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Genetic Diseases

Causes, Effects and Results of Genetic Malfunctions

Elise McDonnell

on 3 April 2013

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Transcript of Genetic Diseases

Genetics and Disease:
<various effects of mutations and malfunctions> Classified into three major categories determined by thier derivation: Genetically determined diseases Polygenic Diseases result from the interaction of multiple genes (more minor effects genetically, but are commonly altered by environmental/population factors)
most common diseases:
diabetes, hypertension, schizophrenia, congenital defects like cleft lip/palate, heart diseases Chromosomal Disorders These are the result of an addition or deletion of entire chromosomes/parts of chromosomes, and are mainly characterized by growth abnormalities, mental retardation as well as phenotypic deformities
NOTE: the loss or gain of whole chromosomes (other than sex chromosomes) is incompatible with life
-->major cause of natural abortions/miscarriages
(15% of recognized pregnancies result in miscarriage, 50% of all conceptions do as well--> 1/4 of conceptions may suffer from major chromosome problems) Single Gene Defects These are diseases due to single gene mutants that affect many other genes/functions
Major diseases of this category are sickle cell anemia and cystic fibrosis
**Also are called "molecular level genetic diseases" because they are small changes that manifest on a massive scale
inherited in Mendelian fashion-->autosomal dominant, autosomal recessive or X-linked fashion Prominence Genetic diseases and malfunctions are far from rare, and are a significant cause of illness and death in human offspring
**approximately 3% of all pregnancies result in birth of a child with a significant genetic disease major example of a chromosomal disorder: Trisomy 21 or Down Syndrome There is less information about these diseases because of the number of factors contributing to their manifestation.
Though many of them are inherited, their possession may be based on a number of other factors that can affect the existing mutations Autosomal Dominant Inheritance *heterozygous offspring has one copy of the mutant allele, one of the normal allele; the mutant is dominant
*onset of these diseases is most commonly delayed, or manifested in adult life
Major examples:
familial cholesterodemia
colonic polyposis (colon polyps)
polycystic kidney disease
Ehler's Danlos Syndrome (I have this) Autosomal Recessive Inheritance In this scenario of inheritance, the homozygous offspring possess two copies of the recessive mutant allele (passed to them by the heterozygous parent carrier)
*1 in 4 inheritance rate
most common diseases:
cystic fibrosis
Sickle-cell anemia X-Linked Inheritance *often only fully expressed in males, because they posses one X>>if that X carries the disease, it is automatically dominant (no dormancy of a male X)
**in females, the somatics (active vs. dormant X) create variation>>if the X carrying the disease is dormant, the disease will be dormant; if the X is active, disease is manifested
Major examples:
hemophilia A
Duchenne muscular dystrophy: muscle tissue breaks down because of missing muscle protein, dystrophin. [transmission of this is through a heterozygous female (healthy) carrier, and it therefore is only manifested in boys] Cytogenetics [Most recognized abnormalities:Trisomy 21, Fragile-X syndrome, Turner's, and Klinefelter's]
Chromosomal disorders can either originate from numerical or structural "mess-ups" ANEUPLOID PENCENTRIC and PARACENTRIC INVERSION "Ploidy" numerical issues can occur during meiosis or mitosis
>> result from nondisjunction, or failure to properly separate
meiotic nondisjunction:
@ the 1st division = gamete has both or neither parental chromosomes that failed to separate properly
@ 2nd division = gamete has 2 copies of the chromosomes that have failed to separate
mitotic nondisjunction: occurs post fertilization if an unbalanced gamete fertilizes a balanced one
*** mosaicism--> occurs when populations of chromosomal defective cells exist and often because there are so many mutations, the zygote is aborted Structural abnormalities Single chromosome
--deletion: one section of the chromosome is missing
duplication: a portion of the chromosome is repeated.
>>>yields either a pericentric (involving the centromere, creating a ring shape) or paracentric inversion (isochromosome creation)
two chromosomes:
material insertions from one to another can occur; translocation or exchange of material between 2 or more chromosomes = reciprocal translocation and Robertsonian/centric fusion Common "ploidy" defects arising from nondisjunction (tetrad homologous chromosomes don't come apart) @ meiosis/anaphase I:
triploidy 69: 3 copies-->this will commonly result in spontaneous abortion
trisomy 21: 1 extra copy of chromosome 21 (down's syndrome)--arises from nondisjunction
monosomy: missing one copy of the sex chromosome (turner's syndrome/ 45X) 1. CHROMOSOMAL DISORDERS
3. POLYGENIC DISEASES So what... --> Because so many complications can arise from genetic malfunctions, it is important to consider the spectrum of their effects; further, when considering all of the things that can go wrong at any given stage during reproduction, it is fascinating how any "normal" offspring are produced.
**people may even possess malfunctions in their DNA that are undetected/unnoticeable (like me--if I didn't say I had Ehler's Danlos, you never would have known)
Research into the field of genetically determined disease also offers insight into just how dramatically a malfunction can affect an offspring as well as the susceptibility to inheritance or development of a disease in populations or families **at 6-7 weeks of development for the zygote, sexual differentiation occurs
**X-linked testicular feminization & klienfelter's disease:
manifested when differentiation goes wrong/stops because of an extra chromosome possessed
(Klienfelter = 47 XXY)
XXX: metafemale syndrome (extra female sex chromosome)
XYY syndrome + 48 XXYY-->found in inmates; Y aneudiploidy possibly is associated with criminal/behavioral abnormalities
*arises from paternal meiotic nondisjunction
Turner's syndrome: 45X>>characterized by female immaturity/short stature (X has no counterpart)
X-linked mental retardation: fragile X (X chromosome has breaks/gaps in it, causes a deprivation of the folate) there are also rare chromosomal rearrangements involving a material exchange that can result in deletion of X chromosome material (this is how something like Duchenne muscular dystrophy can be manifested in females) reciprocal translocation can occur between one of the autosomes and the X chromosome
>>>during the breakage & rejoining of chromosomal segments, there is either loss of some material or breakage within a gene on the X chromosome; normal X chromosome is preferentially inactivated in cases of X autosome translocation. **females with a heterozygous genotype for translocation may express X linked recessive diseases otherwise observed only in males mutations: mistranslations/deletions of DNA instructional information
Autosome/autosomal: refers to Mendelian diseases (those that are inherited) encoded on pairs of autosomes, or non-sex chromosomes Vocabulary associated with genetics
(scientific jargon) mutations are based on scale
gene mutations = have to do with what is inside of a chromosome-->can be caused by a substitution, insertion or deletion
chromosome mutations = severity depends on whether the structure or the number has been altered Sex-Chromosome disorders consider:
Can an offspring survive without a Y chromosome?
Can an offspring survive with only 1 chromosome? Yes!
-->what is NOT viable is a missing X chromosome, which is why none of the chromosomal disorders correspond to a 'missing x' and why things like trisomy 14 cannot exist (this video is super goofy, but it gets the point across)
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