GENETICS: Understanding Patterns of Inheritance

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GENETICSUnderstanding Patternsof Inheritance
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PART IIINON-Mendelian Genetics
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• NON-MENDELIAN GENETICS 
• Despite the importance and power of Mendels
  work, there are important exceptions to some of
  his principles.
• Examples
• Not all genes have only a dominant or recessive
  form.
• A majority of genes have two or more alleles.
• Many important traits are controlled by more than
  one gene.
• If 2 genes are close together on the same
  chromosome, they may always move together (no
  independent assortment for 2 such alleles)

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• Incomplete Dominance One allele is not able to
  completely silence the other.
• Produces intermediate outcomes
• Example a red flower crossed with a white flower
  produces pink flowers.
• RR red flowers WW white flowers
  RW pink flowers

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Incomplete DominanceSnap-dragon Flower Color
No Dominant or recessive alleles
Both alleles are always expressed.
Intermediate outcomes (blending) result.
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• Codominance Both alleles are always expressed as
  phenotype
• PProduces mixed outcomes
• EExample a red flower crossed with a white
  flower produces flowers that are speckled (red
  and white)
• RR red flowers WW white flowers
  RW flowers with pure red pure white areas
• (True-breeding White Rooster Red Hen Dappled
  Young)

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MULTIPLE ALLELES

• Several alleles (variations in a gene) are
  possible for a single trait.
• This does not mean that an individual can have
  more than two alleles, just that more than two
  alleles are available in the population.
• Produces a spectrum of outcomes.

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EXAMPLE (OF MULTIPLE ALLELES)

• A A hypothetical plant population has 3 different
  alleles for color, red, white, and blue. Red is
  dominant to white red and white are both
  dominant to blue.
• AAlleles R or rW or rb

RR  
RrW All produce red flowers
Rrb  
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EXAMPLE (OF MULTIPLE ALLELES, contd.)
rw rw Produce white flowers rw rb
rb rb Produces blue flowers
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(Blood Types are controlled by a gene with
multiple alleles that exhibit codominance)
MULTIPLE ALLELES PLUS CODOMINANCE The Special
Case of Human Blood Typing
Multiple Alleles usually work like this
T gt tA gt tb (that
is, the first allele is dominant to the second
and third, while the second is only dominant
to the third)

• But in some cases (like human blood typing)
  IA IB gt i
• Note that superscripts are both capitalized
  having either A or B form causes you to always
  make one cell surface marker and another
  antibody.
• TWO copies of i with no dominant allele to cover
  them results in NO PROTEIN AT ALL (blood type O)

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Blood Type Genotype(s) Description
A IA IA or IA i Makes cells with A markers only makes antibodies to B surface proteins on blood cells
B IB IB or IB i Makes cells with B markers only antibodies to A surface proteins
AB IA IB only Makes cells with both A and B surface markers makes no antibodies (universal recipient)
O i i only Makes cells with neither A nor B surface markers makes antibodies to both A and B cells
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• Polygenic traits
• Phenotype is determined by several genes
  working together

• Genes may be on the same chromosome or on
  separate chromosomes

• Example height is a single trait, but determined
  by many genes
• Length of legs Length of torso
  Length of neck, etc.

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Human Skin Color a Polygenic Trait
3 genes are involved all work together to
determine phenotype.
Each gene has a dominant and recessive form.A or
a B or b C or c
Each human is conceived with 6 alleles for this
trait (3 from each gamete).
A wide range of graded outcomes result.
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One unusual polygenic process is epistasis

• One gene acts as a switch enables or inhibits
  expression of another

• Example coloring of Labrador retrievers.

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Epistasis One Allele acts as a Switch for
Another
Epi over stasis standingepistasis
standing over
Example Purple vs. white pea plant
flowers. C or c stands on the purple/white
allele (controls its expression).
Purple (P) is dominant but controlled by C.
So a flower can only be white if the plant is
PP or Pp and this outcome is accompanied by
cc (OR if pp combination occurs).
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Linked Genes Genes located on the same
chromosome especially those close to
each other

• Even closely linked genes separate sometimes (due
  to cross-over during meiosis).

• Relative frequency of gene separation during
  meiosis may be used to map exact location of a
  specific base sequence

• Genes located farther apart separate more
  frequently.
• Disease genes (such as Huntingtons) are
  sometimes linked to marker genes tracked this
  way.

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X-Linked (Sex-linked) Traits Some genetic
traits are expressed in males only (or rarely
expressed in females)

• Example White eyes in fruit flies

• Genetic coding for these traits is usually
  recessive Coding is carried on X chromosome
  has no counterpart on Y

• Phenotype occasionally seen in females (if they
  receive 2 copies of allele)

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Human Examples of Sex-linked Traits
hemophilia red-green color blindness
Huntingtons disease.
Barr-Bodies

• The cells of all female Humans de-activate one
  X chromosome (Occurs randomly unpredictable and
  arbitrary for each cell)
• De-activated chromosome visible as
  darkly-staining object in nucleus does not
  synthesize proteins.

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Nondisjunction (Failure of chromosomes to
separate completely during production of
gametes)

• Results in missing or multiple copies of genes
  (or sometimes of entire chromosome) in cells of
  offspring

• Trisomy one gamete brings 2 copies of
  chromosome zygote has 3
• Monosomy one gamete brings NO copy of coding
  zygote ends up with only 1

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Examples of Nondisjunction

• Down Syndrome trisomy in chromosome 21, results
  in developmental cognitive problems

• XXY (Klinefelter Syndrome) incomplete sexual
  development, rangy males

• XO (Turner Syndrome) short, undeveloped females

• XXX Syndrome limited fertility, cognitive
  developmental deficits.

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End of PART IIINON-Mendelian Genetics