all spherical, yellow seeds  F2 generation: expressed 4 combinations of traits
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– spherical, yellow/ spherical, green/ wrinkled, yellow / wrinkled, green

Mendel’s 2nd law
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law of independent assortment: segregation of mother’s (or father’s) units of inheritance is independent when forming gametes

Support for Mendel’s 2nd law
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if units were “linked” all F2 offspring would be either spherical, yellow seeds or wrinkled, green (like the P1 generation) in a 3:1 ratio if units segregated independently, gametes would have 4 different combinations (SY, Sy, sY, and sy), and would, thus, produce 4 combinations in the F2 generation


				
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