Genetics chapter 14

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Genetics (chapter 14)
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One of the great ideas of biology begs the
question why do offspring resemble their
parents.

• Genetics is a field of biology dealing with the
  study of heritable characteristics. Gregor
  Mendel conducted studies (breeding garden peas)
  and greatly increased our knowledge of
  inheritance
• In 1865, Mendel had worked out the rules or laws
  of inheritance (see below)
• In 1903, W. S. Sutton and T. Boveri formulated
  the chromosomal theory of inheritance that
  contends that the process of meiosis caused the
  patterns of inheritance Mendel observed and
  asserts that the hereditary factors called genes
  are located on chromosomes.

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• Genes are units of heredity (composed of DNA).
  The location of each gene on a chromosome is
  referred to as its locus.
• Each diploid individual has a pair of genes for
  each trait. One is inherited from the mother
  the other is inherited from the father.

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• Alleles are one or more alternative states of a
  gene.
• If the two alleles for the trait code for the
  same protein, then we say that they are
  homozygous.
• If the two alleles code for different proteins,
  then we say that they are heterozygous.
• Dominant genes mask the expression of genes that
  are recessive.
• Homozygous dominant (AA), Heterozygous (Aa), or
  Homozygous recessive (aa) are the possible
  genotypes of individuals

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Alleles
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• The genotype of an individual is the actual
  genetic makeup. The phenotype is the observable
  trait that is controlled by the genotype. if red
  is dominant to white (red and white are
  phenotypes), then RR (or homozygous dominant
  individuals) red, Rr (heterozygotes) red, and
  rr (homozygous recessives) white. But there
  are exceptions (see below).
• One of the tools that we use to determine the
  possible genotypes and phenotypes of offspring is
  the Punnett square.
• In crosses, P parental generation, F1 the
  first generation of offspring, and F2 the
  second generation of offspring.

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• Mendels Laws are based on his work with garden
  peas, Pisum sativum (why garden peas? earlier
  studies had shown that hybrids could be produced,
  large numbers of true breeding varieties were
  available, they are small and easy to grow, and
  they have a short generation time). Mendel set
  out address the question what are the basic
  patterns in the transmission of traits from
  parents to offspring

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Law (Principle) of Segregation. Diploid
organisms inherit a pair of genes for each trait
and these genes segregate (separate) during
meiosis and end up in different gametes.

• Four ideas of Mendel involving the Law of
  Segregation
• 1. Alternate versions of genes (different
  alleles) account for variations in inherited
  characteristics.
• 2. For each character, an organism inherits two
  alleles (one from each parent)
• 3. If the two alleles differ, then one, the
  dominant allele is fully expressed in the
  organisms appearance, the other, the recessive
  allele, has no noticeable effect on the
  organisms appearance.
• 4. The two alleles for each character segregate
  (separate) during gamete production.

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Law of Segregation
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• Law (Principle) of Independent Assortment. Each
  gene pair tends to assort into gametes
  independently of other gene pairs located on
  other homologous pairs of chromosomes. Mendel
  determined this law by following two traits, a
  dihybrid cross.

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Law of Independent Assortment
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• Sources of genetic variation include independent
  assortment, crossing over, and random
  fertilization

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• Break for monohybrid and dihybrid crosses on board

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Not all situations are as simple as the examples.
There are variations.

• Pleiotropy is the influence of a single gene on
  unrelated traits.
• Epistasis is where two alleles of a gene mask the
  alleles of another gene and as a consequence, the
  expected phenotypes associated with the latter
  are not present.
• Polygenic inheritance or continuous
  variationoccurs when multiple genes act jointly
  to determine a trait such as height or weight and
  thus it is difficult to determine the
  contribution of an individual gene.
• Incomplete dominance involves the ability of two
  alleles to produce a heterozygous phenotype that
  is different from either homozygous phenotype.
• Codominance is a situation where both alleles are
  expressed.

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• More board work

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• Human Genetics

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• Autosomal Recessive Inheritance. Conditions
  associated with this type of inheritance occur
  when an individual has two copies of a recessive
  allele (they are homozygous recessives)

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• Cystic fibrosis is a disease where mucus clogs
  the lungs, liver and pancreas, that results from
  the failure of a chloride ion transport
  mechanism. The frequency is 1/2500 Caucasions
  and it is rare in African Americans. One in 20
  people is a carrier

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• Phenylketonuria (when undetected) results in
  brain damage due to the presence of a defective
  form of an enzyme associated with the amino acid,
  phenylalanine. Frequency 1/12,000. All babies
  born in the US are screened after birth.

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• Sickle-cell anemia is a unique case
  (Co-dominance). Remember that we talked about
  the heterozygotes for hgb. S being at an
  advantage in areas where malaria is endemic?
  These heterozygotes possess both types of hgb.
  So actually, hgb. A (normal hgb) is codominant
  with hgb. S (in heterozygotes) But two copies of
  hgb. S results in sickle-cell anemia. An
  interesting phenomenon is that 9 of African
  Americans are heterozygous and 0.2 are
  homozygous, whereas 45 of some Africans are
  heterozygous and 6 are homozygous. why?
  because of malaria and the increase in survival
  for heterozygotes over either homozygote

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• Brachydactyly is a condition that occurs in
  people who are heterozygous for a lethal gene.
  The normal gene is incompletely dominant over
  this recessive lethal gene. Thus, individuals
  who are heterozygous possess a blended
  phenotype. Children who are born with two copies
  of this recessive lethal gene die early in life
  due to skeletal deformities

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Almost absent fingers (brachydactyly) on one hand
in a boy of India
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• Tay-Sachs is a disease that results in the death
  of children due to a nonfunctional form of the
  enzyme, hexosaminidase A (it normally breaks down
  lipids in brain cells, but the nonfunctional form
  does not and the lipids build up). This is rare
  in U.S. one in 300,000 births but in Central
  Europe it occurs in 1 in 3,500 births (in
  Ashkenazi Jews)

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• More board work

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• Sex-linked inheritance. This involves situations
  where a recessive allele is expressed in males
  even though the male possesses only one copy of
  the allele. This occurs because the allele is
  present on the X chromosome and the Y chromosome
  does not have a complimentary allele (and thus
  there is no dominant or possibility of a dominant
  allele to mask the recessive allele).

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• Classic hemophilia results from a Factor VIII
  deficiency. The frequency is 1/10,000 Caucasian
  (generally males). Females can be carriers

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• Muscular dystrophy (Duchenne) is associated with
  a degeneration of muscle tissue that results from
  the degradation of myelin coating of nerves that
  stimulate muscles (1/3,700).

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• More board work

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• Autosomal Dominant Inheritance Some diseases are
  caused by dominant alleles that occur on
  autosomal chromosomes. Thus, it does not matter
  if the person inherits one or two copies of the
  allele.

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• Huntingtons disease involves a gradual
  deterioration of brain tissue that begins after
  age 30, it results from the production of an
  inhibitor of brain cell metabolism (1/24,000)

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Figure 1. Samples of coronal and sagittal
magnetic resonance imaging from a patient with
Huntington's disease (top row) and a normal
control (bottom row) showing the outlines of
caudate and putamen (left), cerebral (center) and
cerebellar volumes (right).
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• http//video.google.com/videoplay?docid-761788849
  0585459224qhuntington27sdiseasetotal46start
  0num10so0typesearchplindex6

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• Hypercholesterolemia is associated with an
  excessively high level of cholesterol in the
  blood that leads to heart disease. It is the
  result of an abnormal form of cholesterol surface
  receptors. (1/500). Heterozygotes often develop
  Coronary disease later in life (but not
  necessarily old age) but homozygous individuals
  often die extremely early in life (as children).

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• Achondroplasia is a form of dwarfism with a
  frequency of 1/10,000. Thus 99.99 of the
  population is homozygous recessive

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• More board work

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ABO blood groups

• Red blood cells have surface markers (often
  referred to as antigens). Although there are
  many kinds of these markers, three of them are of
  major clinical importance resulting in the ABO
  and Rh blood groups.

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• Concerning the ABO blood groups, A and B are both
  dominant over O but A and B are codominant with
  each other. Thus the possible ABO blood types
  are A, B, O, and AB. Note that a person can have
  either two A alleles or one A allele and one O
  allele and still be type A.

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• Table for board

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• The second major blood group is the Rh blood
  group. Most individuals possess the D surface
  marker (Rh antigen) and thus would not make
  anti-D. About 15 of caucasians do not have the
  D surface marker and upon exposure to Rh blood,
  they will produce anti-D. Thus, if one of the Rh
  - persons were to get two transfusions with Rh
  blood they could have a transfusion reaction. An
  additional problem occurs when an Rh- mom is
  pregnant with an Rh fetus

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• More board work

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Genetic Counseling and Testing

• Carrier recognition by genetic tests for carriers
• Fetal testing
• 1. amniocentesis
• 2. chorionic villus sampling
• 3. newborn screening
• Some of the genetic diseases and disorders could
  possibly be treated or prevented by somatic cell
  and germ-line gene therapy.

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• Chromosomal Abnormalities result from
  non-disjunction during meiotic divisions. This
  means that one gamete gets zero of a chromosome
  and the other gets both members of the pair.
  Although non-disjunction is more common in the
  gametes of older women, it can also occur in
  younger women or in males of any age. When an
  egg or sperm that is missing a chromosome forms a
  zygote, the embryo often fails to develop. When
  an egg or a sperm that has both members of a pair
  form a zygote, the embryo may fail to develop or
  in other cases, the offspring develops but is
  born with three of that chromosome (trisomy)

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• Down syndrome (trisomy 21) 1/750 births overall,
  but the frequency increases for older women
  because their eggs have been arrested in Meiosis
  longer and the spindles can be faulty. For women
  over 45, the risk is as high as 1/16

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• When X chromosomes fail to separate, some gametes
  have 2 Xs and some have 0 Xs. Thus an offspring
  can be XXX (sterile but otherwise normal), X0
  (sterile, short stature, webbed neck, Turners
  syndrome), XXY (male, sterile with many female
  secondary sex characteristics, Klinefelters
  syndrome).

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If you are interested, a site with the
experiences of a young man with Klinefelters
syndrome

• http//images.google.com/imgres?imgurlhttp//www.
  geocities.com/WestHollywood/Castro/4998/xxy.jpgim
  grefurlhttp//www.geocities.com/WestHollywood/Cas
  tro/4998/klinefel.htmlh330w504sz73hlensta
  rt4tbnid31hVdzASwEJ7fMtbnh85tbnw130prev

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• The Y chromosome can also be present in two
  copies in the sperm. XYY males are fertile, but
  may be more aggressive or antisocial.

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• Changes in the structure of chromosomes also
  occur deletions (cri-du-chat), inversions and
  translocations (a form of cancer), and
  duplications (FMR gene on X chromosome sometimes
  700 repeats causing Fragile X Syndrome).

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cri-du-chat
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• THE END