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Non-disjunction disorders - myclass.peelschools.org

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					Defects in Meiosis
  ALTERATIONS OF CHROMOSOME
     NUMBER AND STRUCTURE

• To study human chromosomes
  microscopically, researchers stain and
  display them as a karyotype


•A karyotype is a photographic inventory of an
individual’s chromosomes
  –A karyotype usually shows 22 pairs of
  autosomes and one pair of sex chromosomes
• Preparation of a karyotype
                                                                 Fixative
                           Packed red       Hypotonic solution
           Blood           And white
           culture         blood cells                                  Stain
                                                         White
              Centrifuge                                 Blood
                                                         cells


                                  2                       3

       1       Fluid




                                         Centromere


                                         Sister
                                         chromatids

                                          Pair of homologous
                                          chromosomes



   4                                             5
 Alterations of Chromosome Number

• Nondisjunction
                      Nondisjunction
  results in abnormal in meiosis I
  chromosome
  numbers: a failure in
  chromosome
  separation          Normal
                          meiosis II

  –Either
  homologous
  pairs fail to           Gametes
  separate
  during                               n+1     n+1       n–1         n–1
  meiosis I                                  Number of chromosomes
– Or sister chromatids fail to separate during
  meiosis II

             Normal
             meiosis I




       Nondisjunction
       in meiosis II




       Gametes

                         n+1     n–1        n          n
                               Number of chromosomes
• Fertilization after nondisjunction in the
  mother results in a zygote with an incorrect
  number of chromosomes



      Egg
      cell


                n+1




      Sperm                          Zygote
      cell                           2n + 1


              n (normal)
 Down syndrome is caused by an
   extra copy of chromosome 21

• This
karyotype
shows three
number 21
chromosomes
• The chance of having a Down syndrome child
  goes up with maternal age
• Down Syndrome occurs in about 1/700 live births.
It is possible to have trisomy of
chromosomes 13, 15, 18, 21 & 22.
This happens infrequently.



Unfortunately (?) is it usually fatal if
the embryo has trisomy of any of the
other autosomal chromosomes
= spontaneous abortion.
      Abnormal numbers of sex
            chromosomes

• Nondisjunction can also produce gametes
  with extra or missing sex chromosomes

– Unusual numbers of sex chromosomes does not
upset the genetic balance as much an unusual number
of autosomes
–Usually offspring are viable (meaning - can survive)
• A man with Klinefelter syndrome has an
  extra X chromosome, genotype XXY
        Poor beard
        growth


        Breast
        development




        Under-
        developed
        testes
• A woman with Turner syndrome lacks an X
  chromosome, genotype X0, or X.
             Characteristic
             facial
             features

             Web of
             skin
             Constriction
             of aorta

             Poor
             breast
             development




             Under-
             developed
             ovaries
Common phenotypes of
women with Turner
syndrome
              Mutation
• An altering of a gene that is accompanied
  by a loss or change in the functioning of
  the genetic information.
• Most mutations are recessive - not
  expressed in a heterozygote.
• Any factor that can cause a mutation is a
  mutagen
• Mutations in somatic cells will usually go
  unnoticed unless they effect many cells
  - not passed down!

• More serious mutations are found in
  gametes of parent organism.

• Congenital defects are mutations that
  are visible at birth - can be caused by
  genes, mutagens (teratogens)
         Sickle Cell Anemia
• Autosomal recessive blood disorder

• An abnormal form of hemoglobin causes red
  blood cells to have a “sickle” shape. Cells can
  clog up blood vessels.

• Caused by just one amino acid that is different
  from what it is supposed to be!

• Treatment - transfusions, monitoring, bone
  marrow transplants.
          Cystic Fibrosis
• Autosomal recessive

• Effects lungs - excessive mucus
  production

• Breathing is very difficult
            Homework!
• Page 209 #1 - 3, 5

				
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