Class Notes Human Genetics Chapter 14

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Class Notes Human GeneticsChapter 14

• Beyond Dominant Recessive Alleles

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Incomplete dominance

• a case in which one allele is not completely
  dominant over another. In incomplete dominance,
  the heterozygous phenotype is somewhere in
  between the 2 homozygous phenotypes.

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Codominance

• a case in which both alleles contribute to the
  phenotype of the organism. The phenotypic result
  is similar to that of incomplete dominance.

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Multiple Alleles

• many organisms have more than 2 alleles for a
  given trait i.e. more than 2 alleles exist in
  the population.

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Polygenic Traits

• these are traits that are controlled by 2 or more
  genes.

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Human Heredity

• A picture of chromosomes arranged in this way is
  known as a karyotype.
• The karyotype to the left is from a typical human
  body cell, which contains 46 chromosomes (23
  pairs).
• Two of these 46 chromosomes are known as sex
  chromosomes, because they determine an
  individuals sex.
• Females have 2 copies of a large X chromosome and
  males have one X and one small Y chromosome.
• To distinguish them from sex chromosomes, the
  remaining 44 chromosomes are known as autosomes.

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Pedigree Chart

• A pedigree chart is a chart that shows the
  relationships within a family. These are useful
  to biologists when studying how a particular
  trait is passed from one generation to the next
  within a given family.

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Pedigree Chart
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Sex Linked Genes

• There is a special pattern of inheritance for
  genes located on the X and Y (sex) chromosomes.
  Genes located on these chromosomes are known as
  sex-linked genes (x-linked) for short. Many
  (most) sex-linked genes are found on the X
  chromosome.

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Sex Linked Genes

• More than 100 sex-linked genetic disorders have
  now been mapped to the X chromosome.
• The human Y chromosome is much smaller than the X
  chromosome and appears to contain only a few
  genes.
• Males have just one X chromosome, thus ALL
  SEX-LINKED ALLELES ARE EXPRESSED IN MALES, even
  if they are recessive.
• In order for a recessive trait to be expressed in
  females, there must be 2 copies of the allele,
  one on each of the two X chromosomes.
• This means that the recessive phenotype of a
  sex-linked genetic disorder tends to be much more
  common in males than females.
• Complete a punnett square below that shows the
  passage of color blindness from parents to child
  (text page 350).

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Human Blood Types

• Human blood comes in a variety of genetically
  determined blood groups. The best known are the
  ABO blood groups and the Rh groups.
• The Rh blood group is determined by a single gene
  with 2 alleles ( and -)
• the allele is dominant and the allele is
  recessive.
• A person can be Rh positive has alleles RhRh
  or RhRh-
• A person can be Rh negative has alleles Rh-Rh-

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Human Blood Types

• With the ABO blood group, there are 3 alleles for
  the gene (IA, IB, and i)
• the alleles IA and IB are codominant both
  alleles produce antigens that can be recognized
  by the immune system on the surface of red blood
  cells (an antigen is a substance that triggers an
  immune system response)
• individuals with alleles IA and IB produce both
  antigens, making them blood type AB
• the i allele is recessive
• individuals with alleles IAIA or IAi produce only
  antigen A, making them blood type A
• individuals with alleles IBIB or IBi produce only
  antigen B, making them blood type B
• individuals who are homozygous for the i allele
  (ii) produce no antigen, and are said to have
  blood type O.

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The Human Genome Project

• The Human Genome Project is an attempt to
  sequence all human DNA. In June 2000, scientists
  announced that the DNA sequence of the human
  genome was essentially complete.
• DNA Fingerprinting is an analysis of sections of
  DNA that have little or no known function, but
  vary widely from one individual to another thus
  making it an excellent way to identify
  individuals
• Gene therapy is the method of replacing and
  absent or faulty gene by a normal, working gene.

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Disorders in Humans

•  Albinism autosomal, lack of pigment in skin,
  hair, and eyes

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Cystic Fibrosis

• Autosomal, excess mucus in lungs, digestive
  tract, liver death at childhood unless treated

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Tay-sachs disease

• Autosomal, lipid accumulation in brain cells
  mental deficiency blindness death in early
  childhood

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Osteogenesis Imperfecta
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Other Disorders

• Achondroplasiaautosomal, dwarfism (one form)
• Huntington's Diseaseautosomal, mental
  deterioration and uncontrollable movements
  appears in middle age
• Sickle Cell diseaseautosomal, sickle shaped red
  blood cells damage to many tissues
• Hemophilia--sex-linked, protein in blood
  necessary for clotting is missing uncontrolled
  bleeding from minor injuries leads to death
  unless treated
• Duchenne Muscular Dystrophy--sex-linked,
  progressive weakening and loss of skeletal
  muscle rarely live past early adulthood
• Down Syndromechromosomal, individual born with
  too many copies of a chromosome (failure to
  separate during meisos) - mild to severe mental
  retardation