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Human Genetics & Inheritance Patterns
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• In the space below list a number of differences and similarities found among humans.
Differences:
Similarities:
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• You may be able to list more similarities than differences.
• Many (but not all) of the features we listed are genetic mutations.
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Note to me…
• Do the taster/non-taster lab!
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Mutations
• We have learned that DNA is responsible for the production of proteins that direct an organism’s metabolism and development.
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• What would happen if there were changes in our DNA?
• On a rare occasion it may result in NEW PHENOTYPES.
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• A change in DNA is called a MUTATION • These changes can affect an entire chromosome or specific
genes. Any cells in our bodies are subject to mutations. – Mutations in our SEX CELLS (egg or sperm) are called GERM CELL
MUTATIONS.
• These mutations do not affect the organism but are passed to offspring.
• Most of the mutations that occur in germ cells are lethal and do not allow the individual to develop past the zygote stage.
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• Mutations that occurs in other body cells are called SOMATIC MUTATIONS . These mutations are passed to daughter cells through mitosis.
• These mutations do not affect the offspring of the affected individual.
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CHROMOSOMAL MUTATIONS:• During cell division mutations may occur in
chromosomes. There are four types of chromosomal mutations:1. DELETION
• 2. INVERSION
• 3. TRANSLOCATION
• 4. NONDISJUNCTION
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__Deletion_ :
• When a piece of the chromosome breaks off. This results in the information on that chromosome piece being lost.
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__INVERSION____ :
• When a piece breaks off from a chromosome and reattaches itself to the chromosome in reverse order.
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_TRANSLOCATION:
• When a broken piece attaches to a non-homologous chromosome.
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Nondisjunction
• A fourth type of chromosomal mutation is called NONDISJUNCTION .
• This occurs when a replicated chromosome pair fails to separate during cell division.
• The daughter cells will result in one having an extra copy of a chromosome and the other cell lacking that chromosome entirely.
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GENE MUTATION:• Scientist have found several mutations
that can occur in the DNA sequence.
• If the mutation only affects one nitrogen base is called a POINT MUTATION.
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• If one nitrogen base is replaced by another
nitrogen base the DNA may code for different AMINO ACID . – If one nitrogen base is added or deleted a
FRAME SHIFT MUTATION occurs. This usually results in the inability of the DNA to code for the correct amino acid.
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Some mutations happen by CHANCE. There are other times when mutations are caused by MUTAGENS, environmental factors that result in that damage of DNA strands.
MUTAGENS:
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• A few well known mutation causing factors are :
– cigarette tars, – asbestos, and – viruses. – Radiation is also an environmental factor that
causes mutations in both germ and somatic cells.
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Spontaneous vs Induced Mutations:• All mutations are described as either
spontaneous or induced.
• We have already discussed the mutations that are induced.
• Spontaneous mutations are considered as those that arise in nature. No specific agent other than natural forces is associated with their occurrence.
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• These mutations are assumed to arise randomly as in changes in the nucleotides sequence of genes.
• All mutations have a cause , but some mutations occur in the absence of mutagens, or mutation-causing agents.
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Some of these mutations are the result of miscues of the DNA REPLICATION MACHINERY. Sometimes the proofreading mechanism fails and the mistake is not caught.
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SPONTANEOUS MUTATIONS • These type of mutations can occur in several
ways: – (1) As DNA is replicating, mistakes are made on
occasion that go uncorrected. – (2) The bases in the DNA template strand or in the
newly inserted nucleotide can shift to an alternate form that base-pairs incorrectly.
– (3) Too many or too few bases can be inserted , causing frame-shift mutations.
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CHEMICAL MUTAGENESISInduced Mutatuions• Different chemicals induce different kinds
of DNA damage.
• Nitrous acid and bisulfite cause deamination – (the removal of an amino group [NH2] that
converts cytosine to uracil) of bases.
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RADIATION- INDUCED MUTATIONS
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• Ultraviolet, gamma, and X-radiation are the common types of mutagenic radiation.
• Ultraviolet radiation is relatively week so the damage it causes is relatively modest: it cross-links adjacent pyrimidines on the same DNA strand, – Forming a dimer, usually a
THYMINE DIMER.
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What’s a dimer? (FYI only, not in notes)
• A dimer is a chemical or biological entity consisting of two subunits called monomers, which are held together by either intramolecular forces (covalent bonds) or weaker intermolecular forces.
• Molecular dimers are often formed by the reaction of two identical compounds
• An example of an intermolecular or physical dimer is acetic acid wherein hydrogen bonds hold the two molecules together. The water dimer is another such dimer.
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• This blocks DNA replication because the replication machinery cannot tell which bases to insert opposite the dimers.
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– Other times an alternate form of a nitrogen base is used that bonds with the alternate form of nitrogen bases.
– Example there are two forms of Thymine one will bond with the usual Adenine and the other form will bond with Guanine:
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• Replication sometimes proceeds anyway and bases are inserted at random .
• If these are the wrong bases, a mutation results.
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Radiation:
• The kind of radiation that is most damaging to DNA has a wavelength of about 260 nm, – which is not surprising since this is the wavelength
of radiation that is most absorbed most strongly by DNA.
– This type of radiation is also abundant in sunlight, so most forms of life are exposed to this type of radiation to some extent.
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– This type of mutation explains why sunlight can cause skin cancer.
– We have a shield from this type of radiation. – The natural shield is the OZONE layer in the earth’s
upper atmosphere. This ozone absorbs the bulk of such radiation.
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Gamma and X rays. • Gamma and X rays have
more energy that ultraviolet radiation. These types of radiation can interact directly with DNA molecule. These types of radiation can cause the DNA to break apart.
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Effects on Organism
• Scientist also classify mutations on the basis of their effect on the organism: – 1. Morphological
trait mutations: • These mutations
affect the morphology or shape of the organism:
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– 2. Biochemical mutations: These mutations affect how an organism accomplishes nutrition. One example is the inability of an organism to synthesize an amino acid.
• Also these types of mutations may affect an organisms chemical processes such as gas exchange: i.e. sickle cell anemia.
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• 3. Lethal mutations: these mutations will cause the death of the organism. – One example of a lethal mutation is
Tay-Sachs or Huntingtons disease.
• Sometimes the lethal mutations will cause immediate death
• or as in the above mentioned disease death may come at a later time in the life of the human.
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Morphological Mutations: • Complex multicellular organisms carrying
• MORPHOLOGICAL MUTATIONS (visible mutations), can usually be distinguished from wild-type (most common variant of a gene).
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– –One example , albino mammals have a mutation in a gene that is responsible for dark coat (or skin) pigment.
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–The mutation is usually in the gene that codes for TYROSINASE, the key enzyme that leads to the production of MELANIN the black pigment in hair, eyes, and skin.
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• A mutated tyrosinase gene may produce no active enzyme, so no melanin can be made.
• As a result humans have very fair skin and light blue eyes, albino mice have white fur and pink eyes.
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LETHAL MUTATIONS:
• Some mutations are so severe that an organism carrying them cannot survive at all.
• These types of mutations are called LETHAL mutations.
• When a genetic defect causes 100% mortality it is termed lethal allele.
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Lethals Cont.
• Lethals are generally recessive resulting in the death of the zygote that is homozygous recessive.
• n 1904 French geneticist Lucien Cue’not carried out crosses on coat color in mice.
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• He obtained results that were not consistent with Mendelian predictions.
• He observed that yellow body color alleles were dominant. When he crossed two heterozygous yellow mice he observed 2:1 ratio of yellow to wild type (brown- agouti). What ratio did he expect:
• 3:1 Wild to yellow
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• He learned that all the Homozygous yellow mice died in utero.
• Histological observations validated this conclusion which demonstrated 1/4 of the embryos from yellow x yellow crosses failed to develop.
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• This demonstrates that lethal mutations in diploid organisms are recessive .
• If one parent contributes a defective gene for an essential protein and the other contributes a wild-type gene, the latter will usually allow the cell to make enough protein to compensate.
• It is only when two defective genes come together in a individual that lethality results.
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Conditional Mutations: • There are certain mutations that can be lethal
under certain conditions.
• These mutations are called CONDITIONAL LETHALS.
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• Temperature Sensitive: – There are certain mutations called TEMPERATURE-
SENSITIVE (or “ts”) mutations.
• This mutation allows for growth at low temperatures but not at normal growth temperatures.
• Lethality in this case is conditional on temperature.
• It is important to realize that it is the PROTEIN PRODUCT that is temperature sensitive, NOT THE GENE ITSELF.
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• This type of mutation creates an altered protein that is easily DENATURED. (EGG WHITE).
• This type of mutations can be observed in humans. – CYSTIC FIBROSIS is caused by a mutant gene
whose protein product cannot fold properly at normal body temperature, so it remains inactive.
– At lower temperatures it functions normally.
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• Genetic mutations can be conditional without being lethal.
• One example is Siamese cats. – These animals have a mutation
in the gene for DARK coat color.
• Siamese cats have dark patches on their feet , faces and ears can you explain this mutation.
Conditional without lethality
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• Patches on their feet, faces , and ears are normal where the temperature is somewhat lower than the rest of the body
• where the protein has been denatured.(lighter in color)
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MISSENSE and NONSENSE MUTATIONS• Many point mutations are MISSENSE
MUTATIONS, in which a base change alters the sense of a codon from one amino acid to another. – This causes an improper amino acid to be
inserted into the protein product of the mutated gene. for example a missense mutation might change the proline codon CCG to ARGININE codon CGG. .
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• An example of such a defect is SICKLE-CELL DISEASE, a true genetic disease.
• People who are homozygous normal for this condition have normal looking red blood cells when their blood is rich in oxygen.
• The shape of normal cells is disc shape that is concave.
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• When people with this disorder exercise there is a depletion of oxygen in their blood this creates a change in the morphology of their red blood cells.
• The sickling is caused when mutated hemoglobin precipitates under low oxygen conditions.
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• The red blood cells forms a crescent (sickle) shape.
• The sickle cells cannot fit through tiny capillaries so they clog and rupture the capillaries.
• This causes internal bleeding and pain. The sickle blood cells also burst and leave the patient ANEMIC.