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Extensions of Mendelian Genetics

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Extensions of Mendelian Genetics Mendelian genetics seems to be relevant to only a small set of heritable features For only a few characters there are – PowerPoint PPT presentation

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Title: Extensions of Mendelian Genetics


1
Extensions of Mendelian Genetics
  • Mendelian genetics seems to be relevant to only a
    small set of heritable features
  • For only a few characters there are
  • Only 2 versions of an allele (green or yellow)
  • 1 gene codes for a single external character
  • 1 allele is completely dominant to the other
  • The basic patterns of segregation independent
    assortment apply to more complex patterns of
    inheritance

2
Different Types of Dominance
  • Complete dominance
  • Phenotype of heterozygote HomoD are
    indistinguishable
  • The pattern with which you are already familiar
  • Incomplete dominance
  • Phenotype of heterozygote is in between the 2
    Homo phenotypes
  • Example pink snapdragons

3
Snapdragons Incomplete Dominance
4
So incomplete dominance does NOT provide evidence
for blending theories
5
Codominance
  • Codominance
  • Phenotype of heterozygote is separate
    distinguishable from Homozygous Dominant
    Homozygous Recessive
  • Example AB blood type or Rhododendron flower

6
Dominance Phenotype
  • The observed dominance/recessiveness of alleles
    depends on the level of the investigation
  • Consider Tay-Sachs disease
  • Brain cells of the baby do not metabolize certain
    lipids
  • As lipids accumulate, seizures, blindness, and
    mental degeneration
  • Death occurs within a few years of conception

7
Tay-Sachs Disease
  • At the Organismal level, the disease is recessive
  • Only children with 2 copies of the recessive
    trait will have the malady
  • Heterozygote is not afflicted they produce some
    lipid-metabolizing enzyme, though not as much as
    in HomoD
  • So intermediate enzyme production
  • This suggests that _at_ the biochemical level, the
    disease is an example of incomplete dominance
  • Which is Tay-Sachs dominance or incomplete
    dominance?

8
Prevalence Dominance
  • Polydactyly
  • Extra fingers or toes
  • 1 of 400 in the US
  • The allele for polydactyly is dominant, but
    rarely present
  • Recessive homozygotes (HomoR) are found 399 out
    of 400 instances

9
Blood Typing
  • Only 2 alleles existed for Mendels peas, but
    this is not typical for most traits
  • Consider ABO blood group in humans
  • A refers to the A membrane carbohydrate type
    A blood
  • B refers to (seriously, Im not writing this
    down)
  • O means neither A or B carbohydrate is found
  • AB means both A B are found
  • BUT the A and B alleles are codominant and are
    both expressed if an individual inherits both
    alleles

10
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11
Epistasis
  • A gene at one locus alters the phenotypic
    expression of a gene at a second locus
  • Example Mouse fur color
  • Bb or BB Black bb brown
  • If HomoR for (C) gene cc, then no fur color
    (albino or white)
  • Regardless of fur color specified by brown-black
    gene
  • If NOT HomoR for (c) gene Cc or CC, then can be
    brown (bb) or black (Bb or BB)

12
What is the phenotype of
  • BBcc?
  • BbCc?
  • bbCC?
  • Bbcc?
  • BBCC?
  • bbcc?

13
Pleiotrophy
  • Single gene has multiple effects
  • Should be unsurprising given intricate molecular
    and cellular interactions for development of an
    organism
  • Phenylketonuria
  • Mental Retardation
  • Reduced skin and hair pigmentation

14
Polygenic Inheritance
  • The additive effect of 2 or more genes on 1
    phenotypic character
  • Called quantitative characters since there is a
    continuum of gradations
  • Normal curve of phenotypes
  • Example human skin pigmentation is determined by
    at least 3 separately inherited genes
  • AABBCC Dark
  • AaBbCc Intermediate
  • aabbcc Light

15
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16
Pedigree Analysis
  • Family tree describing the interrelationships of
    parents children across the generations

17
Recessively Inherited Disease
  • Requires 2 copies of the recessive allele
    (Homozygous Recessive) to express the mality
  • Heterozygotes are called carriers
  • Normal phenotype, but may transmit disease to
    offspring
  • Examples
  • Cystic Fibrosis
  • Tay Sachs disease
  • Sickle-cell disease

18
Cystic Fibrosis
  • Recessive autosomal disease
  • Common in those of European descent
  • 1 of 2,500 affected, but 1 of 25 are carriers
  • Affects Chloride ion transport between a cell and
    extracellular fluid
  • If untreated, most die before 5th birthday
  • Typically, patients live until their 20s or 30s
    with efficacious treatment

19
Sickle-Cell Anemia
  • Recessive autosomal disease
  • African descent
  • Affects Hemoglobin protein in RBCs
  • Low blood oxygen hemoglobin molecules clump
    together forming sickle shaped RBCs
  • Sickle-celled RBCs clump together creating
    chronic vascular occlusion of small vessels
  • Example of incomplete dominance
  • Heterozygotes are usually normal but will show
    some symptoms during prolonged periods of reduced
    blood oxygenation

20
Dominant Alleles
  • Most harmful alleles are recessive, but some
    human diseases are due to dominant alleles
  • Only require one copy of the allele to be
    expressed
  • Hypothesis if there is a lethal disease carried
    on a dominant allele, it would have burned out
    its carriers by now.
  • UNLESS, the lethal disease carried by a dominant
    allele is one that affects organisms of advanced
    age
  • Like Huntingtons disease

21
Examples of Dominant Allele Disease
  • Achondroplasia form of dwarfism
  • Heterozygous individual dwarf
  • 1 in 25,000 have achondroplasia, so 99 of the
    population are HomoR
  • Huntingtons disease
  • Caused by a lethal dominant allele
  • Degenerative disease of nervous system
  • Usually only affects those gt 40 yrs old

22
Genetic Testing
  • Pedigree analysis gives some info about risk to
    offspring
  • There are also tests to identify carriers of
    certain genetic diseases
  • Amniocentesis amniotic fluid is removed and
    then cells contained in the fluid are cultured to
    identify certain chromosomal defects via
    karyotype
  • Chorionic villus sampling (CVS) placental
    tissue is removed for same purpose as amnio, but
    results are available far faster, can be
    performed earlier in pregnancy
  • Karyotype can be immediately obtained

23
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