Polygenic Inheritance

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Polygenic Inheritance

• When a trait is influenced by many genes
• The more genes that contribute to a single trait,
  the greater the number of categories
• Continuous variation
• Dominant allele has quantitative (additive)
  effects on the phenotype
• Height, body build, IQ color of hair, skin and
  eyes
• Disorders include cleft lip, alcoholism,
  allergies, schizophrenia

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What is the color of your eyes?
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Variations
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Multiple Alleles

• Several allelic forms of the same gene, but each
  individual has only two of the possible alleles
• ABO blood group (can have AO, BO or AB only)

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Gene Interaction

• For some traits, there are genes whose actions
  are required for other genes to be expressed
• Example Human, synthesis of melanin pigment
• Epistasis- covering up the affect of the
  phenotypic expression

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Epistasis

• Epistatic genes affect the phenotypic expression
  of alleles that are at a different gene locus
• AA or Aa color
• aa albino
• It doesnt matter what genes for eye color and
  hair color are inherited

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Pleitropism

• Opposite of Polygenic inheritance
• One gene confers more than one character
• Ex. Same gene can be responsible for color of
  skin, hair and eyes (3 different characters)

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Cell Divisions

• Mitosis
• Happens only in normal somatic (body) cells
• One diploid (2n 46 chromosomes) cell divides to
  give rise to 2 daughter cells each of which are
  identical to the mother cell
• Includes 4 stages Prophase, Metaphase, Anaphase,
  Telophase and Cytokinesis (division of
  cytoplasm)

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Prophase

• Condensation of chromosomes
• Disappearance of nuclear membrane

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Metaphase

• Alignment of chromosomes along equator of the
  cell
• Formation of spindle fibers

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Anaphase

• Chromosomes begin moving to the poles (pulled by
  the spindle fibers)

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Telophase

• Alignment into two daughter cells identical to
  parents
• Process called Cytokinesis (splitting of cell)

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Meiosis

• Happens only in sex chromosomes
• Gives rise to the gametes (sperm and ovum)
• In two stages
• Meiosis I (Reduction division where in a diploid
  cell (2n) becomes haploid (n))
• Meiosis II (Equational division, where in the
  haploid cells undergo mitosis to result in two
  haploid cells

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Chromosomal Aberrations

• Chromosomal errors either in
• Structure of chromosomes
• Deletion, Inversion, Duplication and
  Translocation
• of chromosomes
• Euploidy (Polyploidy)
• Alterations in whole chromosome sets (addition of
  a whole set of chromosomes to the genome)
• Aneuploidy (Polysomics)
• Alteration in the number of chromosomes may be
  chromosome number being less than or more than 46

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Nondisjunction of sex chromosomes

• Nondisjunction is the failure of chromosomes or
  chromatid pairs to separate in Meiosis I or II
• Abnormal chromosome movement during meiosis.
  Gametes too few or too many chromosomes
• Age of mother influences
• See Study guide

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Polyploids

• Individuals have 3 or more of each chromosome
• i.e., in place of being diploid (2n), they end up
  being triploid (3n), tetraploid (4n), hexaploid
  (6n) etc.
• Lethal in humans
• This is how we get most of our cereals

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Polysomics

• Alteration in the number of autosomes
• Downs syndrome (Trisomy 21 as there are 3 copies
  of chromosome 21 in Downs kids as opposed to
  normal 2 copies)
• Cri-du-chat
• Fragile X syndrome
• Alterations of the number of sex chromosomes
• XYY Jacobs syndrome
• XXY Klinefelters syndrome
• XXX Poly-X syndrome
• XO Turners syndrome, (no Y)

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Monosomics (deficiency of chromosome)

• XO Turner Syndrome
• Female resulting from non-disjunction during egg
  or sperm formation
• Sterile, failure to enlarged breasts hips,
  shortness, no menstruation
• YO not viable

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Trisomics (excess of chromosomes)

• XXY Klinefelters Syndrome
• sterile, small testes, enlarged breasts
• How many ever X in the genome, still if there
  is a Y, the zygote develops into a male
• XXXY, XXYY, XXXXY are more mentally challenged
  than the XXY

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Trisomics (excess of chromosomes)

• XXX Metafemale
• limited fertility but otherwise normal
• Some mental retardation possible
• XYY Jacobs syndrome
• male, may be taller than usual

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Alterations of Chromosome Structure

• Inversion
• A segment of the chromosome is turned 180 degrees
• Changes linkage group
• Extremely damaging that most embryos die
• ABCD becomes DCBA

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Alterations of Chromosome Structure

• Translocation
• A broken piece of a chromosome attaches to itself
  to another
• Can change gene expression
• It is seen in some forms of cancer, when a
  segment of chromosome 8 is translocated to 14
• Can lead to Downs Syndrome

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Alterations of Chromosome Structure

• Deletion
• A segment (portion) of the chromosome is missing
• Caused by viruses, chemicals or irradiation
• Loss of a portion of chromosome 5 causes
  Cri-du-chat
• Rounded moonlike face, cat like cry, mental
  physical retardation

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Alterations of Chromosome Structure

• Duplication
• A segment of the chromosome is repeated
• Fragile X syndrome, results in a form of mental
  retardation
• Myotonic dystrophy and Huntingtons Disease

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• Mechanism
• Most mutations usually involve recessive alleles
• Phenylketonuria
• Tay-Sachs Disease
• Dominant lethal allele
• Huntington Disease
• Always expressed, though at midlife
• Always lethal

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Genes and Behavior

• Mechanism
• Product from gene-specific proteins
• Proteins have specific functions leading to
  phenotypes
• Protein functions
• hormones, enzymes, structural, neurotransmitters

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Modifying DNA

• Recombinant DNA technology
• cutting, splicing and copying DNA, Polymerase
  chain reaction (PCR)
• Genetic engineering
• Microorganisms factories for human proteins,
  vaccines, environmental applications
• New plants for agriculture
• New uses for domestic animals
• Human gene therapy