INHERITANCE PATTERNS AND HUMAN GENETICS

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CHAPTER 12

• INHERITANCE PATTERNS AND HUMAN GENETICS

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SEX DETERMINATION

• XX FEMALE
• XY MALE
• MALE GAMETE DETERMINES SEX AT FERTILIZATION

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SEX LINKAGE

• Thomas Hunt Morgan worked with fruit flies
  confirmed that  genes were on chromosomes a.
  Fruit flies are cheaply raised in common
  laboratory glassware b. Females only mate once
  and lay hundreds of eggs c. Fruit fly
  generation time is short, allowing rapid
  experiments

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• Experiments involved fruit flies with XY system
  similar to human system
• Besides genes that determine sex, sex chromosomes
  carry many genes for traits unrelated to sex

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• X-linked gene is any gene located on the X
  chromosome that is not on the Y chromosome
• X-linked alleles are designated as superscripts
  to X chromosome

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• Heterozygous females are carriers that do not
  show the trait but can pass it on
• Males are never carriers but express the one
  allele on the X chromosome- so they have the
  condition

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Examples of x-linked conditions

• Red-green color-blindness is X-linked recessive
• In humans, another well-known X-linked traits is
  hemophilia (free bleeders that lack clotting
  factors in their blood)
• One of the most famous genetic cases involving
  hemophilia goes back to Queen Victoria who was a
  carrier for the disorder and married Prince
  Albert who was normal
• Their children married other royalty, and spread
  the gene throughout the royal families of Europe

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PEDIGREES

• Also called a family tree
• Squares represent males and circles represent
  females
• Horizontal lines connecting a male and female
  represent mating
• Vertical lines extending downward from a couple
  represent their children
• A shaded symbol means the individual possess the
  trait
• Half-shaded symbols are carriers

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Example Sex-Linked Problems

• 1. What are the results of crossing a colorblind
  male with a female carrier for colorblindness?
• Trait     Red-Green ColorblindnessAlleles    
• XC    normal vision
• Xc    colorblindness
• XCXc       x    Xc Y

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• Genotypes   XCXC ,XCY, XCXc, XcY, Genotypic
  Ratio 1111
• Phenotypesnormal vision female, normal vision
  male, female carrier, colorblind male

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• What are the results of crossing a colorblind
  male with a colorblind female? 
• XC    normal vision
• Xc    colorblindness
• XcXc       x    Xc Y

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• Genotypes      XcXc , XcY 
• Genotypic Ratio 11 ratio
• Phenotypes      colorblind female,
  colorblind male
• Phenotypic ratio 11 ratio

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SINGLE-ALLELE TRAITS

• Controlled by a single allele of a gene
• Example
• Huntingtons disease (HD)- dominant allele on an
  autosome
• Look at table 12-1

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POLYGENIC TRAITS

• A trait controlled by 2 or more genes lots of
  variation
• Example skin, hair, eye color

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SEX INFLUENCED TRAITS

• Presence of male or female sex hormones influence
  the expression of these
• Male and females have different phenotypes even
  if same genotype
• Gene expression occurs because of presence of
  certain hormones
• Ex patterned baldness and testosterone

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SINGLE-ALLELE TRAITS

• Individual has to have 2 copies of the gene-
  homozygous recessive
• Examples
• Cystic fibrosis
• Sickle cell anemia

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MULTIPLE ALLELE TRAITS

• Controlled by 3 or more alleles of the same gene
  that code for a single trait-ABO blood type. A
  and B are codominant to each other and dominant
  to i
• IAIA A
• IAi A
• IBIB B
• IBi B
• IAIB AB
• ii O

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LINKED GENES

• Each chromosomes has 1000's of genes
• All genes on a chromosome form a linkage group
  that stays together except during crossing-over
• Some genes located on the same chromosome tend to
  be inherited together 

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• Linked genes were discovered by Thomas Hunt
  Morgan while studying fruit flies
• Linked alleles do not obey Mendel's laws because
  they tend to go into the gametes together
• Crosses involving linked genes do not give same
  results as unlinked genes

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Chromosome Mapping

• Recombinants result from chromosome crossing over
  during prophase I of meiosis
• Geneticists can use recombination data to map a
  chromosome's genetic loci (position on a
  chromosome)
• A genetic map lists a sequence of genetic loci
  along a particular chromosome

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• Alfred Sturtevant, a student of Morgan, reasoned
  that different recombination frequencies reflect
  different distances between genes on a chromosome
  
• The farther apart genes are, the greater
  likelihood of crossing-over
• The closer together two genes are, the less
  likely of crossing-over occurring

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• A map unit equals 1 recombination frequency
• If 1 of crossing-over equals one map unit, then
  6 recombinants reveal 6 map units between genes
• To determine the frequency of recombinants, the
  following formula is used
• Recombination Number of recombinants x
  100
• Frequency   ---------------------------
  --------      Total Number of
  Offspring

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Chromosome Mutations

• Mutations are changes in genes or chromosomes
  that can be passed on to offspring
• Mutations increase the number of variations that
  occur
• Chromosomal mutations include changes in
  chromosome number and/or structure
• Monosomy occurs when an individual has only one
  of a particular type of chromosome
• Turner syndrome (X0) is an example of monosomy

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• Trisomy occurs when and individual has three of a
  particular type of chromosome
• Examples of trisomy include Klinefelter's
  Syndrome (XXY) and Down Syndrome or Trisomy 21
  where the individual has three 21st chromosomes
• Both monosomy trisomy result when chromosomes
  fail to separate during meiosis called
  nondisjunction
• Monosomy and trisomy (aneuploidy) occur in plants
  and animals and may be lethal (deadly)
• Polyploidy where the offspring have more than two
  sets of chromosomes occurs often in plants (3n,
  4n )

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• Environmental factors including radiation,
  chemicals, and viruses, can cause chromosomes to
  break causing a change in chromosomal structure
• Inversion occurs when a piece of a chromosome
  breaks off reattaches to the same place but in
  the reverse order
• Translocation occurs when a chromosome segment
  breaks off attaches to a different chromosome
• Deletions occur when the end of a chromosome
  breaks off is lost

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• Cri du chat syndrome (results in retardation a
  cat-like cry) is due to a deletion of a portion
  of chromosome 5
• Duplications occur when a section of a chromosome
  is doubled
• Fragile X Syndrome caused by an abnormal number
  of repeats (CCG) results in retardation long,
  narrow face which becomes more pronounced with
  age

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Child with Fragile X syndrome
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Gene Mutations

• Change in genes caused by change in structure of
  the DNA
• DNA bases may be substituted, added, or removed
  to cause gene mutation
• When genes are added or removed, the mutation is
  called a frame shift mutation

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• Adding or Removing genes is called a point
  mutation

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• Sickle cell anemia (red blood cells are C-shaped
  so can't carry as much oxygen) is an example of a
  gene mutation in African Americans

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• Tay-Sachs (a disorder where the nervous system
  deteriorates) is a fatal gene mutation in Jewish
  people of Central European Descent
• Phenylketonuria or PKU occurs from the inability
  of a gene to synthesize a single enzyme necessary
  for the normal metabolism of phenylalanine and
  results in death