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V. Pedigrees & Detecting Disorders *Human inheritance is hard to study because humans mature slowly & produce few offspring. A. Pedigree Charts -Most knowledge concerning human inheritance comes from examining the heredity in human populations & families (as shown in family trees & pedigree charts). Pedigree chart – a chart which shows the relationships within a family. -Pedigrees help to determine the inheritance of some alleles/disorders & predict them in offspring, ex : Recessive hitchhiker’s thumb. -If a person has a recessive trait that is not expressed they are called a carrier of that trait, ex : heterozygous female with normal vision having the recessive trait for colorblindness. Reading a Pedigree Chart 1. Circles represent females & squares represent males. Below is a pedigree chart of a family showing four generations. A total of 20 2. A horizontal line connecting a male & individuals. Generations are identified by Roman female represents a marriage. numerals. Individuals in each 3. A vertical line & a bracket connect generation are identified by Arabic the parents to their children. numerals numbered from the left. 4. A shaded circle or square represents Therefore the affected individuals are II3, IV2 and IV3. a person who expresses the trait/disorder. 5. A circle or square that is not shaded represents a person who does not express the trait/disorder. 6. If used in a pedigree chart, a half-shaded circle or square represents a carrier of the trait/disorder. Pedigree Charts What is the genotype of person #1? How is person #1 related to person # 5 & person # 11? Genetic Counseling *Couples with genetic disorders in their family histories may seek genetic counseling to predict the likelihood of having children with the disorders. Genetic counselor – a specialist who analyzes a pedigree chart to infer the genotypes of family members. -Example : One can infer that if a man passes a dominant trait onto two of three children, and has a third child who is recessive for the trait; that he MUST be heterozygous for that trait. -Genetic counselors trace both family histories, look for inherited disorders, develop a pedigree, & provide the probabilities of passing on the disorders to offspring. B. Detecting Disorders Methods of Detection 1. Allele testing – may be done by : a. Labeled DNA probes – specific DNA base sequences that detect complementary base sequences found in disease-causing alleles. b. Testing for changes in restriction enzyme cutting sites. Restriction enzyme – enzyme that cuts DNA at a specific sequence of nucleotides. c. Comparing lengths of normal vs. abnormal chromosomes. Detecting Disorders 2. Karyotype - picture of grouped pairs of chromosomes. 3. Ultrasound – non-invasive procedure of bouncing sound waves off of a fetus to produce its image. a. Can detect abnormalities in bone & muscle but may be disruptive to developing cells (ADD/ADHD). 4. Amniocentesis – invasive procedure by which a long needle is placed into the amniotic sac of cells shed by a fetus & a sample of amniotic fluid is withdrawn for testing. *May cause a fetus to abort. 5. Fetoscopy – invasive procedure of inserting a needle with a light and special viewing scope into the amniotic sac. Detecting Disorders Possible Solutions : *The Human Genome Project (an ongoing effort to analyze the human DNA sequence) has made gene therapy a possibility. Gene Therapy – replacement of an absent or faulty gene with a normal, working gene. A new gene is injected into an adenovirus vector, which is used to introduce the modified DNA into a human cell. If the treatment is successful, the new gene will make a functional protein.