Genetic Inheritance

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Genetic Inheritance
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Mendels Laws

• Gregor Mendel was an Austrian monk who in 1860
  developed certain laws of heredity after doing
  crosses between garden pea plants.

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Gregor Mendel

• Gregor Mendel combined his farmers skills with
  his training in mathematics.
• Mendels law of segregation states that each
  individual has two factors (called genes today)
  for each trait.
• Alternative forms of a gene affecting the same
  trait are now referred to as alleles.

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• The factors segregate during the formation of the
  gametes and each gamete has only one factor from
  each pair.
• Fertilization gives each new individual two
  factors again.

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The Inheritance of a Single Trait

• A capital letter indicates a dominant allele,
  which is expressed when present.
• An example is W for widows peak.
• A lowercase letter indicates a recessive allele,
  which is only expressed in the absence of a
  dominant allele.
• An example is w for continuous hairline.

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Widows peak
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Genotype and Phenotype

• Genotype - the genes of an individual.
• Homozygous - both alleles are the same for
  example, WW stands for homozygous dominant and ww
  stands for homozygous recessive.

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• Heterozygous means that the members of the
  allelic pair are differentfor example, Ww.
• Phenotype refers to the physical or observable
  characteristics of the individual.
• Both WW and Ww result in widows peak, two
  genotypes with the same phenotype.

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One-Trait Crosses

• In one-trait crosses, one trait is considered.
• When performing crosses, the original parents are
  called the parental generation, or the P
  generation.
• All of their children are the filial generation,
  or F generation.

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• If you know the genotype of the parents, it is
  possible to determine the gametes and use a
  Punnett square to determine the phenotypic ratio
  among the offspring.
• This ratio is used to state the chances of a
  particular phenotype.

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Monohybrid cross
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• In the cross of Ww x Ww, what is the chance
  having a homozygous dominant child? what is the
  chance of having a heterozygous child? what is
  the chance of having a child with widows peak?
• Chance of W ½, or chance of w ½
• The probability of these genotypes is
• The chance of WW ½ x ½ ¼
• The chance of Ww ½ x ½ ¼
• The chance of wW ½ x ½ ¼
• The chance of ww ½ x ½ ¼
• The chance of widows peak (WW, Ww, wW) is ¼ ¼
  ¼ ¾ or 75.

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The One-Trait Testcross

• A testcross is used to determine the genotype of
  a phenotypically dominant individual.
• A testcross crosses the dominant phenotype with
  the recessive phenotype.
• Why?

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The Inheritance of Many Traits

• The Law of Independent Assortment
• -each pair of alleles segregates independently
  and all possible combinations of alleles can
  occur in the gametes.
• What gametes can a individual with the genotype
  AaBb produce?

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• Answer
• AB, Ab, aB, ab
• What is the probability that a person homozygous
  dominant for two traits will have a child that is
  phenotypically dominant for both traits?

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What is the expected phenotypic ratio if you
cross two individuals that are heterozygous for
widows peak and short fingers? First- determine
the possible gametes then draw a Punnett square.
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What is the expected phenotypic ratio if you
cross an individual heterozygous for widows peak
and short fingers with a person with continuous
hairline and long fingers? First- determine the
possible gametes then draw a Punnett square.
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Two-Trait Crosses and Probability

• It is possible to use the laws of probability to
  arrive at a phenotypic ratio for a two-trait
  cross without using a Punnett square.
• If you flip two coins, what is the probability of
  getting two heads?
• If you flip two coins, what is the probability of
  getting one head?

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• The probabilities for the dihybrid cross between
  heterozygotes
• Probability of widows peak and short fingers ¾
  x ¾ 9/16
• Probability of widows peak and long fingers ¾
  x ¼ 3/16
• Probability of straight hairline and short
  fingers ¼ x ¾ 3/16
• Probability of straight hairline and long fingers
  ¼ x ¼ 1/16

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Probability and the Hexahybrid Cross

• What is the probability that a cross between two
  individuals heterozygous for each of six traits
  will have an offspring dominant for each of those
  seven traits?

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Genetic Disorders

• Patterns of Inheritance
• pedigree charts shows pattern of inheritance of
  a characteristic within a family.

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• males are squares
• females are circles.
• Filled squares or circles indicate that the
  individual has the condition
• Patterns often indicate the mode of inheritance

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Autosomal Recessive Disorders

• Tay-Sachs Disease
• Tay-Sachs disease is common among United States
  Jews of central and eastern European descent.
• An affected infant develops neurological
  impairments and dies by the age of three or four.
  
• Tay-Sachs results from a lack of hexosaminidase A
  and the storage of its substrate in lysosomes.

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Cystic Fibrosis

• Cystic fibrosis is the most common lethal genetic
  disorder among Caucasians.
• A chloride ion transport protein is defective in
  affected individuals.
• Normally when chloride ion passes through a
  membrane, water follows.
• In cystic fibrosis patients, a reduction in water
  results in a thick mucus which accumulates in
  bronchial passageways and pancreatic ducts.

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Phenylketonuria (PKU)

• Individuals with phenylketonuria lack an enzyme
  needed for the normal metabolism of
  phenylalanine, coded by an allele on chromosome
  12.
• Newborns are regularly tested for elevated
  phenylalanine in the urine.
• If the infant is not put on a phenylalanine-restri
  ctive diet in infancy until age seven when the
  brain is fully developed, brain damage and severe
  mental retardation result.

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Autosomal Dominant Disorders

• Neurofibromatosis
• Small benign tumors, made up largely of nerve
  cells, occur under skin or on various organs.
• The effects can range from mild to severe, and
  some neurological impairment is possible this
  disorder is variably expressive.
• The gene for this trait is on chromosome 17.

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Huntington Disease

• Individuals with Huntington disease experience
  progressive degeneration of the nervous system
  and no treatment is presently known.
• Most patients appear normal until middle age.
• The gene coding for the protein huntingtin
  contains many more repeats of glutamines than
  normal.

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Huntington disease
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Beyond Simple Inheritance Patterns

• Polygenic Inheritance
• Polygenic traits are governed by more than one
  gene pair.

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Skin Color

• The inheritance of skin color, determined by an
  unknown number of gene pairs, is another example
  of polygenic inheritance.

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Multiple Allelic Traits

• more than two alternative alleles exist for a
  particular gene locus.
• blood type is an example

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ABO Blood Types

• Alleles IA,IB,I
• Genotype Phenotype
• IA IA A
• IA i A
• IB IB B
• IB i B
• IA IB AB
• i i O

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• What are the possible blood types of children
  from a mother with type A blood and a father with
  type B blood?

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Incompletely Dominant Traits

• Codominance -both alleles are equally expressed
  in a heterozygote. (blood type)
• Incomplete dominance - heterozygote shows an
  intermediate phenotype (curly vs. wavy hair
  sickle cell anemia)

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• Many genetic disorders and other traits are
  inherited according to laws first established by
  Gregor Mendel.
• Inheritance is often more complex, providing
  exceptions to Mendels laws but helping to
  explain an even wider variety in patterns of gene
  inheritance.

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Sex-Linked Traits

• Sex Determination-
• XX female
• XY male
• Traits controlled by genes on the X or Y
  chromosomes are sex-linked
• An allele is termed X-linked.

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• Duchenne muscular dystrophy involves the absence
  of a protein called dystrophin that is involved
  in the release of calcium from the sarcoplasmic
  reticulum of muscle cells.
• The lack of dystrophin causes calcium to leak
  into the cell, which promotes the action of an
  enzyme that dissolves muscle fibers.
• A test is now available to determine the carriers
  of Duchenne muscular dystrophy.

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Hemophilia

• Hemophilia refers to the lack of one of several
  clotting factors that leads to excessive bleeding
  in affected individuals.
• Hemophiliacs bleed externally after injury, but
  also bleed internally around joints.
• Hemorrhages can be stopped with blood
  transfusions or a biotechnology clotting factor.

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Muscular Dystrophy

• Muscular dystrophy is characterized by the
  wasting of muscles.
• The most common form is Duchenne muscular
  dystrophy this is an X-linked disorder,
  occurring in 1 of 3,600 males.
• Muscles weaken, frequent falls and difficulty in
  rising occur early death occurs by age 20.

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Color Blindness

• Three types of cones are in the retina detecting
  red, green, or blue.
• Genes for blue cones are autosomal those for red
  and green cones are on the X chromosome.
• Males are much more likely to have red-green
  color blindness than females.
• About 8 of Caucasian men have red-green color
  blindness.

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X-Linked Disorders
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X-Linked Alleles

• The key for an X-linked problem shows the allele
  attached to the X as in
• XB normal vision
• Xb color blindness.
• Females with the genotype XBXb are carriers
  because they appear to be normal but each son has
  a 50 chance of being color blind depending on
  which allele the son receives.
• XbXb and XbY are both colorblind.

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• Why are there so few Y-linked or X-linked
  dominant disorders?

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Linked Genes

• Alleles are inherited as a group, rather than
  according to the law of independent assortment
• What are the linkage groups
• Changes expected ratios (example)
• Are the basis of some genetic tests.
• Crossing over can tell us how far apart genes
  are.