Complex Inheritance

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Complex Inheritance
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Simple Recessive Heredity

• Most genetic disorders are caused by recessive
  alleles the person must be homozygous recessive
  in order to express the disorder
• Cystic Fibrosis
• Tay-Sachs
• PKU

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Simple Dominant Heredity

• Only one dominate allele must be present for the
  person to express the phenotype
• Some examples of traits that follow this pattern
  are
• Hitchhikers Thumb
• Tongue rolling
• Free earlobes
• Mid-digital hair

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Incomplete Dominance

• Phenotype of the heterozygote is an intermediate
  between the two homozygous parents
• Neither allele is dominantthe alleles blend
  together if offspring is heterozygous

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Incomplete Dominance

• In snapdragons, neither red or white is
  completely dominant
• Red is represented by R and white by R (R prime)
• RRred flowers
• RR white flowers
• RRpink flowers

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Incomplete Dominance in Humans

• Curly hair is incompletely dominant
• HH straight hair
• HH curly hair
• HH wavy hair

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Codominance

• Phenotypes of both homozygous parents are
  expressed in the heterozygous offspring in equal
  amounts
• BOTH alleles are dominant AND BOTH alleles will
  be seen if genotype is heterozygous

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Codominance

• Chickens with black feathers are homozygous for
  the B allele (BB)
• Chickens with white feathers are homozygous for
  the W allele (WW)
• A heterozygous chicken (BW) would have black and
  white feathers

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Codominance in Humans

• Sickle Cell Anemia
• Common in African-American populations
• Hemoglobin crystallizes, changing the shape of
  RBC
• Sickle shaped cells slow blood flow and block
  small vessels
• Sickle cells have short life span causing low
  blood cell count and anemia

10
Multiple Alleles

• The presence of 3 or more alleles for a genetic
  trait
• 3 or more genotypes are possible for one traitso
  you get 3 or more phenotypes are possible for one
  trait
• Ex Pigeons
• There are three alleles that code for feather
  color, but each individual can have only two.
• BAash red gt Bgrey gt b brown

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Multiple Alleles

• BABA or BAB Ash red feathers
• BB or Bb Grey feathers
• bb Brown feathers

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Multiple Alleles in Humans

• In humans, there are 3 alleles that code for
  blood type
• 3 alleles (IA, IB, and i) determine 4 blood types
  (A, AB, B, and O).
• IAIA or IAi result in type A blood
• IBIB or IBi result in type B blood
• IAIB results in type AB blood
• ii results in type O blood
• If a person is given the wrong blood during a
  transfusion, incompatible blood cells can clump
  together and cause death.

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Sex-Linked Inheritance

• Traits are controlled by genes located on sex
  chromosomes
• Sex chromosomes determine sex
• XX female
• XY male
• Y chromosome doesnt have same genes as X
• Disorders affect males more than females

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Sex-linked Inheritance

• Ex Fruit flies
• XRRed Eyes
• XrWhite Eyes
• Females
• XRXR or XRXr Red Eyes
• XrXr White Eyes
• Males
• XRY Red Eyes
• XrY White Eyes

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Sex-Linked Inheritance in Humans

• Red-green color blindness
• Person cannot tell the difference between red and
  green.
• Recessive allele affects the red and green
  receptors in the eye
• Inherited on the X chromosome

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Sex-Linked Inheritance in Humans

• Hemophilia
• Missing clotting factor so cannot stop bleeding
• Treatments include clotting factor injections and
  blood transfusions
• Inherited on the X chromosome

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Polygenic Inheritance

• Inheritance pattern of a trait controlled by two
  or more genesgenes may be on the same chromosome
  or different chromosomes
• 2 or more genes control the trait
• Genes can be on different chromosomes

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Polygenic Inheritance in Humans

• Involves 3-4 different genes
• Offspring look like an intermediate or
  combination of parents skin color
• Eye and Hair Color are also polygenic

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Environmental Influences

• Temperature
• Siamese Cats body temp determines fur color
• Warm areaswhite fur
• Cooler areasblack/brown fur
• Light
• Sun exposure increases melanin production,
  resulting in tan skin
• Chemicals
• Pesticide use can result in mutations and
  resistance

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Environmental Influences

• Hormones
• Feathers of peahens versus peacocks
• Females are dull and males have bright,
  attractive plumage
• Sex changing fish
• Age
• Cells cannot divide indefinitely, eventually
  cannot repair themselves