Human%20Chromosomes

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Human Chromosomes

• The 46 human chromosomes contain 6 billion base
  pairs (bp) of DNA.
• Genes make up only 2 percent of the DNA.
• Average human gene is about 3,000 bp. The largest
  gene is more than 2 million bp.

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Genetic Disorders

• Recessive alleles phenotype expressed in
  homozygous recessive individuals only.
• Dominate alleles phenotype expressed if the
  dominate allele is present (homozygous and
  heterozygous dominant).
• Codominant alleles both alleles are dominant.

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Human Genes and Chromosomes

• Chromosome 22 approx. 43 million bp 545
  different genes. Genetic disorders include a form
  of leukemia and neurofibromatosis. Also contains
  long stretches of repetitive DNA that are
  unstable sites where rearrangements can occur
  (breakage).

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• Chromosome 21 approx. 32 million bp about 225
  genes. Genetic disorders include amyotrophic
  lateral sclerosis (ALS). Also contains many
  regions with no genes at all.
• Many genes that are located close together on the
  same chromosome are linked, meaning that they
  tend to be inherited together. May be separated
  by crossing-over during meiosis.

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Chromosomal Disorders

• Errors in chromosomal separation can occur during
  meiosis.
• Nondisjunction homologous chromosomes fail to
  separate. If this occurs, abnormal numbers of
  chromosomes may find their way into gametes, and
  a disorder of chromosome numbers may result.

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Nondisjunction
Section 14-2
Homologous chromosomes fail to separate
Meiosis I Nondisjunction
Meiosis II
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Nondisjunction
Section 14-2
Homologous chromosomes fail to separate
Meiosis I Nondisjunction
Meiosis II
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Nondisjunction
Section 14-2
Homologous chromosomes fail to separate
Meiosis I Nondisjunction
Meiosis II
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Down Syndrome

• Trisomy 21 an individual is born with three
  copies of autosomal chromosome 21.
• In U.S., occurs in about 1 in 800 babies born.
• Produces mild to severe mental retardation and an
  increased susceptibility to many diseases and a
  higher frequency of birth defects.

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Sex Chromosome Disorders

• Turners syndrome (karyotype 45,X) female that
  inherits only one X chromosome. The women are
  sterile and their sex organs do not develop at
  puberty.
• Klinefelters syndrome (karyotype 47,XXY) Males
  with an extra chromosome X. (Y chromosome has
  sex-determining region.) Cannot reproduce.
• XXXY and XXXXY have been reported.

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Super Male or XYY syndrome

• Affected individuals are usually very tall and
  thin.
• Many experience severe acne during adolescence.
• Additional symptoms may include antisocial or
  behavioral problems and learning disabilities.
  Intelligence is usually normal, although IQ, on
  average, is 10 to 15 points lower than siblings.

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Karyotyping
The Human Genome

• To analyze chromosomes, cells are photographed
  during mitosis when chromosomes are fully
  condensed and easy to see.
• Chromosomes are cut out of photographs and lined
  up with homologous chromosome and in order of
  largest to smallest.

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• Sex chromosome one of two chromosomes that
  determine an individuals sex females have two X
  chromosomes males have one X chromosome and one
  Y chromosome.
• Autosomes chromosome that is not a sex
  chromosome.
• 46,XX female
• 46,XY male

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Polygenic traits

• Trait controlled by more than one gene.
• Examples height, skin color, and eye color.

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• None of the genes for a polygenic trait are
  dominant. Each gene has an active allele and an
  inactive allele.
• Active alleles have an additive effect on the
  phenotype.
• Inactive alleles do not affect the phenotype.
• Because of the additive effects, a continuous
  range of phenotypes is possible.

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Sex-Linked Genes

• Genes located on the X or Y chromosomes.
• Males have one X chromosomes. All X-linked
  alleles are expressed in males, even if they are
  recessive.
• Because males pass their X chromosomes along to
  their daughters, X-linked genes move from fathers
  to daughters and then to sons of the daughters.

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Colorblindness

• Three human genes associated with color vision
  are located on the X chromosome.
• In males, a defective version of any one of these
  genes produces colorblindness.
• The most common form, red-green colorblindness,
  is found in about 1 in 10 males and 1 in 100
  females.

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Figure 14-13 Colorblindness
Section 14-2
Father (normal vision)
Normal vision
Colorblind
Male Female
Daughter (normal vision)
Son (normal vision)
Mother (carrier)
Daughter (carrier)
Son (colorblind)
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Figure 14-13 Colorblindness
Section 14-2
Father (normal vision)
Normal vision
Colorblind
Male Female
Daughter (normal vision)
Son (normal vision)
Mother (carrier)
Daughter (carrier)
Son (colorblind)
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Hemophilia

• X-linked disorder. Two genes that help control
  blood clotting are found on the X chromosome.
• Recessive allele of either gene causes
  hemophilia. A protein necessary for normal blood
  clotting is missing.
• Occurs in about 1 in 10,000 males.
• Treated by injections of clotting proteins.

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Duchenne Muscular Dystrophy

• Sex-linked disorder that results in the
  progressive weakening and loss of skeletal
  muscles.
• In the U.S., 1 in 3,000 males affected.
• Caused by a defective version of the gene that
  codes for a muscle protein.

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Concept Map
Section 14-1
Autosomol Disorders
caused by
include
include
include
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Blood Types A, B, AB, O Rh factor , -
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Blood Types Multiple alleles Codominance
Section 14-1
Safe Transfusions
Antigen on Red Blood Cell
Phenotype (Blood Type
Genotype
From
To
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Transfusion Reaction

• Treatment
• transfusion is stopped
• corticosteroids
• antihistamines
• acetominophen (Tylenol)

• Symptoms
• fever
• chills
• rash
• flank pain or back pain
• bloody urine
• fainting or dizziness

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Figure 14-3 A Pedigree
Section 14-1
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.
A pedigree chart shows the relationships within a
family.
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From Gene to Molecule

• Cystic fibrosis caused by the deletion of three
  bases in the DNA of the gene resulting in an
  amino acid (phenylalanine) missing from the
  protein. The protein produced by the gene is a
  chloride ion pump. The deletion of phenylalanine
  prevents the protein from being transported to
  the cell membrane.

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Figure 14-8 The Cause of Cystic Fibrosis
Section 14-1
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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• Sickle cell disease characterized by
  sickle-shaped red blood cells that are more rigid
  than normal cells and tend to get stuck in
  capillaries leading to damage to cells, tissues,
  and organs.
• The normal allele differs from the sickle cell
  allele by one DNA base leading to a substitution
  of valine for glutamic acid.

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Sickle cell disease and malaria

• In parts of the world where malaria is a major
  health threat, the presence of the sickle cell
  allele in heterozygous individuals confers
  resistance to malaria.

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X-Chromosome Inactivation

• In female cells, one X chromosome is randomly
  switched off. The turned-off chromosome forms a
  dense region in the nucleus called a Barr body.
• Also occurs in cats and other mammals (ex, white
  cat with orange and black spots).

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Internet
Go Online

• Links from the authors on DNA samples
• Career links on geneticists
• More information on the Human Genome Project
• Interactive test
• For links on pedigrees, go to www.SciLinks.org
  and enter the Web Code as follows cbn-4141.