The Human Genome

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The Human Genome

• Chapter 14

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Human Chromosomes

• A Karyotype is a picture of the chromosomes in
  order form largest to smallest.
• Pair 23 are the sex chromosomes. (X Y)
• The other 22 pair are called the autosomes.
• Egg and Sperm with 23 chromosomes each join to
  make the diploid 46 chromosome cell.
• Sex of the baby is determined by the males sperm
  containing either an X or Y chromosome.

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Human Traits

• To study a trait it must be on a single
  chromosome and truly be inherited not influenced
  by the environment.
• Pedigrees can be used to trace a trait in a
  family.
• Pedigrees can be used to determine genotypes in
  families.

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Figure 14-3 A Pedigree
A circle represents a female.
A square represents a male.
A horizontal line connecting a male and female
represents a marriage.
A vertical line and a bracket connect the parents
to their children.
A half-shaded circle or square indicates that a
person is a carrier of the trait.
A circle or square that is not shaded indicates
that a person neither expresses the trait nor is
a carrier of the trait.
A completely shaded circle or square indicates
that a person expresses the trait.
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Human Genes

• Blood Groups In humans three alleles determine
  blood type
• The Rh factor is a protein made in Rh people
  that was first discovered in the rhesus monkey.
• ABO are the alleles that make the human blood
  antigens which determine blood type
• See Fig. 14-4 page 344.

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Figure 14-4 Blood Groups
Safe Transfusions
Phenotype (Blood Type
Antigen on Red Blood Cell
Genotype
To
From
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Recessive Alleles

• Genetic disorders often appear because of the
  double recessive alleles nonfunctioning protein.
• PKU was the first recessive genetic disease
  understood and tested at birth.
• Tay-Sachs is another disease usually in the
  Jewish population. May also test.
• Fig 14-6 page 345 shows others.

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Dominant Codominant Alleles

• Dominant genetic disorders are caused by a single
  dominant allele
• Huntington disease and achondroplasia are two
  examples of dominant disorders.
• Codominant diseases have two alleles that make a
  protein. One is a good protein the other is
  nonfunctioning protein.
• Sickle-cell Anemia is a codominant.

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Sickle Cell Disease

• This disease causes disk shaped red blood cells
  to change shape and stick to the arteries of the
  body.
• There is only one nucleotide changed which
  changes just one amino acid in the protein which
  makes hemoglobin.
• This is a codominant disease which means carriers
  will not suffer the disease and becomes resistant
  to malaria.

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Cystic Fibrosis

• CF is caused by a recessive allele on chromosome
  7.
• A deletion of 3 nucleotides causes a protein
  which moves Cl- ions not to work.
• The cells in the lungs use the Cl moving protein
  to remove mucus in the lung tissue
• See Fig 14-8

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Figure 14-8 The Cause of 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 Chromosomes

• Only 2 of the human DNA codes for protein. The
  98 have a control function.
• Chromosome 21 22 were the first sequenced.
• 22 contains 545 genes with alleles for leukemia
  and neurofibromatosis.
• 21 contains 225 genes with alleles which cause
  amyotrophic lateral sclerosis (ALS) or Lou
  Gehrigs disease.

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Sex Linked Genes

• Genes on the sex chromosomes are called sex
  linked genes.
• The X is much bigger than the Y and carries most
  of the genes.
• Since females have XX they are less likely to get
  a recessive. Sex linked disorders like color
  blindness, Muscular Dystrophy Hemophilia are
  more common in male.

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Figure 14-13 Colorblindness
Father (normal vision)
Normal vision
Colorblind
Male Female
Daughter (normal vision)
Son (normal vision)
Mother (carrier)
Daughter (carrier)
Son (colorblind)
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X-Chromosome Inactiviation

• Mary Lyon discovered in females one X is randomly
  shut off and form a Barr Body.
• Calico cats are always female so there can be
  three colors.
• This occurs in most animals

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Chromosomal Disorders

• Nondisjuction (chromosomes not separating) is the
  most common cause of chromosomal disorders.
  (polyploidy)
• Down syndrome is caused by triplet chromosome 21
  and leads to mental retardation. 1-800 babies
  have it.
• Turners (45,XO) Klinefelters ( 47 ,XXY)
  Syndrome are nondisjunction of the sex
  chromosomes and are infertile.

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Nondisjunction
Homologous chromosomes fail to separate
Meiosis I Nondisjunction
Meiosis II
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DNA Analysis

• Doctors can test for the difference in alleles to
  see if recessives exist.
• DNA Fingerprinting can identify individuals like
  fingerprints.
• The Human Genome Project has sequenced the entire
  Human DNA strand.
• Rapid Sequencing allowed for pieces to be
  sequenced and then put together.
• Promoters help to find the start of genes.

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Figure 14-18 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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Gene Therapy

• Gene Therapy replaces an absent or faulty gene
  with a working version.
• Viruses or injections can get the modified DNA
  into the cells that need them.
• There are numerous ethical and moral issues
  involved with our increased knowledge and
  abilities in genetics.