The Human Genome

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The Human Genome

• Chapter 14

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Human Heredity

• 14-1

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NOVA Cracking the Code of Life

• Chapter 1 Instructions for Making a Human Being

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Human Chromosomes
A karyotype is a picture of chromosomes from a
cell arranged in homologous pairs.
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Human Chromosomes

• Humans have 46 chromosomes.
• 44 autosomes
• 2 sex chromosomes
• Normal female 46 XX
• Normal male 46 XY
• Sperm 23 X or 23 Y
• Egg 23 X

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Human Chromosomes
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Human Chromosomes

• What determines the sex of a child?
• The sperm

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Human Traits

• To study the inheritance of human traits genetic
  counselors use a pedigree chart.
• From this, geneticists can infer genotypes of
  family members.

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Human Traits

• Draw a pedigree chart.

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Human Traits

• Associating an observed human trait with a gene
  is difficult.
• Many human traits are polygenic.
• The environment influences the expression of a
  trait.

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Human Genes

• Humans have 4 blood types caused by 3 alleles.
• A B codominant
• O recessive
• Types A, B, AB, O
• Blood types must be matched for a safe
  transfusion.
• Draw blood type chart..

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Human Genes

• Rh factors represent another group of antigens
  found on some red blood cells.
• Rh present.
• Rh- absent.

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Human Genes

• Most genetic disorders are caused by recessive
  alleles.
• To have the disorder an individual must inherit
  two recessive alleles.
• Ex PKU, Tay-Sachs, Cystic Fibrosis, Albinism

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Figure 14-8 The Cause of Cystic Fibrosis
Cystic Fibrosis
Chromosome 7
CFTR gene
The most common allele that causes cystic
fibrosis is missing 3 DNA bases. As a result,
the amino acid phenylalanine is missing from the
CFTR protein.
Normal CFTR is a chloride ion channel in cell
membranes. Abnormal CFTR cannot be transported
to the cell membrane.
The cells in the persons airways are unable to
transport chloride ions. As a result, the airways
become clogged with a thick mucus.
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Human Genes

• A few are caused by dominant alleles.
• Only one allele needs to be inherited to be
  expressed.
• Ex Huntingtons disease, Dwarfism

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Human Genes

• Sickle cell anemia is caused by a codominant
  allele.
• One normal allele and one abnormal allele are
  inherited for making the protein hemoglobin.
• The sickle cell allele is inherited among many
  African Americans.
• Carrying this allele produces resistance to the
  Malaria parasite.

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Sickle Cell Anemia
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NOVA Cracking the Code of Life

• Chapter 2 Getting the Letters Out

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Human Chromosomes

• 14-2

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Human Genes and Chromosomes

• Only 2 of your DNA functions as genes.

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Human Genes and Chromosomes

• Genes located on the same chromosome are linked.
• They tend to be inherited together.
• They can be separated by crossing-over during
  meiosis.

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Crossing-Over
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Sex-Linked Genes

• Sex-linked genes are located on the sex
  chromosomes.
• More genes are on the X than the Y.

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Sex-Linked Genes

• Genes on the X chromosome are always expressed in
  males even if recessive.
• Males get sex-linked disorders more often than
  females.
• Colorblindness, hemophilia, muscular dystrophy

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X-Chromosome Inactivation

• In females, one X chromosome is randomly switched
  off.
• Calico cats are always female.

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X-Chromosome Inactivation

• The switched off X chromosome becomes a dense
  region in the nucleus known as a Barr body.
• Barr bodies arent found in males.

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Olympics and Barr Bodies

• Sex testing was introduced in competitive sports
  in the mid-1960s, amid rumor that some
  competitors in women's events were not truly
  female - especially two Soviet sisters who won
  gold medals at the 1960 and 1964 Olympics, and
  who abruptly retired when gender verification
  testing began.

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Olympics and Barr Bodies

• The first tests, at the European Championships in
  1966 and the Pan-American Games in 1967, required
  female competitors to undress before a panel of
  doctors. Other methods used during this period
  included manual examination or close-up scrutiny
  of the athlete's genital region.

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Olympics and Barr Bodies

• When athletes complained that these tests were
  degrading, the IOC at the Mexico City Olympics in
  1968 introduced genetic testing in the form of a
  sex chromatin (Barr body) analysis of cells from
  a buccal smear. The procedure was further
  modified at the Barcelona games, using the
  polymerase chain reaction to amplify
  the DNA extracted from a specimen to allow
  detection of a Y chromosome gene, SRY, that codes
  for male determination.
• Journal of the American Medical Association, July
  17, 1996, vol. 276, no. 3, pp. 177-178

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Chromosomal Disorders

• Caused by nondisjunction during meiosis.
• Homologous chromosomes fail to separate resulting
  in sex cells with one too many or one too few
  chromosomes.

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Nondisjunction
Non disjunction
Section 14-2
Homologous chromosomes fail to separate
Meiosis I Nondisjunction
Meiosis II
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Chromosomal Disorders

• Downs syndrome 47,XX or 47,XY
• Turners syndrome 45,X
• Klinefelters syndrome 47,XXY

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Downs Syndrome
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Turners Syndrome
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Kleinefelters Syndrome
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Human Molecular Genetics

• 14-3

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Human DNA Analysis

• Even though the human genome is over 6 billion
  bases long, genetic tests exist for detecting the
  presence of defective recessive genes.
• Prospective parents can now be tested to
  determine if they carry recessive alleles for
  hundreds of disorders.
• Is this a good idea?

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DNA Fingerprinting

• All humans share the same genes.
• The 98 of DNA that is not encoded in genes is
  what makes us different from each other.
• DNA fingerprinting makes use of this difference
  to identify individuals.

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DNA Fingerprinting4-18 DNA Fingerprinting
Restriction enzyme
Chromosomes contain large amounts of DNA called
repeats that do not code for proteins. This DNA
varies from person to person. Here, one sample
has 12 repeats between genes A and B, while the
second sample has 9 repeats.
Restriction enzymes are used to cut the DNA into
fragments containing genes and repeats. Note that
the repeat fragments from these two samples are
of different lengths.
The DNA fragments are separated according to size
using gel electrophoresis. The fragments
containing repeats are then labeled using
radioactive probes. This produces a series of
bandsthe DNA fingerprint.
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The Human Genome Project

• In 2000, the entire human genome was sequenced.
• Dr. Francis Collins, Dr. Eric Lander and Dr.
  Craig Venter (NOVA Cracking the Code of Life)
• Advances in technology allowed this to happen in
  several years rather than the predicted 20.

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Rapid Sequencing
Gene Sequence
Promoter
Start signal
Gene
Stop signal
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Gene Therapy

• The process of changing a gene that causes a
  disorder.
• An absent or defective gene is replaced by a
  normal functioning gene.
• Viruses are often used to deliver functional
  genes to affected cells.

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Gene Therapy
Bone marrow cell
Nucleus
Normal hemoglobin gene
Chromosomes
Bone marrow
Genetically engineered virus
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Ethical Issues

• Genetic cures
• Designer babies
• Medical costs
• Research costs