Heredity and Evolution

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Heredity and Evolution

• Prepared by
• Gowri Baskar

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Heredity

• Also called inheritance or biological
  inheritance.
• It is the passing on of traits from parents to
  their offspring either through asexual
  reproduction or sexual reproduction
• The offspring cells or organisms acquire the
  genetic information of their parents.
• Through heredity, variations between individuals
  can accumulate and cause species to evolve by
  natural selection.
• The study of heredity in biology is genetics.

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Gene

• Gene is the unit of inheritance/heredity.
• Gene is the part of a chromosome which controls
  the appearance of a set of hereditary
  characteristics.

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Evolution

• Evolution is the sequence of gradual changes
  which take place in the primitive organisms over
  millions of years and new species are produced.
  Since, the evolution is of the living organisms,
  so it is called Organic Evolution.
• Darwins theory of Evolution Charles Robert
  Darwin gave the theory of evolution in his famous
  book, The Origin of Species. The theory of
  evolution proposed by Darwin is known as The
  Theory of Natural Selection. It is also called
  Darwinism.

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Darwins theory of evolution

• There is natural variation within any population
  and some individuals have more favourable
  variations than others.
• Population remains fairly constant even though
  all species produce a large number of off
  springs.
• This is due to competition or struggle for
  existence between same and different species.
• The struggle for survival within population
  eliminates the unfit individuals and those with
  favourable variations survive and pass on these
  variations to their progeny to continue. This is
  called natural selection.
• The favourable variations are accumulated over a
  long time period leading to the origin of a new
  species.

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Genotype and Phenotype

• In humans, eye color is an example of an
  inherited characteristic an individual might
  inherit the "brown-eye trait" from one of the
  parents.
• Inherited traits are controlled by genes and the
  complete set of genes within an organism's genome
  is called its genotype.
• The complete set of observable traits of the
  structure and behavior of an organism is called
  its phenotype.
• These traits arise from the interaction of its
  genotype with the environment. As a result, many
  aspects of an organism's phenotype are not
  inherited.

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• Sexual reproduction
• The mode of reproduction that involves two
  individuals one male and one female.
• They produce sex cells or gametes which fuse to
  form a new organism.

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Father of Genetics
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Moravian monk Gregor Mendel

• The idea of particulate inheritance of genes can
  be attributed to the Moravian monk Gregor Mendel
  who published his work on pea plants in 1865.
• However, his work was not widely known and was
  rediscovered in 1901.
• It was initially assumed that Mendelian
  inheritance only accounted for large
  (qualitative) differences, such as those seen by
  Mendel in his pea plants and the idea of
  additive effect of (quantitative) genes was not
  realised until R.A. Fisher's (1918) paper, "The
  Correlation Between Relatives on the Supposition
  of Mendelian Inheritance" Mendel's overall
  contribution gave scientists a useful overview
  that traits were inheritable.
• His pea plant demonstration became the foundation
  of the study of Mendelian Traits. These traits
  can be traced on a single locus.

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Dominant traits

• The traits that express themselves in an organism
  in every possible combination and can be seen are
  called Dominant traits.
• In Mendels experiment, we see that tall trait in
  pea plants tends to express more than the short
  trait.
• Therefore, the tall trait of the plant is said to
  be dominant over the short trait.

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Recessive traits

• A trait which is not expressed in presence of a
  dominant allele is known as recessive.
• So, recessive character/trait is present in an
  organism but cannot be seen if a dominant allele
  exists.

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Monohybrid cross

• When only one character is considered while
  crossing two organisms, then such a cross is
  known as monohybrid cross.
• The ratio of characters, arising out of this
  cross, at F2 generation is called monohybrid
  ratio.
• E.g., If tall plant (TT) is crossed with a dwarf
  plant (tt), we get 3 tall1 short plant at the
  end of the F2 generation.
• So, 31 is monohybrid ratio.
• Here, the height of the plant is considered at a
  time.

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Dihybrid cross

• When two characters are considered while crossing
  two organisms, then such a cross is known as a
  dihybrid cross.
• The ratio of characters, arising out of this
  cross, at F2 generation is called dihybrid ratio.
• E.g., If a plant with round and green pea is
  crossed with a plant with wrinkled and yellow
  pea,
• The first generation plants would all have round
  and green pea.
• On crossing the same for an F2 generation, we
  would observe four combinations of characters in
  the ratio of 9331.
• Thus, 9331 is the dihybrid ratio.

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Sex determination in humans

• Mechanism of Sex Determination in Human
  BeingsIn human beings, the sex of the
  individual is genetically determined.
• Sex determination is the process by which sex of
  a new born individual can be determined.
•  Human beings have 1 unpaired sex chromosome. Sex
  chromosome of male is XY and of female is XX.
•  Sex of a child depends on what happens at
  fertilisation.
• Father is responsible for the determination of
  the sex of a child.

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Sex determination in humans