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CHROMOSOMES

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					CHROMOSOMES
What Is a Chromosome?
  A long, continuous strand of DNA, plus several
    types of associated proteins, and RNA.
In X-Y system, male
  determines sex of
  offspring.
Inheritance of Genes on Sex Chromosomes

 • X-Linked traits are carried on the X
 chromosome and most are recessive
     Examples: color blindness, hemophilia

 • More common in females; affected males
 are hemizygous, cannot be carriers

 • Male inherits X-linked condition from
 carrier or affected mother

 • Y-linked traits are very rare because Y has
 few genes
Hemophilia: recessive X-linked trait
Genotype          Phenotype
XHXH              non-carrier female
XHXh              carrier female
XhXh              female with hemophilia
XHY               normal male
XhY               male with hemophilia
What is the probability that a carrier female
 and a normal male will have a son with
 hemophilia?




                        ¼ or 25%




What is the probability that a non-carrier
 female and a hemophiliac male will have a
 son with hemophilia?
X Inactivation
  Female mammals have 2 alleles for every gene on
    the X chromosome, while males have only 1.


  This inequality is balanced by “turning off” one X
    chromosome in each cell of a 3 week old female
    embryo.
  – some cells turn off paternal X
  – some cells turn off maternal X
                         Inactivated X
                         appears as a dark-
                         staining structure
                         called a Barr body.




How many Barr bodies would cells of a
male possess?
X inactivation is responsible for the appearance
  of calico cats.




The earlier X inactivation occurs, the larger the
  patches.
Chromosome Abnormalities
1. Polyploidy - extra full sets of
   chromosomes.

  –   animal polyploids spontaneously abort or die
      shortly after birth
  –   plant polyploids are relatively common (wheat,
      lilies)
Aneuploidy - an extra (trisomy) or missing
 (monosomy) chromosome.
 Aneuploidy is usually due to a meiotic
 error called nondisjunction.
• Autosomal aneuploids
  – trisomy 13
  – trisomy 18
  – trisomy 21 (Down
      syndrome)


• Sex chromosome aneuploids
  –   Turner syndrome        XO female
  –   Triplo-X               XXX female
  –   Klinefelter syndrome   XXY male
  –   Jacobs syndrome        XYY male
 Deletion - part of a
   chromosome is
   missing.

Duplication - part of a
  chromosome is
  present twice.


 Inversion - part of a
   chromosome is
   reversed.
Translocation - nonhomologous
 chromosomes exchange parts
 (reciprocal translocation) or combine
 (Robertsonian translocation).

				
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posted:3/23/2012
language:English
pages:15