What makes us human?

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What makes us human?
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Human Chromosomes

• To analyze chromosomes, cell biologists
  photograph cells in mitosis, when the chromosomes
  are fully condensed and easy to see.
• The biologists then cut out the chromosomes from
  the photographs and group them together in
  homologous pairs.
• A picture of chromosomes arranged in this way is
  known as a karyotype (Homologous pairs)

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Human Chromosomes

• A Human body cell contains 46 chromosomes.
• A haploid sperm, carrying just 23 chromosomes,
  fertilized a haploid egg, also with 23
  chromosomes.
• 22 autosomes ( 1-22)
• 1 sex chromosome (X or Y)
• The diploid zygote, or fertilized egg, contained
  the full complement of 46 chromosomes.

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Karyotype

• These human chromosomes have been cut out of a
  photograph and arranged to form a karyotype.

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Human Chromosomes

• Two of your 46 chromosomes are known as sex
  chromosomes, because they determine an
  individuals sex. (Chromosomes 23)
• Females have two copies of a large X chromosome.
  Males have one X and one small Y chromosome.
• The remaining 44 chromosomes are known as
  autosomal chromosomes, or autosomes. (Chromosomes
  1-22)

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Sex Chromosomes

• All egg cells carry a single X chromosome (23X).
• Half of all sperm cells carry an X chromosome
  (23X) and half carry a Y chromosome (23Y).
• This ensures that just about half of the zygotes
  will be 46XX and half will be 46XY.

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Human Traits

• A pedigree chart, which shows the relationships
  within a family, can be used to help with this
  task.
• Many human traits are polygenic (controlled by
  many genes)
• Environmental effects on gene expression are not
  inherited genes are.

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Interest Grabber
A Family Tree
Section 14-1

• To understand how traits are passed on from
  generation to generation, a pedigree, or a
  diagram that shows the relationships within a
  family, is used. In a pedigree, a circle
  represents a female, and a square represents a
  male. A filled-in circle or square shows that the
  individual has the trait being studied. The
  horizontal line that connects a circle and a
  square represents a marriage. The vertical
  line(s) and brackets below that line show the
  children of that couple.

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Interest Grabber continued

• 1. This pedigree shows the inheritance of
  attached ear lobes. Which parent has attached ear
  lobes?
• 2. How many children do the parents have? Which
  child has attached ear lobes?
• 3. Which child is married? Does this childs
  spouse have attached ear lobes? Do any of this
  childs children have attached ear lobes?

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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.
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Human Genes

• Some of the very first human genes to be
  identified were those that control blood type.
• The Rh blood group is determined by a single gene
  with two alleles positive and negative. Rh
  stands for rhesus monkey, the animal in which
  this factor was discovered.
• The positive (Rh) allele is dominant, so persons
  who are Rh/Rh or Rh/Rh- are said to be
  Rh-positive.
• Individuals with two Rh- alleles are Rh-negative.

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Section 14-1
Blood Groups
Safe Transfusions
Antigen on Red Blood Cell
Phenotype (Blood Type
Genotype
From
To
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Human Alleles

•  Many human genes have become known through the
  study of genetic disorders.
• Genetic Disorders can be caused by
• recessive alleles
• dominant alleles
• Codominant alleles
• What makes an allele dominant, recessive, or
  codominant?
• It all depends on the nature of a genes protein
  product and its role in the cell.

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Concept Map
Section 14-1
Autosomol Disorders
caused by
include
include
include
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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 

• Sickle cell disease is a common genetic disorder
  found in African Americans.
• Sickle cell disease is characterized by the bent
  and twisted shape of the red blood cells

• These sickle-shaped red blood cells are more
  rigid than normal cells and tend to get stuck in
  the capillaries, the narrowest blood vessels in
  the body.
• As a result, blood stops moving through these
  vessels, damaging cells and tissues beyond the
  blockage.
• Sickle cell disease produces physical weakness
  and damage to the brain, heart, and spleen. In
  some cases, it may be fatal.

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Sickle Cell Disease 

• Hemoglobin is the protein that carries oxygen in
  the blood.
• Mutation the amino acid valine in place of
  glutamic acid.
• As a result, the abnormal hemoglobin is somewhat
  less soluble than normal hemoglobin. Blood gets
  stuck in cappillaries.

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Why do so many African Americans carry the sickle
cell allele?

• Most African Americans can trace their ancestry
  to west central Africa.
• Malaria, a serious parasitic disease that infects
  red blood cells, is common in this region of
  Africa.
• People who are heterozygous for the sickle cell
  allele are generally healthy and are resistant to
  malaria.

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Checkpoint Questions

• What are sex chromosomes? What determines whether
  a person is male or female?
• Using an example, explain how a small change in a
  persons DNA can cause a genetic disorder.
• 3. How does studying genetic disorders such as
  PKU help biologists understand normal alleles?
• 4. What are some problems biologists face in
  studying human inheritance?
• 5. If a woman with type O blood and a man with
  type AB blood have children, what are the
  childrens possible genotypes?

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Sex-Linked Genes

• Is there a special pattern of inheritance for
  genes located on the X chromosome or the Y
  chromosome?
• The answer is yes. Because these chromosomes
  determine sex, genes located on them are said to
  be sex-linked genes.

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Sex-Linked Genes

• Males have just one X chromosome. Thus, all
  X-linked alleles are expressed in males, even if
  they are recessive.
• In order for a recessive allele, such as the one
  for colorblindness, to be expressed in females,
  there must be two 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 among males than among females.
• In addition, because men pass their X chromosomes
  along to their daughters, sex-linked genes move
  from fathers to their daughters and may then show
  up in the sons of those daughters.

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Expression of X-Linked Alleles Colorblindness 

•  X-linked alleles are always expressed in males,
  because males have only one X chromosome.
• Males who receive the recessive Xc allele all
  have colorblindness.
• Females, however, will have colorblindness only
  if they receive two Xc alleles.

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Sex-Linked Genes Hemophilia

•  Hemophilia is another example of a sex-linked
  disorder.
• Two important genes carried on the X chromosome
  help control blood clotting.
• A recessive allele in either of these two genes
  may produce a disorder called hemophilia
• In hemophilia, a protein necessary for normal
  blood clotting is missing.

• About 1 in 10,000 males is born with a form of
  hemophilia.
• People with hemophilia can bleed to death from
  minor cuts and may suffer internal bleeding from
  bumps or bruises.
• Fortunately, hemophilia can be treated by
  injections of normal clotting proteins.

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Sex-Linked Genes Duchenne Muscular Dystrophy

•  
• Duchenne muscular dystrophy is a sex-linked
  disorder that results in the progressive
  weakening and loss of skeletal muscle.
• People with Duchenne muscular dystrophy rarely
  live past early adulthood. In the United States,
  one out of every 3000 males is born with Duchenne
  muscular dystrophy.

• Duchenne muscular dystrophy is caused by a
  defective version of the gene that codes for a
  muscle protein.
• Researchers in many laboratories are trying to
  find a way to treat or cure this disorder,
  possibly by inserting a normal allele into the
  muscle cells of Duchenne muscular dystrophy
  patients.

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X-Chromosome Inactivation

• Females have two X chromosomes, but males have
  only one.
• If just one X chromosome is enough for cells in
  males, how does the cell adjust to the extra X
  chromosome in female cells?

• In female cells, one X chromosome is randomly
  switched off.
• That turned-off chromosome forms a dense region
  in the nucleus known as a Barr body.
• Barr bodies are generally not found in males
  because their single X chromosome is still
  active.

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X-Chromosome Inactivation Cats

• In cats, for example, a gene that controls the
  color of coat spots is located on the X
  chromosome.
• One X chromosome may have an allele for orange
  spots and the other may have an allele for black
  spots.
• In cells in some parts of the body, one X
  chromosome is switched off. In other parts of the
  body, the other X chromosome is switched off.

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X-Chromosome Inactivation Cats

• As a result, the cats fur will have a mixture of
  orange and black spots, as shown in the figure
  below.
• Male cats, which have just one X chromosome, can
  have spots of only one color.
• By the way, this is one way to tell the sex of a
  cat. If the cats fur has three colorswhite with
  orange and black spots, for exampleyou can
  almost be certain that it is female.

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Chromosomal Disorders

• The most common error in meiosis occurs when
  homologous chromosomes fail to separate.
• This is known as nondisjunction, which means not
  coming apart. Nondisjunction can occur either
  during meiosis I, as shown in the figure below,
  or in meiosis II,

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Chromosomal Disorders

• Nondisjunction causes gametes to have abnormal
  numbers of chromosomes.
• The result of nondisjunction may be a chromosome
  disorder such as Down syndrome.

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Down Syndrome

•  If two copies of an autosomal chromosome fail to
  separate during meiosis (nondisjunction) an
  individual may be born with three copies of a
  chromosome.
• This is known as a trisomy, meaning three
  bodies. The most common form of trisomy involves
  three copies of chromosome 21 and is called Down
  syndrome.

• In the United States, approximately 1 baby in 800
  is born with Down syndrome.
• Down syndrome produces mild to severe mental
  retardation.
• It is also characterized by an increased
  susceptibility to many diseases and a higher
  frequency of some birth defects.

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Sex Chromosomal Disorders

• Disorders also occur among the sex chromosomes.
• Two of these abnormalities are Turners syndrome
  and Klinefelters syndrome.
• In females, nondisjunction can lead to Turners
  syndrome.

• A female with Turners syndrome inherits only one
  X chromosome (genotype XO).
• Women with Turners syndrome are sterile because
  their sex organs do not develop at puberty.

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Sex Chromosomal Disorders

• In males, nondisjunction causes Klinefelters
  syndrome (genotype XXY).
• The extra X chromosome interferes with meiosis
  and usually prevents these individuals from
  reproducing.
• Cases of Klinefelters syndrome have been found
  in which individuals were XXXY or XXXXY.
• There have been no reported instances of babies
  being born without an X chromosome, indicating
  that the X chromosome contains genes that are
  vital for normal development.

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Checkpoint Questions

1. Why are sex-linked disorders more common in males
   than in females?
2. How does nondisjunction cause chromosome number
   disorders?
3. List at least two examples of human sex-linked
   disorders.
4. Describe two sex chromosome disorders.
5. Distinguish between sex-linked disorders and sex
   chromosome disorders.