Modern

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• Modern
• Mendelian
• Genetics

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Gene Expression

• Every organism has at least two alleles that
  govern every trait- one from the mother and one
  from the father- to the offspring.
• The genes encode information that is expressed as
  traits of the organism, which is called gene
  expression.
• A single gene (one set of alleles) may control
  one or several traits.

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• In any given cell within an organism, only some
  of the genes are expressed, while all other genes
  are inactivated.
• Genes that are on are expressed
• Genes that are off are not expressed.
• Mechanisms that can switch genes on and off
  include
• 1. Intracellular chemicals
• 2. Enzymes
• 3. Environment

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Heredity and the Environment

• The development and expression of inherited
  traits can be influenced by environmental factors
  such as
• 1. Nutrients
• 2. Temperature
• 3. Sunlight

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• Ex Temperature affects fur color in the
  Himalayan rabbit.
• Under normal circumstances, these rabbits are
  white with black ears, feet, and nose.
• If you shave the fur off its back and place an
  ice pack there, the hair will grow back in the
  color black-not white.

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(I) Gene-Chromosome Theory

• Genes exist in a linear fashion on chromosomes.
• Two genes associated with a specific
  characteristic are known as alleles and are
  located on homologous chromosomes.

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Gene Linkage

• Genes for different traits are located on the
  same chromosome pair, and are said to be linked.
• Therefore they are usually inherited together.

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Crossing-Over

• This occurs during synapsis in the first meiotic
  division.
• The chromatids of homologous chromosomes often
  twist around each other, break, exchange segments
  and rejoin.
• This exchange of segments, called crossing-over,
  results in a rearrangement of linked genes and
  produces variations in offspring.
• Crossing-over is an important source of genetic
  variation in sexual reproduction.

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(II) Sex-Linked Traits

• 1. Each human cell contains 46 chromosomes
• 22 pairs of autosomes
• 1 pair of sex chromosomes
• a) sex chromosomes are designated as
• X and Y
• XX female
• XY male

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2. The sex of a human is genetically determined
at fertilization when a sperm cell containing
either the X or Y chromosome unites with an egg
cell containing the X chromosome.

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• 3. Sex-Linkage
• a) Morgans work with Drosophila demonstrated
  that genes for certain traits are located on the
  X chromosome and do not appear on the Y
  chromosome.
• b) genes found on the X chromosome are said to
  be sex-linked genes.

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• Recessive traits that are sex-linked occur
  more frequently in males than in females. For
  the recessive trait to show in a female, she must
  be homozygous recessive. The gene must be present
  on both of her X chromosomes ( X X ).

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• For the recessive trait to show in males, the
  recessive gene on the X chromosome will be
  expressed since it does not appear on the Y
  chromosome
• ( X Y).

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Human Disorders associated with Sex-Linked Genes

• Hemophilia- disease in which the blood does not
  clot properly.
• Colorblindness- inability to see certain colors,
  most commonly red and green.
• Both of these disorders are more common in
  males than in females because a female will not
  show the disorder as long as she has one normal
  gene. Females who are heterozygous for a
  sex-linked trait are said to be carriers for that
  trait.

(Colorblindness Test) http//www.umds.ac.uk/physio
logy/daveb/brainday/
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(III) Mutations

• Changes in genetic material are called mutations.
• If a mutation occurs in the sex cell, it may be
  transmitted to the offspring.
• Mutations occurring in body cells may be passed
  on to new cells of the individual due to mitosis,
  but will not be transmitted to the offspring by
  sexual reproduction.
• Ex cancer
• Mutations can be classified as chromosomal
  alterations or gene mutations.

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Chromosomal Alterations

• Is a change in the number of chromosomes or in
  the structure of the chromosome.
• The effects of chromosomal alteration are often
  visible in the phenotype of an organism because
  many genes are usually involved.
• An example of chromosomal alteration is
  nondisjunction.

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Nondisjunction

• One or more pairs of homologous chromosomes fail
  to separate.
• This results in gametes with more (or less) than
  their normal haploid chromosome number.
• http//raptor.lmc.cc.mi.us/liberal/bio/bio212/cyt
  ogene.html

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If these gametes are involved in fertilization,
the resulting zygote may have more (or less) than
the normal diploid chromosome number.

• Examples
• 1. Downs Syndrome- results from the possession
  of an extra chromosome. This is due to the
  nondisjunction of chromosome 21 in one of the
  parents. Will result in mental retardation or
  physical abnormalities.
• 2. Polyploidy- sometimes all 22 chromosomal
  pairs fail to separate. The resulting 2n gamete
  fuses with the normal n gamete, producing a 3n
  zygote. This is common in plants but rare in
  humans.

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Changes in Chromosome Structure

• Changes in the make-up of chromosomes may result
  from random breakage and recombination of
  chromosome parts.
• Examples
• Deletion
• Inversion
• Translocation

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Deletion

• The deletion of a chromosome segment occurs when
  a portion of a chromosome breaks off and does not
  become attached to any other chromosomes.

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Inversion Translocation

• Inversion- A reversal in the order of genes on a
  chromosome segment.
• Translocation- Is a chromosomal rearrangement in
  which a section of a chromosome breaks off one
  chromosome .

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Gene Mutations

• Involves a random change in the chemical nature
  of the genetic material (DNA).
• Some gene mutations, like albinism are obvious,
  while others are not noticeable.
• Most gene mutations are recessive and hidden by
  the normal, dominant allele.
• However, if both parents carry the same recessive
  mutant gene, there is a chance that their
  offspring will be homozygous recessive and show
  the harmful trait.

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(IV) Mutagenic Agents

• Causes mutations
• Radiation- ultraviolet light, x-rays,
  radioactive substances.
• Chemicals- asbestos fibers

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• Ultra-violet radiation from the sun strike cells
  in a persons skin damaging the DNA in those
  cells.
• UV light, chemicals, tobacco, and x-rays can
  cause mutations in the DNA. All of these
  substances are known as mutagens.

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What happens to your skin when you tan?

• When exposure to the sun occurs, our body has its
  own built-in repair system. This system consists
  of a series of repair enzymes that detect damaged
  pieces of DNA.
• The damaged pieces of DNA are removed, and the
  DNA is repaired.
• If exposure is too great, too much damage occurs.
  The body is unable to fix the damage and
  mutations occur.
• Mutations frequently produce cancers, although
  this disease may occur many years after exposure
  to the mutagen.

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(V) Human Genetic Disorders
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1. Phenylketonuria (PKU)

• A disorder in which the body cannot synthesize an
  enzyme necessary for the normal metabolism of
  phenylalanine.
• Results in mental retardation.

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2. Sickle-cell Anemia

• A gene mutation that results in the production of
  abnormal hemoglobin molecules and abnormal red
  blood cells.
• African descent

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Normal Sickle-cell

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3. Tay-Sachs

• Deterioration of the nervous system due to the
  accumulation of fatty material as a result of the
  inability to synthesize a specific enzyme.
• Jewish people of Central Europe descent.

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(VI) Types of Genetic Disorder Detection
Techniques
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1. Screening

• Chemical analysis of body fluids such as blood
  and urine.
• Detection of PKU and
• Tay-Sachs

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2. Karyotyping

• The preparation of an enlarged photograph of
  chromosomes.

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3. Amniocentesis

• Removal of amniotic fluid for chemical and/or
  cellular analysis.
• Detection of sickle-cell anemia.

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