Polymorphism & Restriction Fragment Length Polymorphism(RFLP) Genome VariationDefinition:In 2 non-related individuals: 0.1% genome variation (1 in each 1500 bases)PolymorphismMutationClinical effectClinically harmless (No effect on phenotypePotentially harmful (results in genetic diseaseincidence 0.1%rare: 0.1%PolymorphismPolymorphism: is a variation in nucleotide sequence from one individual to another, occurring in the non-coding regions of DNA.
RFLP: is a genetic variant that can be examined by cleaving the DNA into fragments (Restriction fragments) with a RE. The length of the restriction fragments is altered if the genetic variant alters the DNA so as to create or abolish a site of RE cleavage. It can be used to detect human genetic variations, e.g. in prospective parents or in fetal tissue
2 types of DNA variation result in RFLP:
SNPVNTRConsidered markers, which, in most cases, have NO known effect on the structure or rate of production of any particular protein.
Single Nucleotide Polymorphism (SNP; 90% of human genome variation)Variable Number of Tandem Repeats (VNTR)Tandem repeats:short sequences of DNA at scattered locations in the genomeRepeated in tandem (one after another)The # of these units varies from person to person, but is unique for any given individualServes as a molecular fingerprint
Variable Number of Tandem Repeats (VNTR)VNTR loci: sites in genome that are frequently showing VNTR, important in DNA fingerprinting analysis (e.g. in forensic & paternity identity)
RFLP of VNTRPrenatal DiagnosisIt is recommended if there is a history of severe genetic disease, (affected previous child or near relative)
Its aim is to determine the presence of the disorder in a developing fetus
Methods available:Visualization of the fetus e.g. by Ultrasound or fetoscopy:If the genetic abnormality gross anatomic defects e.g. neural tube defect
Amniotic fluid biochemical analysis: e.g. level of a fetoprotein (increase in open neural tube defects, & low in Down syndrome.Fetal cells in amniotic fluid or in chrorionic villi biopsy: Karyotyping: the morphology of the metaphase chromosomes (e.g trisomies, translocations abnormal length of chromosome
Fetal DNA analysis: (the most detailed genetic picture; DNA from WBC, Amniotic fluid, or Chorionic villiDNA has to be amplified first: In the past: cell cultureNow: PCRExamples of diseases diagnosed prenatallySickle cell anemia: Could be diagnosed prenatally by:I- Electrophoresis analysis of the amount & type of Hb obtained by hemolyzing fetal blood1- risk in obtaining fetal blood2- Late detection (2nd trimester)
2- Fetal DNA then do RFLP: Advantages: 1- Safe2- early detection
RFLP in a family with a child affected by PKU