Mendel and the Gene Idea

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Mendel and the Gene Idea

• Chapter 14

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• Gregor Mendel - monk - studied pea plants, looked
  at traits.
• Pea plants many varieties with distinct heritable
  features (characters) with different variants
  (traits).
• Pea plants self-fertilize Mendel
  cross-fertilized to study traits.

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• Mendel cross-pollinated (hybridize) 2
  contrasting, true-breeding pea varieties.
• True-breeding parents - P generation hybrid
  offspring - F1 generation.
• F1 hybrids then self-pollinate to produce F2
  generation.

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• Thought genes blended - purple flower crossed
  with white flower result would be light purple
  flowers.
• All the flowers purple.

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• When flowers self-fertilized, white flower
  reappeared in next generation.
• Ratio of purple to white in F2 generation was 31.

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• Developed hypothesis to explain process.
• 1Alternative versions of genes (alleles) account
  for variations in inherited characters.
• 2For each character organism inherits 2 alleles,
  1 from each parent.
• Alleles can be same or different.

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• 32 alleles differ - 1 (dominant allele) fully
  expressed in organism.
• Other (recessive allele) no noticeable effect on
  organisms appearance.
• 42 alleles for each character segregate
  (separate) during gamete production.

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• Found that alleles not linked due to inheritance
  patterns.
• Independent assortment of each pair of alleles
  during gamete formation - law of independent
  assortment.

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• Therefore, Mendel had three different laws.
• The law of dominance and recessiveness states
  that one gene is dominant over the more recessive
  gene.
• The law of segregation states that alleles
  separate during meiosis.
• The law of independent assortment states that
  alleles organize in the gametes regardless of
  other alleles.

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• An organism with two identical alleles for a
  character is homozygous for that character.
• Organisms with two different alleles for a
  character is heterozygous for that character.
• A description of an organisms traits is its
  phenotype.
• A description of its genetic makeup is its
  genotype.

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• A Punnett square predicts the results of a
  genetic cross between individuals of known
  genotype.
• A testcross, breeding a homozygous recessive with
  dominant phenotype, but unknown geneotype, can
  determine the identity of the unknown allele.

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• Mendels experiments focused on monohybrid
  crosses meaning that he looked at only one trait
  at a time.
• Later on he started looking at dihybrid crosses
  involving probabilities of two different traits.

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• Mendels ideas are based on probability.
• If you were to toss a coin 4 times, the coin has
  a ½ chance of coming up heads every time.
• Each toss is independent of the one done before.
• The probability of it coming up heads all four
  times is ½ ½ ½ ½ 1/8.
• This is known as the rule of multiplication.

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• There is also a law of addition that determines
  the chances of an event happening in different
  ways.
• For example, there are two ways that F1 gametes
  can combine to form a heterozygote.
• The dominant allele could come from the sperm and
  the recessive from the ovum (probability 1/4).
• Or, the dominant allele could come from the ovum
  and the recessive from the sperm (probability
  1/4).
• The probability of a heterozygote is 1/4 1/4
  1/2.

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• Incomplete dominance can also occur in offspring.
• In incomplete dominance, heterozygotes have a
  completely different phenotype than homozygotes.
• This happens in snapdragons.
• Homozygous recessive flowers are white
  homozygous dominant flowers are red
  heterozygotes are pink.

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• Another inheritance pattern is codominance in
  which two alleles affect the phenotype in
  separate, distinguishable ways.
• An example of this is blood type.
• If you inherit an A allele and a B allele, your
  blood type will be AB if it is AA or AO, your
  blood type will be A.
• This means that A is dominant to O, B is dominant
  to O, but A is codominant to B.

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• Blood type is important because type A has anti-B
  antibodies.
• If exposed to B blood, it will clump together
  causing a transfusion reaction.
• People with blood type O have both antibodies and
  therefore can donate to any other blood type.
• On the other hand, AB has neither antibodies and
  therefore can receive from any blood type.

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• Dominant genes do not mean that they are more
  popular in a given population.
• Also, most genes do not control only one trait
  but are pleiotropic, affecting more than one
  phenotypic character.
• In epistasis, a gene at one locus alters the
  phenotypic expression of a gene at a second
  locus.

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• In mice, one gene determines whether or not there
  will be a coat color.
• If that gene is turned off, the mouse will be
  white if it is turned on, another locus will
  determine what the color is (brown or black).

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• Quantitative characters vary in a population
  along a continuum.
• This is because of polygenic inheritance which is
  when more than one gene controls a single trait.
• An example of this is skin color which is
  controlled by at least three different genes and
  is responsible for the variety of skin colors.

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• Phenotype also depends on environment.
• For humans, nutrition influences height, exercise
  alters build, sun-tanning darkens the skin, and
  experience improves performance on intelligence
  tests.

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• Genetic experiments cannot ethically be performed
  on humans, so geneticists use pedigrees to look
  at traits found in families.
• A family tree is then created showing the absence
  or presence of a specific trait to determine how
  it is passed.

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• Ethnicity plays a role in genetic disease
  patterns.
• For example, sickle-cell anemia is found
  predominately in African-Americans.
• This disease causes the red blood cells to be
  sickle shaped instead of the normal disk shape
  causing the cells to get stuck in the vessels.

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• Cystic fibrosis affects mostly Caucasians.
• Cystic fibrosis is a multi-system disease that
  causes mucous to build up in various organs,
  especially the lungs.

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• Tay-Sachs affects people of Jewish descent.
• Tay-Sachs affects the brains of small children,
  ultimately causing the death of the child prior
  to 5 years old.

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• Some genetic diseases, such as dwarfism, are
  dominant diseases.
• This means that a child has a 50 chance of
  inheriting the disease because one of the parents
  has the disease.
• Huntingtons disease is also a dominant disease
  that affects the nervous system.
• Most dominant diseases are not lethal
  (Huntingtons disease is).

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• Genetic counseling is a field of study that works
  with people that have a history of genetic
  disease in the family.
• A child with a recessive disease can be born to
  phenotypically normal parents.
• There are several tests that can be performed to
  determine a couples risk.

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• One technique is amniocentesis.
• Cells are extracted from the amniotic fluid
  surrounding the fetus and then analyzed to search
  for potential problems in a technique called
  karyotyping.
• Karyotyping is essentially mapping out the
  chromosomes of an individual.

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• A second technique, chorionic villus sampling
  (CVS) can allow faster karyotyping and extracts a
  sample of fetal tissue from the chorionic villi
  of the placenta.

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• A more routine test is an ultrasound which
  detects only physical abnormalities that are
  present.