Sex determination

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Sex determination

• Sex determination is determined by the presence
  or absence of the Y chromosome.
• Follows the monohybrid cross pattern. 50 female
  and 50 male.

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• NOVA Online Life's Greatest Miracle Watch the
  Program Here go to 5 and 6
• Rediscovering Biology - Unit 11 Biology of Sex
  and Gender Animations and Images

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Human genetics follow Mendelian principles.

• Random mutations occur constantly. The Human
  Gene Mutation Database contains a catalog of 1163
  mutations which cause clinical manifestations-
  some 3 of the genome.
• Think about introns and exons.

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The Human Karyotype

• There are 44 autosomes and 2 sex chromosomes in
  the human genome, for a total of 46.
• Karyotypes are pictures of homologous
  chromosomes lined up together during Metaphase I
  of meiosis.
• The chromosome micrographs are then arranged by
  size and pasted onto a sheet.

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How to do simulation. Homework tonight.

• Karyotyping Activity ( you only need to do one
  patient. You pick but be sure to label which
  you did)
• Heredity and Traits (scroll to Karyotype)

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Human traits

• Heredity and Traits

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• Heredity and Traits scroll to use of karyotype
  in diagnosis

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• A common abnormality is caused by nondisjunction,
  the failure of replicated chromosomes to
  segregate during Anaphase II.
• A gamete lacking a chromosome cannot produce a
  viable embryo. Occasionally a gamete with n1
  chromosomes can produce a viable embryo

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• Result of nondisjuction is aneuploidy.
• http//www.tokyo-med.ac.jp/genet/anm/domov.gif

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• Result of nondisjuction is aneuploidy.
• http//www.tokyo-med.ac.jp/genet/anm/domov.gif

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Most common example is Trisomy 21 or Downs
Syndrom

• 1 per 750 births
• Comparable rates in all ethnic groups
• Non disjunction of chromosome during female
  meiosis. At birth eggs of female are at Prophase
  I. The eggs are as old as the mother.

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Trisomy 21/Downs Syndrome

• characteristic facial features, short stature
  heart defects
• susceptibility to respiratory disease, shorter
  lifespan
• prone to developing early Alzheimer's and
  leukemia
• often sexually underdeveloped and sterile,
  usually some degree of mental retardation.
• Down Syndrome is correlated with age of mother
  but can also be the result of nondisjunction of
  the father's chromosome 21.

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Downs Syndrome
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Symbols used in geneticss.g. 26

• Dominant is upper case
• Recessive is lower case
• Condiment one letter chosen this letter and
  subscript represent each allele.
• Look at study guide page 26
• Sex linked dominant and recessive the letter X
  is used to symbolze the X chromosome and
  subscripts alleles

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Notations for Pedigree charts
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Inheritance Patterns p 189 workbook

• Autosomal recessive disorders.
• Recessive genetic
• disorders - both parents are carriers and each
  contributes an allele to the embryo.

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• Examples include
• include
• cystic fibrosis,
• sickle cell anemia
• Tay Sachs disease.

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Pedigree Chart Autosomal Recessive
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Autosomal Dominant

• An autosomal dominant gene (non-sex determining)
  chromosome.
• The phenotype expressed even if the gene is
  heterozygous.
• The chances of an autosomal dominant disorder
  being inherited are 50 if one parent is
  heterozygous for the mutant gene and the other is
  homozygous for the normal,

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• Examples of dominant genes include the tumor
  suppressor genes BRCA1 and BRCA2.
• Mutations in these genes lead to the development
  of breast cancer, as the tumour-suppressing
  functions of the proteins encoded by the genes
  have been disabled.
• NOVA Online Cracking the Code of Life Watch
  the Program Here ( go to 14)

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• Another example is Huntington's disease,
  neurological disorder - impaired motor function.
• mutant gene -abnormal protein, containing large
  repeats of amino acid glutamine. This defective
  protein is toxic to neural tissue.

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• In these cases the pattern goes on due to the age
  of the person when the influence of the mutation
  is expressed .

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Autosomal dominant
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Nondisjuction

• Interactive Lab Exercises ( go to exploring
  meiosis)

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Sex chromosome non disjuction

• In sex chromosomes, nondisjunction results in a
  number of abnormalities. Klinefelter syndrome
  -males have an extra X chromosome. The genotype
  is XXY. May also have more than one extra
  chromosome resulting in genotypes which include
  XXYY, XXXY, and XXXXY.

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• 1 of 700 men
• the testicles of Klinefelter boys stay very small
  
• few sperms (infertility)
• delayed motor function, speech, and maturation
  development

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• Other mutations - extra Y chromosome genotype of
  XYY.
• These males were once thought to be taller than
  average males and overly aggressive based on
  prison studies. Additional studies however have
  found XYY males to be normal.

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Turners Syndrome

• Tuner syndrome females.
• monosomy X, have a genotype of only one X
  chromosome (XO).
• Trisomy X females have an additional X chromosome
  and are also referred to as metafemales (XXX).

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• 1 out of every 2,500
• heart defects to minor cosmetic issues. short
  stature and loss of ovarian functions
• no puberty
• Kidney problems and osteoporosis
• Appearance
• short neck with a webbed appearance
• low hairline at the back of the neck, and
  low-set

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Sex Linked Recessive Disorders

• These genetic diseases are diseases caused by an
  error in a single DNA gene.
• X chromosome
• called "sex-linked" rather than X-linked.

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• Recessive means that disease only occurs when a
  person has two copies of the bad gene.
• Male (XY) has only one copy of an X, more likely
  to express the disease.
• Females with XX have a heterozygous advantage.
  They may carry the gene and have a mild
  expression of the disorder, due to other X
  chromosome to compensate.

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Typical features of X-linked recessive
inheritance are

• Never passed from father to son.
• Males are much more likely -one copy of the
  mutant allele to express the phenotype.
• Affected males get the disease from their mothers
  and all of their daughters are obligate carriers.
  

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• All that an affected male can pass on to his
  daughters is his X chromosome with the mutant
  allele.
• Sons of heterozygous females have a 50 chance of
  receiving the mutant allele.
• These disorders are typically passed from an
  affected grandfather to 50 of his grandsons.

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A Royal Mishap

• The European Royal Family

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Examples

• Hemophilia, a severe type of blood-clotting
  disorder, occurs almost entirely in males. The
  most common and severe type of muscular
  dystrophy,
• Duchenne muscular dystrophy, affects males
  between the ages of 3 and 7.
• Color blindness Lets check who is color blind.
  http//www.geocities.com/Heartland/8833/coloreye.h
  tml

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Sex Linked Dominant

• The term "sex-linked dominant" means that a
  single abnormal gene on the X chromosome can
  cause the disease.
• The disease is likely to be transmitted to boys
  and girls. However, boys may not survive.
• This is a rare mode of transmission

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Example - Incontinentia Pigmenti

• Incontinentia pigmenti
• cause characteristic patterns of discolored skin
  and also involve the brain, eyes, nails, and
  hair.
• Information page Incontinentia Pigmenti
  Information Page National Institute of
  Neurological Disorders and Stroke (NINDS)

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1 week old. Melanin deposits are asymmetric
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A closer look at mutations. Pages 169- 177

• mutation adds, deletes, alters, or moves
  nucleotides.
• A phenotype that a mutation alters is mutant.
• A gene can mutate spontaneously, particularly if
  it contains regions of repetitive DNA sequences.
• Mutagens are chemicals or radiation that increase
  the mutation rate

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• Life  eLearning ( scroll to 13.4 ) look at
  animations.

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Single base mutations

• A single base, say an A, becomes replaced by
  another, are also called point mutations.

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• Single base mutation sickle cell anemia - an
  evolutionary advantage

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Missense mutations/ Hemophilia

• The new nucleotide alters the codon so as to
  produce an altered amino acid in the protein
  product.

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Nonsense mutations

• With a nonsense mutation, the new nucleotide
  changes a codon that specified an amino acid to
  one of the STOP codons (TAA, TAG, or TGA
• Therefore, translation of the messenger RNA
  transcribed from this mutant gene will stop
  prematurely. The earlier in the gene that this
  occurs, the more truncated the protein product
  and the more likely that it will be unable to
  function. / Cystic Fibrosis

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Insertions and Deletions (Indels)

• Extra base pairs may be added (insertions) or
  removed (deletions) from the DNA of a gene.
• The number can range from one to thousands.

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• Insertions / deletions frameshift .
• Changes the reading during translation

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• a number of inherited human disorders are caused
  by the insertion of many copies of the same
  triplet of nucleotides. Huntington's disease and
  the fragile X syndrome are examples of such
  trinucleotide repeat diseases.

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Translocations

• Translocations are the transfer of a piece of one
  chromosome to a nonhomologous chromosome.

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Acute Mylogenis Leukemia
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Inversion

• an entire section of DNA is reversed. example
• The fat cat ate the wee rat
• The fat tar eew eht eta tac/ Can not be
  translated