V. Pedigrees & Detecting Disorders

1. 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.

2. Reading a Pedigree Chart Below is a pedigree chart of a family showing four generations. A total of 20 individuals. Generations are identified by Roman numerals. Individuals in each generation are identified by Arabic numerals numbered from the left. Therefore the affected individuals are II3, IV2 and IV3. 1. Circles represent females & squares represent males. 2. A horizontal line connecting a male & female represents a marriage. 3. A vertical line & a bracket connect the parents to their children. 4. A shaded circle or square represents 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.

3. Pedigree Charts What is the genotype of person #1? How is person #1 related to person # 5 & person # 11?

4. 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.

5. 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.

6. 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.

7. Detecting Disorders

8. Possible Solutions : 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. *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.