Basic Principles of Heredity

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Basic Principles of Heredity

• Packet 18

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Mendel
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Vocabulary Word Introduction

• Heredity
• Transmission of genetic information from parent
  to offspring
• Genetics
• The science of heredity
• Studies both genetic similarities and genetic
  variation

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Vocabulary II

• Genes
• Located on the chromosome
• Composed of DNA
• Locus
• The location of a gene on the chromosome
• Allele
• Different form, of a particular gene, that is
  located at a specific locus on a specific
  chromosome
• Allele is used when investigation two or more
  forms of a particular gene

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Allele
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Mendels Laws

• When Mendel carried out his research, the
  processes of mitosis and meiosis had not yet been
  discovered.
• Principle of Segregation
• During meiosis, the alleles for each locus,
  separate from each other
• When haploid gametes are formed, each contain
  only one allele for each locus
• Segregation of alleles is a direct result of
  homologous chromosomes separating during meiosis

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Mendels Laws

• Principle of Independent Assortment
• The random distribution of alleles, of different
  loci, into gametes
• Results in recombination
• The presence of new gene combinations not present
  in the parental (P) generation.
• Independent assortment occurs because there are
  two ways in which two pairs of homologous
  chromosomes can be arranged at metaphase I of
  meiosis.
• The orientation of homologous chromosomes on the
  metaphase plate determines the way chromosomes
  are distributed into haploid cells.

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Mendels Laws
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Mendels Laws
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Mendels LawLaw of Independent Assortment
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Vocabulary III

• Dominant Allele
• May mask the expression of the other allele known
  as the recessive allele
• There must be two alleles present
• Recessive Allele
• May only be expressed when paired with another
  recessive allele

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Homozygous vs. Hetereozygous

• Homozygous Dominant
• Two identical alleles that are in a dominant
  state
• Homozygous Recessive
• Two identical alleles that are in a recessive
  state
• Hetereozygous
• Two different alleles
• One dominant
• One recessive

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Genotype vs. Phenotype

• Genotype
• Composition of a specific region of DNA, in an
  individuals genome, that varies within a
  population
• The allele composition found within a cell
• Allows the expression of the phenotype
• Phenotype
• The physical effect of a particular genotype.

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Genotype vs. Phenotype
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Punnett Square

• Punnett Square
• A diagram used in the study of inheritance
• Shows the result of random fertilization in
  genetic crosses.

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Solving Genetics ProblemsTest/Monohybrid/Dihybrid
Cross

• Monohybrid Cross
• A cross, between parents (P generation),
  involving ONE allele
• Test Cross
• A cross between individuals of an unknown
  genotype and a homozygous recessive individual
• Still involving ONE allele
• Dihybrid Cross
• A cross, between parents (P generation),
  involving TWO alleles.
• The first generation of offspring
• F1 generation
• First filial
• The second generation of offspring
• F2 generation
• Second filial

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Punnett Square

• Example 1
• Sex determination

• Sex is determined by sex chromosomes
• X Y
• The Y chromosome determines male sex in most
  species of mammals
• The Y chromosome contains the SRY gene
• Sex reversal on Y gene

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Punnett Square

• Example 2
• Monohybrid cross

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Punnett Square

• Example 3
• Test Cross

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Punnett Square

• Example 4
• Dihybrid cross

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Blood Groups
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Multiple Alleles

• Three, or more alleles, can potentially occupy a
  particular locus.
• A diploid individual any two of the three alleles
• A haploid individual, or gamete, has only one

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Blood Groups II
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Rh Factor

• Determines whether someone has positive or
  negative blood
• A protein antigen that is on the surface of blood
  cells and if that antigen is present, the
  individual is positive
• A B O AB
• If the antigen is not present, then the
  individual is negative
• A- B- O- AB-

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Rh Factor II

• If an RH-negative mother is exposed to blood from
  an Rh-positive fetus, the mothers blood will
  produce antibodies that will attack the blood of
  the fetus--potentially killing the unborn child.
• This is why, blood types should be determined
  before having children
• If, the male and female are negative, and
  positive, the mother must receive medication to
  prevent her immune system from attacking the
  child.

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Punnett Square

• Example 5
• Blood Type Cross
• We WILL NOT be doing Punnett Squares involving
  the Rhesus factor.

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Incomplete Dominance

• Occurs when hybrids have an appearance between
  the phenotypes of the parental varieties.
• The hetereozygote is intermediate in phenotype
• Example
• The color between red and white
• Pink

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Incomplete Dominance
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Incomplete Dominance
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Punnett Square

• Example
• Incomplete Dominance

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Codominance

• Situation in which the phenotypes of both alleles
  are exhibited in a heterozygote
• Hetereozygote simultaneously expresses the
  phenotypes of both parents.
• Example
• Red Flower crossed with a White Flower
• The child will display flowers with red and white
  spots
• Both alleles are exhibited

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Punnett Square

• Example
• Codominance

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Epistasis

• Epistatis occurs when one gene alters the
  expression of another gene
• The genes are independent of each other

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Epistasis
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Linkage

• Each chromosome behaves genetically as if it
  consisted of genes arranged in a linear order
• Linkage is the tendency for a group of genes, on
  the same chromosome, to be inherited together via
  crossing over
• Therefore, groups of genes on the same chromosome
  are linked genes.
• Independent assortment does not apply if two loci
  are linked close together on the same pair of
  homologous chromosomes.
• Normally, they are passed on together.
• However, recombination of linked genes can result
  from crossing-over during Prophase I of Meiosis I

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Linked vs. Unlinked

• Recombination of unlinked genes Independent
  Assortment of chromosomes
• Recombination of Linked genes Crossing Over

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Linkage II

• Measuring the frequency of recombination between
  linked genes may provide an opportunity to
  construct a linkage map of a chromosome.

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Distinguishing Between Independent Assortment and
Linkage(Linked Genes)

• Perform a two-point test cross
• One individual must be hetereozygous for the
  linked genes
• One individual must be homozygous recessive for
  the both characteristics
• Linkage is recognized when there is an excess of
  parental type offspring (majority) and a
  deficiency of recombinant type offspring are
  produced in the two-point cross.

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Two Point Cross

• Parent 1
• BbVv
• Grey with normal wings
• Parent 2
• bbvv
• Black with vestigial wings

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Linked Genes
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Two-Point Cross
BV bv Bv bV
bv BbVv bbvv Bbvv bbVv
Expected Results 575 575 575 575
Actual Results 965 944 206 185

• Calculations
• Parental Genotypes
• 965 (42) 944 (41) 1909
• 1909/2300 83
• Recombinant Genotypes
• 206 (9)185 (8) 391
• 391/2300 17
• If independent assortment was to occur, the
  percentages would be 25 a piece.
• The recombinants arose because of crossing over

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Gene Mapping

• By measuring the frequency of recombination
  between linked genes, it is possible to construct
  a linkage map of a chromosome
• This is how scientists were able to develop a
  detailed genetic map of Neurospora (fungus),
  fruit fly, the mouse, yeast and many plants that
  are particularly important as crops

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Sex-Linked Genetics

• Sex is determined by sex chromosomes
• X and Y
• XX female
• XY male
• The X chromosome contains many important genes
  that are unrelated to sex determination
• These genes are required for both males and
  females
• A male receives ALL of his X-linked genes from
  his mother while a female receives her X-linked
  genes from both parents.

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Sex-Linked Genetics
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Female Mammals

• Display Dosage Compensation
• In females, only one of the two chromosomes is
  expressed in each cell
• Equalizes the expression of x-linked genes for
  both genders.
• The other allele is inactive
• Seen as a dark-staining Barr body at the edge of
  the interphase nucleus.
• A random event that occurs in each somatic cells
• A female that is hetereozygous expresses one of
  the alleles in about half her cells and the other
  allele in the other half

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Dosage Compensation II

• Mice and cats have several alleles that code for
  coat color on the x-chromosome.
• Females that are hetereozygous for such genes may
  show patches of one color in the middle areas of
  the other color.
• Variegation
• Not always visible in other circumstances
• May require special techniques

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Dosage Compensation
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Sex Linked Disorders
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X-Linked Recessive Disorder

• Males will show this trait if they have the
  recessive allele on the X chromosome
• Considered as hemizygous for the trait
• Females will show this trait if they have the
  recessive allele on both X chromosomes
• Homozygous recessive
• Hemophilia
• Inability to have clotting of blood
• xh
• Color blindness
• xc

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X-Linked Dominant Disorder

• Baldness
• XBXb
• This female will not go bald due to lack of
  testosterone
• XBXB
• This individual will start to lose her hair in
  the future

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Pleiotrophy

• The ability of one gene to have several effects
  on different characteristics.
• Normally, can be traced to a single cause
• Defective enzyme

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Autosomal Disorders

• Disorders caused by some form of alteration
  (mutation) on an autosome

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Autosomal DisordersHuntington Disease

• Caused by a rare autosomal dominant allele that
  affects the nervous system
• Gene found at one end of chromosome 4
• No symptoms appear until 30s and 40s
• Symptoms
• Uncontrollable muscle spasms
• Degeneration of the nervous system
• Personality changes
• Ultimately fatal 10-20 years after onset of
  symptoms
• No effective treatment has been found
• Problem with symptoms appearing in the 30s and
  40s
• These individuals have children of their own
  before the disease develops

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Autosomal DisordersSickle Cell Anemia

• Caused by a change in polypeptides found in
  hemoglobin
• Hemoglobin is the protein that carries oxygen in
  red blood cells
• The recessive allele causes the change in the
  polypeptide chain
• Individuals that are hetereozygous display
  co-dominance
• Both alleles are expressed
• Individuals are partially resistant to malaria
• Caused by Plasmodium, a protist (protozoan),
  carried by the Anepheles mosquito
• Mild Symptoms
• Fatigue (feeling tired)
• Paleness
• Jaundice (Yellowing of the skin and eyes)
• Shortness of breath

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Sickle Cell Anemia
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Autosomal DisordersPhenylketonuria (PKU)

• Autosomal recessive disorder
• Lack enzyme that converts amino acid
  phenylalanine to another amino acid
• Tyrosine
• The excess phenyalanine is converted to toxic
  phenylketones
• Damages the developing nervous system
• Can be screened for early in life and lifestyle
  changes made to prevent severe symptoms that
  result in mental retardation

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Autosomal DisordersCystic Fibrosis

• Autosomal recessive disorder
• Gene responsible for the disorder codes for a
  protein that transports chloride ions across cell
  membranes
• Defective protein, found in the epithelial cells
  lining the passageways of lungs, intestines,
  pancreas, liver, sweat glands ad reproductive
  organs result in the production of a thick mucus
• Leads to tissue damage
• What are some treatments available?

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Autosomal DisordersTay-Sachs Disease

• Autosomal recessive disorder
• Caused by abnormal lipid metabolism in the brain
• Results in blindness and severe mental
  retardation
• Symptoms begin in the first year and normally
  result in death before the age of 5 years.
• Lack of enzyme results in the inability to break
  down a lipid in the brain
• Lipids build in the lysosomes
• Lysosomes swell and burst causing the nerve cells
  to malfunction

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Ploidy
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Ploidy

• Degree of repetition of the basic number of
  chromosomes
• Diploidy
• Chromosomes repeat 2X
• Remember, in humans, you have one copy of a
  chromosome from the maternal father and one from
  the maternal mother

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Euploidy

• True ploidy
• Having 2 copies of each chromosome

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Polyploidy

• Definition
• The presence of multiple sets of chromosomes
• Common in plants but rare in animals
• Normally lethal in humans

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Aneuploidy

• Either missing, or having, extra copies of
  certain chromosomes.
• Trisomy
• Indicates the individual has an extra chromosome
• Monosomy
• Indicates that one member of a pair of
  chromosomes is missing

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Non-Disjunction

• Causes trisomy or monosomy
• Causes
• Homologous pairs fail to separate
• During Anaphase I of Meiosis I
• Sister chromatids fail to separate
• During Anaphase II of Meiosis II

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Sex Chromosome AneuploidyTurner Syndrome

• 2n - 1
• 45 XO karyotype
• 44 autosomes 1 X chromosome
• There is the absence of a sex chromosome
• No Barr bodies
• Female in appearance but their female sex organs
  do not develop at puberty and they are sterile
• Ovaries degenerate in late embryonic life
• Short in stature
• Shows normal intelligence but some cognitive
  functions are defective
• There are no Barr bodies
• Due to the lack of the other X chromosome

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Sex Chromosome AneuploidyKlinefelter Syndrome

• 2n 1
• 47 XXY karyotype
• 44 autosomes 3 sex chromosomes
• There is an extra X chromosome
• One Barr body per cell
• Male in appearance and they too are sterile
• Male with slowly degenerating testes
• Female type pubic hair pattern
• May have breast development

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Turner Syndrome vs. Klinefelter Syndrome
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Klienfelter Syndrome
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Sex Chromosome AneuploidyXYY karyotype

• Males that are usually fertile
• Some are unusually tall with heavy acne
• Others may have some mental disabilities
• Predisposition to be more violent in behavior
• Gametes never YY or XY--meiosis is normal
• After age of 35, extra Y chromosome often
  degenerates and is not passed onto offspring

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Sex Chromosome AneuploidyXXX karyotype

• Fertile females
• May be some mental disabilities
• Rare
• Eggs will produce only X after meiosis--not XX

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Autosomal AneuploidyDown SyndromeTrisomy 21

• Caused by an extra copy of chromosome 21
• There are three copies of chromosome 21 in their
  somatic cells
• 0.15 percent of all live births
• Growth failure and mental retardation
• Big toes widely spaced
• Congenital heart disease
• Mean life expectancy is about 17 years and only 8
  survive past age 40

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Trisomy 21
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Autosomal AneuploidyPatau SyndroneTrisomy 13

• Multiple defects
• Death is typical by the age of 3

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Autosomal AneuploidyEdwards SyndroneTrisomy 18

• Ear deformities
• Heart defects
• Spasticity and other damage
• Death is typical by the age of 1
• Some may survive longer

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Abnormalities in Chromosome Structure
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Abnormalities in Chromosome StructureDisorders

• The changes in the shape of the chromosome may be
  due to either of the following
• Translocation
• Deletions
• Fragile sites

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Translocation

• A chromosome fragment breaking off and attaching
  to a non-homologous chromosome
• Reciporcal translocation
• Two non-homologous pairs exchange genetic
  information
• Can result in deletion and/or duplication of genes

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Translocation Down Syndrome

• 4 of Down Syndrome cases
• Individuals actually have 46 chromosomes
• One of copies of chromosome 14 has combined with
  chromosome 21
• The large arm of chromosome 21 has been
  translocated to the large arm of another
  chromosome--usually chromosome 14

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Deletion

• The loss of part of a chromosome
• The abnormal chromosome is known as a deletion
• Sometimes chromosomes break and fail to rejoin

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Cri du Chat Syndrome

• Part of the short arm of chromosome 5 is deleted
• Breakage point varies from case to case
• Infants normally have a small head with altered
  features
• Moon face
• Infants have a distinctive cry that sounds like a
  cat mewing
• Infants normally survive childhood
• Exhibit severe mental retardation

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Fragile Sites

• Weak points at specific locations in chromatids
• Appears to be a place where part of a chromatid
  appears to be attached to the rest of the
  chromosome by a thin thread of DNA
• Have been identified on the X chromosome and
  certain autosomes

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Fragile X Syndrome

• Fragile site occurs near the tip of the X
  chromosome
• Where nucleotide triplet CGG is repeated many
  more times than normal
• Most common cause of mental retardation

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Genetic Screening Genetic Counseling

• Genetic Screening
• Identifies individuals who might carry a serious
  genetic disease
• Screening of newborns is the first step in
  preventative medicine
• Genetic Counseling
• Provide couples, concerned about the risk of
  abnormality in their children, medical and
  genetic information

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Screening
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Pedigrees

• Definition
• A family tree that shows the transmission of
  genetic traits within a family over several
  generations.
• Pedigree Analysis
• Useful in detecting autosomal dominant mutations,
  autosomal recessive mutations, X linked recessive
  mutations and defects due to genomic imprinting
• Genomic Imprinting
• Expressions of a gene based on its parental origin

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Pedigree Analysis
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Pedigree Analysis
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Homework

• Bioinformatics
• Proteomics
• Aminocentesis
• Chronic villus sampling (CVS)
• Preimplantation genetic diagnosis (PGD)
• Know how to discuss (argue for/against)
• Genetic discrimination
• The Human Genome Project