Complex Patterns of Heredity and Human Genetics

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Chapter 8

• Section 4
• Complex Patterns of Heredity and Human Genetics

P.S. Most of this is not in your textbook
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Human Genetics - Pedigrees

• Genetic crosses are difficult to complete in
  humans.
• Geneticists have to rely on family trees,
  pedigrees, which can be constructed to show the
  inheritance of a particular trait in a family.

Pedigree for albinism
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Complex Control of Inheritance

• Although Mendelian genetics can explain
  inheritance of some traits (ex. height in peas),
  there are many traits that dont obey the same
  simple rules.

• The following patterns exist
• Polygenic inheritance
• Intermediate inheritance (incomplete dominance)
• Codominance
• Multiple Alleles
• Environmentally-influenced traits
• Sex-linked traits

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Polygenic Inheritance

• Some traits are determined by multiple genes
  polygenic inheritance
• Examples -
• Height in humans
• Skin color in humans
• Eye color in humans
• There is much more variability in the population
  for these traits
• they show a bell curve type distribution.
• It is much more difficult to predict the results
  of crosses in polygenic traits

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Incomplete Dominance

• In a case of complete dominance, when two parents
  with opposite traits are crossed, all of the
  offspring will show the dominant trait (ex. tall
  x short 100 tall).
• In contrast, some traits show incomplete
  dominance, where the offspring of parents with
  opposite traits produce offspring with an
  intermediate phenotype.
• Examples
• Plumage color in chickens (black white blue)
• Flower color in snapdragons (red white pink)

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Intermediate Inheritance
NOTE Some books use C with a superscript letter
to represent alleles in cases of intermediate
inheritance and codominance. We have chosen to
simplify this method and use two different
capital letters to indicate a lack of dominance.
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Intermediate Inheritance Problem

• Snapdragon flowers occur in three colors, white,
  red and pink.
• Alleles
• R and W
• The three phenotypes are
• White - WW
• Pink - RW
• Red - RR
• What are the genotypic phenotypic ratios of the
  offspring of a cross between red pink parents?

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Intermediate Inheritance Problem

• What are the genotypic phenotypic ratios of the
  offspring of a cross between red pink parents?
• G 50 RR, 50 RW
• P 50 red, 50 pink

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Codominance

• In some traits, when parents with opposite traits
  are crossed, the offspring express both alleles.
• In codominance, neither allele is dominant, nor
  recessive, nor do the alleles blend in the
  phenotype.
• Examples-
• Coat color in horses
• Blood type in humans

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Codominance Examples

• Coat color in horses
• Two alleles R (red - chestnut) and W (white)
• Three phenotypes
• White - WW
• Roan - RW
• Red - RR
• If two roan horses are crossed, what are the
  genotypic and phenotypic ratios of the offspring?

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Codominance example

• If two roan horses are crossed, what are the
  genotypic and phenotypic ratios of the offspring?
• G 25 R R 50 RW 25 WW
• P 25 red 50 roan 25 white

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Multiple Alleles

• Some traits are determined by genes with more
  than two (multiple) alleles.
• Blood type
• In humans, there are four blood types (A, B, AB,
  and O).
• These blood types reflect markers found on these
  cells.
• There are three alleles that determine these
  markers.
• A A markers
• B B markers
• O no marker
• A and B - are codominant
• O - is recessive to both

OO AA or AO BB or BO AB
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Blood type examples

• A woman with blood type AB has children with a
  man with O blood. What will be the genotypic and
  phenotypic ratios of their offspring?

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Blood type examples

• A woman with blood type AB has children with a
  man with O blood. What will be the genotypic and
  phenotypic ratios of their offspring?
• G 50 AO, 50 BO
• P 50 A, 50 B

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Blood types

• Your body can accept blood that has markers that
  your bodys immune system recognizes.
• Your body will reject blood that has any marker
  that are foreign to your body.

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Environment-Influenced traits

• The environment influences how many traits are
  expressed.
• For example
• Height in humans (depends on health/nutrition
  while growing)
• Shape of plants /color of flower
• Coat color in Siamese cats

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Autosomal vs. Sex-linked traits

• If a gene is autosomal, it will appear in both
  sexes equally
• Genes found on a sex chromosome are called
  sex-linked traits.
• Most of these traits (X-linked) are found on the
  X chromosome because it contains so much more DNA
  than the Y chromosome.
• Females (with 2 Xs) have two copies of these
  alleles males (with 1 X and 1Y) have only one
  copy of the allele. So, these diseases are most
  commonly expressed in males because males express
  any allele they have.
• Examples
• Eye color in fruit flies
• Color blindness in humans
• Hemophilia in humans

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Sex-linked traits - Eye color in fruit flies

• Two alleles (on Xs)
• XR (red) and Xr (white)
• Y
• Three female genotypes
• XR XR - red
• XR Xr - red
• Xr Xr - white
• Two male genotypes
• XRY- red
• XrY- white

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Fruit Fly Eye Color Problem

• If you cross a homozygous red-eyed female with a
  white eyed male, what are the genotypic and
  phenotypic ratios of their offspring?

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Fruit Fly Eye Color Problem

• G 50 XRXr, 50 XRY
• P 100 females are red
• 100 males are red
• What will happen if you then cross the F1? (new
  problem on next page!)

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Fruit Fly Eye Color Problem

• If you cross a heterozygous red-eyed female with
  a white eyed male, what are the genotypic and
  phenotypic ratios of their offspring?
• G 25 XRXr, 25XrXr, 25 XRY, 25 XrY
• P 25 red females,
• 25 white females
• 25 red males,
• 25 white males

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Colorblindness

• Red-green colorblindness is a common sex-linked
  disorder that involves the malfunction of
  light-sensitive cells in the eyes.

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Pedigree of Queen Victorias Family - Hemophilia
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Detecting Genetic Disorders

• Pregnant women can have prenatal testing to
  determine the health of the fetus genetic
  screening.
• Amniocentesis and chorionic villi sampling both
  involve taking cells from tissues around fetus to
  determine chromosome number.

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Genetic Errors

• A mutation is any change in the nucleotide
  sequence of DNA in an organism.
• A mutation may
• change the structure of the whole chromosome by
  involving many nucleotides (chromosomal)
• Or it may alter only a single nucleotide (point)
• Larger errors, such as change in chromosome
  number (like trisomy 21) or chromosomal errors,
  are easier to detect with a karyotype than are
  small (point) mutations.

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An interesting genetic family

• The Fugates of Kentucky lived for many years in
  relative isolation in the Appalachian Mountains
  and several of them were blue.
• The matriarch of the family, Mary Fugate, who
  moved to Kentucky in the 1800s, was a carrier of
  a rare disease that prevented her blood from
  carrying the usual amount of oxygen.
• The disease, Methemoglobinemia, is genetically
  inherited and causes an enzyme deficiency that
  results in cyanosis, "a bluish color to the
  skin."

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An interesting genetic family

• Over the course of time, due to the isolation of
  the family, several of the Fugates intermarried.
  This caused the normally recessive disease to
  occur with greater frequency.
• Despite the fact that their blue appearance
  didn't otherwise affect their health, a doctor
  prescribed "methylene blue." The chemical allowed
  a second enzyme to do the work of the missing one
  and caused the Fugates to pink right up!

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Fugates