NOTES: Ch 14, part 2 Extending Mendelian Genetics

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NOTES Ch 14, part 2 Extending Mendelian
Genetics
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The Spectrum of Dominance

• ? Complete dominance occurs when the phenotypes
  of the heterozygote and dominant homozygote are
  identical

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CODOMINANCE

• ? In codominance 2 dominant alleles affect the
  phenotype in separate, distinguishable ways
• ? Example the human blood group MN

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CODOMINANCE

• ? inheritance characterized by full expression of
  both alleles in the heterozygote
• Example in chickens, BB black feathers, bb
  white, Bb speckled (both black and white
  feathers)

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INCOMPLETE DOMINANCE

• ? inheritance where one allele is not completely
  dominant over the other, so the heterozygote has
  a phenotype that is intermediate between the
  phenotype of the 2 homozygotes
• Ex RR red flowers,
• rr white, Rr pink

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• The Relationship Between Dominance and Phenotype
• ? Dominant and recessive alleles do not really
  interact
• ? they lead to synthesis of different proteins
  that produce a phenotype

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• Frequency of Dominant Alleles
• ? Dominant alleles are not necessarily more
  common in populations than recessive alleles

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Multiple Alleles

• ? Most genes exist in populations in more than
  two allelic forms

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• ? The ABO blood group in humans is determined by
  multiple alleles

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Polygenic Inheritance

• ? Many human characters vary in the population
  along a continuum and are called quantitative
  characters

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POLYGENIC INHERITANCE

• ? mode of inheritance in which the additive
  effect of 2 or more genes determines a single
  phenotypic character
• Examples height
• hair color (4 gene pairs)
• eye color (2 gene pairs)
• skin color

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• ? Quantitative variation usually indicates
  polygenic inheritance
• (An additive effect of 2 or more genes on a
  single phenotype)

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PLEIOTROPY

• ? the ability of a single gene to have multiple
  phenotypic effects
• Examples sickle cell anemia
• Siamese cats tigers

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EPISTASIS

• ? interaction between 2 nonallelic genes in which
  one modifies the phenotypic expression of the
  other
• Example gene for pigment deposition is
  epistatic to gene for melanin production in mice

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• ? An example of epistasis

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Nature and Nurture The Environmental Impact on
Phenotype

• ? Another departure from simple Mendelian
  genetics arises when the phenotype for a
  character depends on environment as well as on
  genotype

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• ? The norm of reaction is the phenotypic range of
  a particular genotype that is influenced by the
  environment

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MULTIFACTORIAL INHERITANCE

• ? A trait depends on many factors a variety of
  genotypes as well as environmental influences
  (such as certain chemicals, medicines, or diet)
• Examples of disorders that may be a result of
  multifactorial inheritance diabetes, heart
  disease, neural tube defects, autism, Alzheimer
  disease, ALS, and many cancer syndromes

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Same genotype Phenotype depends on acidity of
soil
height controlled by several genes height
influenced by amount of nourishment
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Integrating a Mendelian View of Heredity and
Variation

• ? An organisms phenotype includes its physical
  appearance, internal anatomy, physiology, and
  behavior
• ? the phenotype reflects its overall genotype
  and unique environmental history

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Inheritance Patterns for Genetic Diseases in
Humans
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Pedigree Analysis

• ? A pedigree is a family tree that describes the
  interrelationships of parents and children across
  generations

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• ? Inheritance patterns of particular traits can
  be traced and described using pedigrees

Figure 14.14 A, B
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1) Autosomal Recessive

• ? recessive alleles that cause human disorders
  are usually defective versions of normal alleles
• ? defective alleles code for either a
  malfunctioning protein or no protein at all

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• ? heterozygotes can be phenotypically normal, if
  1 copy of the normal allele is all that is needed
  to produce sufficient quantities of the good
  protein
• Examples cystic fibrosis, Tay-Sachs, sickle
  cell anemia

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• ? Carriers are heterozygous individuals who
  carry the recessive allele but are phenotypically
  normal

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Cystic Fibrosis

• ? Symptoms of cystic fibrosis include
• -Mucus buildup in the some internal organs
• -Abnormal absorption of nutrients in the small
  intestine

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Sickle-Cell Disease

• ? Sickle-cell disease
• -Affects one out of 400 African-Americans
• -Is caused by the substitution of a single amino
  acid in the hemoglobin protein in red blood cells
• ? Symptoms include
• -Physical weakness, pain, organ damage, and even
  paralysis

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2) Autosomal Dominant

• ? only 1 dominant allele is needed in order to
  produce the effects of these diseases
  (heterozygous)
• ? Lethal homozygous dominant condition results in
  spontaneous abortion of fetus

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• ? homozygous recessives are of normal phenotype
• Examples achondroplasia, Huntingtons Disease

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• ? ACHONDROPLASIA a form of dwarfism that is
  lethal when homozygous for the dominant allele

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• ? HUNTINGTONS DISEASE a degenerative disease of
  the nervous system
• -it has no obvious phenotypic effects until
  about 35 to 40 years of age

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Genetic Testing and Counseling

• ? Genetic counselors can provide information to
  prospective parents concerned about a family
  history for a specific disease

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Counseling Based on Mendelian Genetics and
Probability Rules

• ? Using family histories genetic counselors help
  couples determine the odds that their children
  will have genetic disorders

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Tests for Identifying Carriers

• ? For a growing number of diseases tests are
  available that identify carriers and help define
  the odds more accurately

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Fetal Testing

• ? AMNIOCENTESIS the liquid that bathes the fetus
  is removed and tested
• ? CHORIONIC VILLUS SAMPLING (CVS) a sample of
  the placenta is removed and tested

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