Patterns of Heredity and Human Genetics

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Patterns of Heredity and Human Genetics

• Chapter 12

• Objectives
• Interpret a pedigree
• Determine human genetic disorders that are caused
  by inheritance of recessive alleles
• Predict how a human trait can be determined my a
  simple dominant allele

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Mendelian Inheritance of Human Traits

• How can we trace traits in our family?
• Geneticists use a tool called a pedigree (graphic
  representation of genetic inheritance)

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• Male
• Female
• Mates
• Heterozygous or carrier

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• I, II, III represent generations
• pedigrees are used by genetic counselors to help
  trace genetic disorders and other traits

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Simple Recessive Heredity

• most genetic disorders are caused by recessive
  alleles (rare) you have to receive 2 alleles, 1
  from mom and 1 from dad in order to inherit the
  disorder

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Examples

• Cystic Fibrosis (CF)
• 1/2000 are affected by CF, 1/200 are carriers
• characterized by a defective protein that causes
  mucas to build up in the lungs

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• Tay Sachs Disease
• common in Ashkenazi Jews
• characterized by the absence of an enzyme that
  breaks down fat in the brain
• most children die by the age of 5

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Simple Dominant Heredity

• only need a single dominant allele for the trait
  to be expressed

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• examples of dominant traits in humans
  unattached ear lobes, tongue rolling, hitchhikers
  thumb, mid digit hair

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Examples of dominant genetic disorders

• Huntingtons Disease
• doesnt affect individuals until their 30-50
  years of age
• characterized by the breakdown of areas of the
  brain

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12-2 Whey Heredity follows different rules

• Distinguish between incompletely dominant and
  codominant alleles
• Compare multiple allelic and polygenic
  inheritance
• Analyze the pattern of sex-linked inheritance
• Summarize how internal and external environments
  affect gene expression
• SCS

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Complex Patterns of Heredity

• most traits observed are not a result of simple
  dominant/ recessive inheritance

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Types

• Incomplete dominance- heterozygous condition
  results an intermediate phenotype or mixing
  neither allele is completely dominant over the
  other

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• Example Snap dragons. Homozygous Red and white
  flowers are crossed, and they produce all pink
  offspring

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• Codominance- both alleles are expressed equally
• Example Cows. Homozygous Red and white cows
  are crossed (mated), and they produce offspring
  that appear pink from a distance because they
  both red and white hairs

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• Multiple Alleles- traits controlled by two or
  more alleles

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• Sex linked inheritance- traits controlled by
  genes located on the sex chromosomes
• Thomas Morgan was the first to discover
• Worked with fruit flies

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Problems involving sex linkage

• In fruit flies, red eyes are dominant over white
• example of a cross
• XRXr x XrY

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Polygenic Inheritance-

• traits that are controlled by more than 1 gene
• Genes might be on the same or different
  chromosomes

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• Other factors influence the expression of genes
• Environmental Influences
• External Environmental (temp. nutrition, light,
  chemicals, infectious agents)
• Internal Environment (hormones, age)
• genetic makeup determines the potential to
  develop and function

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12-3 Complex Inheritance of Human Traits

• Compare codominance, multiple alleleic,
  sex-linked, and polygenic patterns of inheritance
  in humans
• Distinguish among conditions in which extra
  autosomal or sex chromosomes exist

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Complex Inheritance of Human Traits

• Codominance-
• Sickle Cell Anemia
• most common in black Americans

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• Characterized by the sickling of red blood cells,
  which hampers circulation and affects oxygen
  levels of the blood
• In malaria prone areas people with this gene are
  protected against malaria

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• Homozygous conditions results in full blown SC,
  and medication has to be taken
• Heterozygous condition results in a mild case,
  may not need medication

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Multiple Alleles

• Blood types
• There are 4 blood types in humans (A, B, AB, and
  0)
• the type of marker (protein on your red blood
  cells determine your blood type)

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• Phenotypes Genotypes
• Type A IAIA or IA i
• Type B IBIB or IBi
• Type AB IAIB
• Type O ii

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• knowing your blood type is important
• blood transfusions, child birth, blood donations

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• Type Blood
• Can Receive
• A A, O
• B B, O
• AB AB, O, A, B Universal recipient AB
• Universal Donor O

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Sex Linkage

• Some human traits are determined by genes found
  on the sex chromosomes (mostly X)
• Examples of sex linked traits
• Red/Green Color Blindness- cant differentiate
  red from green
• recessive disorder
• More males have it because they only need one
  allele

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• Hemophilia
• bleeders disease blood does not clot naturally
  so a person can bleed to death
• 1/10,000 males are afflicted, only 1/ 100 million
  females are affeced

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Polygenic Traits

• example include skin color, eye color,

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Changes in chromosome number

• Normally humans have 46 chromosomes, but
  sometimes during replication, chromosomes do not
  separate correctly

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• Geneticist use a tool called a karyotype to
  determine if an extra chromosome is present
• Take a sample of a persons DNA and line up the
  metaphase chromosomes

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Examples of Disorders

• Downs Syndrome (Trisomy 21)
• a result of an extra chromosome in the 21st pair
• some degree of mental retardation

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• Turners Syndrome
• XO, female, infertile
• Kleinfelters
• XXY, Male, infertile