Variation in human beings as a quality of life and a genetic phenomenon

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Variation in human beings as a quality of life
and a genetic phenomenon
Ass. Nedoshytko Khrystyna
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VARIATION The term variation describes the
difference in characteristics shown by organisms
belonging to the same natural population or
species. It was the amazing diversity of
structure within any species that caught the
attention of Darwin and Wallace during their
travels. The regularity with which these
differences in characteristics were inherited
formed the basis of Mendel's research.
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Why does one organism look different to another ?

• Do the members of this family have any similar
  features ?
• Are the members different in any ways?
• What reasons can you think of to explain these
  differences ?

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Inherited Differences in Humans.

• Genes control the characteristics that develop.
• 1/2 the instructions come from the father and 1/2
  come from the mother.
• The new individual is genetically unique

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So why are Identical twins not identical ?

• Twins have identical genes in their bodies.
• Yet they do not have identical characteristics.

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Why arent fruits from the same plant identical ?
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What about this wheat grown in the same field
from the same parent plants?
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• A study of differences in any large population
  shows that two forms of variation
• phenotypic variation and genetic variation.
• Phenotypic variation continuous discontinuous
• The physical and biochemical characteristics of
  an organism make up the phenotype
• The genotype determines the potential of an
  organism, environmental factors determines to
  what extent it will occur.
• Continuous variation - differences are grade into
  each other Ex. human height and weight.
• Discontinuous variation - differences are
  discrete usually qualitative. Example dwarf or
  tall.

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• There are certain characteristics within a
  population, which exhibit a limited form of
  variation. Variation in this case produces
  individuals showing clear-cut differences with no
  intermediates between them, such as blood groups
  in humans.
• Characteristics showing discontinuous variation
  are usually controlled by one or two major genes
  which may have two or more allelic forms and
  their phenotypic expression is relatively
  unaffected by environmental conditions.

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• CONTINUOUS VARIATION
• Gradual or not so clear-cut variation
• The classes are artificial and have been decided
  upon by us to make it easier to draw a graph.
• May be caused by genes or environment or both.
• Examples - weight, leaf length, height, skin
  colour.
• Many characteristics in a population show a
  complete gradation from one extreme to the other
  without any break.
• This is illustrated most clearly by
  characteristics such as mass, linear dimension,
  shape and color of organs and organisms. The
  frequency distribution for a characteristic
  exhibiting continuous variation is a normal
  distribution curve.

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• Most of the organisms in the population fall in
  the middle of the range with approximately equal
  numbers showing the two extreme forms of the
  characteristic. Characteristics exhibiting
  continuous variation are produced by the combined
  effects of many genes (polygenes) and
  environmental factors. Individually each of these
  genes has little effect on the phenotype but
  their combined effect is significant.

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• ENVIRONMENTAL INFLUENCES
• The ultimate factor determining a phenotypic
  characteristic is the genotype. At the moment of
  fertilization the genotype of the organism is
  determined, but the subsequent degree of
  expression allowed to this genetic potential is
  influenced greatly by the action of environmental
  factors during the development of the organism.

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GENETIC VARIATION Genetic variation arises in
two principal ways I. Formation of new
combinations (genotypes) by shuffling of parental
genes, and II. Modification in
chromosomes and genes (DNA) called mutation.
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• I. New Combinations genotypes arise in 3 ways
• (a) Independent assortment of
  chromosomes during gamete formation
• (b) Reciprocal recombination of linked genes in
  chromosomes by crossing over in the prophase of
  meiosis I
• and
• (c) Random fertilization.

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• II. MUTATION
• Hugo De Vries introduced the term mutation in
  1901.
• Scientific study of mutation was started by
  Thomas Hunt Morgan in 1910 with his work on
  Drosophila melanogaster.
• The first mutation he reported was a white-eyed
  male fly in a population of normal red-eyed
  males.

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• A mutation is a change in the amount or the
  structure of the DNA of an organism.
• This produces a change in the genotype, which may
  be inherited by cells derived by mitosis or
  meiosis from the mutant cell.
• A mutation may result in the change in appearance
  of a characteristic in a population.
• Mutations occurring in gamete cells are
  inherited, whereas those occurring in somatic
  cells can only be inherited by daughter cells
  produced by mitosis. We are known as somatic
  mutations.

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• There are 3 main types of mutations
• 1.Chromosomal mutations (changes in number of
  chromosomes).
• 2.Chromosomal aberrations (changes in
  structure of chromosomes).
• 3.Gene (point) mutations (changes in
  structure of the nucleotides).

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• 1. Chromosomal Mutations
• Chromosomal mutations may be the result of
  changes in the number or structure of
  chromosomes. Certain forms of chromosomal
  mutation may affect several genes and have a more
  profound effect on the phenotype than gene
  mutations.

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• Changes in the number of chromosomes are usually
  the result of errors occurring during meiosis but
  they can also occur during mitosis.
• ?) These changes may involve the loss or gain of
  single chromosomes, a condition called
  aneuploidy
• b) or the increase in entire haploid sets of
  chromosomes, a condition called polyploidy.

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Chromosome Number Problems

• Polyploidy
• Individuals have three or more of each type of
  chromosome (3n, 4n)
• Common in flowering plants
• Lethal for humans
• 99 die before birth
• Newborns die soon after birth

• Aneuploidy
• Individuals have one extra or less chromosome
• (2n 1 or 2n - 1)
• Major cause of human reproductive failure
• Most human miscarriages are aneuploids

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Nondisjunction
n 1
n 1
n - 1
chromosome alignments at metaphase I
n - 1
nondisjunction at anaphase I
alignments at metaphase II
anaphase II
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• Aneuploidy
• In this condition half the daughter cells
  produced have an extra chromosome, (2n 1) and
  so on, whilst the other half have a chromosome
  missing, (2n - 1) and so on.
• Aneuploidy can arise from the failure of a pair,
  or pairs, of homologous chromosomes to separate
  during anaphase I of meiosis.
• One of the commonest forms of chromosomal
  mutation in humans resulting from non-disjunction
  is a form of trisomy called Down's syndrome (2n
  47).

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• 47,XY (21)

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(No Transcript)
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• b) Euploidy (Polyploidy)
• Gamete and somatic cells containing multiples of
  the haploid number of chromosomes are called
  polyploids, and the prefixes tri-, tetra- and so
  on indicate the extent of polyploidy,
• for example 3n is triploid,
• 4n is tetraploid, 5n is pentaploid and so on.
• Polyploids is much more common in plants than in
  animals.

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• A modified form of polyploidy can occur in
  animals and give rise to cells and tissues, which
  are polyploid. This process is called endomitosis
  and involves chromosome replication without cell
  division.
• The giant chromosomes in the salivary glands of
  Drosophila and
• tetraploid cells in the human liver are produced
  by endomitosis.

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• 2. Chromosomal Mutations or Aberrations
• These mutations affect large portions of the
  chromosomes and are observable under a
  microscope. Crossing over during prophase I of
  meiosis involves the reciprocal transfer of
  genetic material between homologous chromosomes.

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• Morphological Modifications in chromosomes
• They are of two types
• a) intrachromosomal
• b) interchromosomal

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• Intrachromosomal modifications.
• These changes affect a single chromosome.
• They occur in two ways - deletion
• -
  inversion.
• In both cases, the process involves breakage and
  reunion of segments of chromosomes.

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- Deletion

• A segment of a chromosome separates and is lost.
  The affected chromosome loses certain genes, and
  becomes shorter than normal.

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- Inversion

• Occurs when a region of a chromosome breaks off
  and rotates through 180 before rejoining the
  chromosome.
• No change in genotype occurs as a result of
  inversion but phenotypic changes may be seen.

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b) Interchromosomal modifications.

• These changes affect two chromosomes
  simultaneously.
• They also occur in two ways
• translocation
• duplication.

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- Translocation.

• A segment of chromosome breaks off and joins a
  nonhomologous chromosome. Both the affected
  chromosomes get modified.
• The donor suffers deletion and becomes shorter
  than normal.
• The recipient has an extra set of genes and
  becomes longer than normal.
• Translocation may be reciprocal.

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• In some cases of Down's syndrome, where the
  diploid number is normal, the effects are
  produced by the translocation of an extra G21
  chromosome onto a larger chromosome, usually D15.

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46, XY, t (15q21q)
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- Duplication.

• A fragment of a chromosome joins a homologous
  chromosome.
• In some cases a region of a chromosome becomes
  duplicated so that an additional set of genes
  exists for the region of duplication.

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3. Gene mutations

• A gene mutation or point mutation is the result
  of a change in the nucleotide sequence of the DNA
  molecule in a particular region of the
  chromosome.
• Gene mutations occurring during gamete formation
  are transmitted to all the cells of the offspring
  and may be significant for the future of the
  species.
• Somatic gene mutations which arise in the
  organism are inherited only by those cells
  derived from the mutant cells by mitosis.

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Point Mutations

• Silent mutation
• UAC is changed to UAU, there is no noticeable
  effect, because both of these codons code for
  tyrosine.

• Nonsense mutation
• If UAC is changed to UAG, however, the result
  could very well be a drastic one because UAG is a
  stop codon. If this substitution occurs early in
  the gene, the resulting protein may be too short
  and may be unable to function.

Missense mutation Finally, if UAC is changed to
CAC, then histidine is incorporated into the
protein instead of tyrosine.
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• A change in one amino acid may not have an effect
  if the change occurs in a noncritical area or if
  the 2 amino acids have the same chemical
  properties. In this instance, however, the
  polarities of tyrosine and histidine differ
  therefore, this substitution most likely will
  have a deleterious effect on the functioning of
  the protein. Recall that the occurrence of valine
  instead of glutamate in the beta (B) chain of
  hemoglobin results in a sickle-cell disease.

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Hemoglobin and Sickle Cell Anemia

• Single base mutation in DNA
• A to T transversion
• Single amino acid change in the protein
• Glutamine to Valine
• Polar charged R group to non-polar R group

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Sticky Situation
Low Oxygen
Hemoglobin Polymerizes
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Sickling Cells
Polymers of hemoglobindeform red blood cells
Normal
Sickle
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Sickle Cell Anemia

• Recessive trait
• Symptoms-
• Chronic hemolytic anemia
• Severe pain
• Rapid septicemia (infection)
• Asplenia (no spleen left)

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• Implications of Mutation
• The effects of chromosome and gene mutations are
  very variable. In many cases the mutations are
  lethal and prevent development of the organism.
• Some forms of chromosomal mutation may bring
  certain gene sequences together, and that
  combined effect may produce a beneficial
  characteristic.
• Another significance of bringing certain genes
  closer together is that they are less likely to
  be separated by crossing over and this is an
  advantage with beneficial genes.

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• Origin of Mutations.
• Mutations may arise spontaneously due to certain
  intracellular factors or be induced by
  environmental factors. The latter are called
  mutagens or mutagenic agents.
• Spontaneous Mutations.
• These occur at random and their frequency is
  rather low. They are thought to arise generally
  by errors in the process of replication. Many
  cell products such as formaldehyde, peroxides act
  as mutagens.

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• Induced Mutations.
• The mutagens that induce mutation may be physical
  or chemical. The physical mutagens include
  radiation and temperature.
• Radiations. High-energy radiations such as
  X-rays, gamma rays, alpha and beta rays, cosmic
  rays, ultraviolet light, etc.. have been found to
  be mutagenic in almost all organisms. They
  produce mutations by disrupting the chemical
  structure of DNA.
• (b) Temperature. It is reported that rise in
  temperature increases the rate of mutation.
  Temperature probably affects the thermal
  stability of DNA and the rate of reaction of
  other substances with DNA.

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• (?) Chemicals. A variety of chemicals act as
  mutagens. These include nitrous acid,
  formaldehyde, peroxides, mustard gas,
  5-bromouracil, etc. Colchicine induces polyploidy
  by inhibiting the formation of spindle in cell
  division. This doubles the number of chromosomes
  as the cell fails to divide.

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• Thank you for attention!
•