Outline and Chapter notes to accompany chapter 3 HUMAN GENETICS Jan., 2001 A. GENES CARRIED ON SEX CHROMOSOMES DETERMINE SEX AND SEX-LINKED TRAITS. SEX DETERMINATION: Humans and most other species have an XX / XY form of sex determination: XX usually produces female (with two copies genes on the X- chromosome); XY usually produces male (only one copy of most sex-linked genes) Occasional anomalies resulted in discovery of the sry gene carried on the Y chromosome; this gene determines maleness, presumably by regulating production of the hormone TESTOSTERONE. Two other forms of sex determination are WW (male) versus WZ (female) in birds and haplodiploidy (males are haploid and females are diploid) in the insect order Hymenoptera. SEX-LINKED TRAITS are those carried on the X chromosome. Because males only have one X chromosome, they only have one copy of any sex-linked gene, and thus only a single allele. The product of this allele is always displayed phenotypically. Females have two copies of each sex-linked genes. If a sex-linked allele is recessive, females will not exhibit the phenotype unless they have two copies of that allele. Females heterozygous for a recessive sex-linked trait are called carriers. They do not exhibit the trait phenotypically, but they can pass it on to their descendents. Recessive sex-linked alleles for uncommon conditions show up much more often in males and only rarely in females. Males with such traits may have affected grandfathers or great-grandfathers, and the connecting individuals in the intervening generations are carrier females. Red-green colorblindness and hemophilia are examples of sex-linked traits in humans. CHROMOSOMAL VARIATION: The pattern of chromosomes visible under a light microscope is called a KARYOTYPE. Most people have 23 pairs of chromosomes. These include the sex chromosomes and 22 pairs of other chromosomes, called autosomes. Trisomy is an uncommon condition in which an extra chromosome is present, making a triple instead of the usual pair. The most frequent form of trisomy is trisomy of chromosome 21, resulting in one form of Down's syndrome. Other examples of trisomy include Patau's syndrome (trisomy #13) Trisomy of the sex chromosomes can result in: XXY (sterile males with Klinefelter's syndrome), XYY (males with an extra Y), or XXX (sterile females with an extra X). Chromosome numbers may also be lower than 46. Females with only one X chromosome (XO) have Turner's syndrome and are sterile. Both Turner's and Klinefelter's syndomes result from a type of abnormal cell division called nondisjunction. Chromosomal translocations occur when a piece of one chromosome is attached to another. B. SOME DISEASES AND DISEASE PREDISPOSITIONS ARE HEREDITARY IDENTIFYING GENETIC CAUSES FOR TRAITS. PEDIGREES of large families (or of many families) can help identifiy whether a genetic trait is caused by a dominant or a recessive allele, and whether the trait is sex-linked or autosomal. Many human traits are affected by single genes. Examples include: Brown eyes (dominant to blue eyes) Ability to curl or roll up tongue (dominant to inability) Ability to taste the chemical PTC (dominant to nontasting) SOME HEREDITARY DISEASES ARE ASSOCIATED WITH KNOWN GENES: Huntington's disease (dominant to absence of the disease) NOTE: phenotypic effects develop late in life Metabolic diseases controlled by recessive alleles are called "INBORN ERRORS OF METABOLISM" Albinism (inability to make melanin pigment) Phenylketonuria (PKU) (inability to break down the amino acid phenylalanine) Alkaptonuria (inability to break down homogentisic acid) Porphyria (inability to break down certain purines) C. MOLECULAR TECHNIQUES HAVE LED TO NEW USES FOR GENETIC INFORMATION. THE HUMAN GENOME PROJECT A GENOME is the complete hereditary material of an organism. The Human Genome Project hopes to sequence the whole human DNA genome and map the location of all genes. (Many DNA sequences have not yet been matched to any known genetic function.) The RFLP technique (available since 1980) has allowed rapid advances in gene mapping: Cut DNA with restriction enzymes. Use electrophoresis to separate RESTRICTION FRAGMENTS differing in length; variation in the lengths of particular fragments is a RESTRICTION FRAGMENT LENGTH POLYMORPHISM (RFLP). Try to find a family with a genetic condition and a RFLP that accompanies the condition; the responsible gene and the RFLP will therefore be nearby on the same chromosome. Ethical issues raised by the Human Genome Project (and by human genetics in general) include: privacy; misuse of medical information; knowing one's fate (actually one's predispositions) early in life; doctors and insurers "playing God" USING DNA MARKERS TO IDENTIFY INDIVIDUALS (forensic uses): Matching DNA samples between suspects and evidence from a crime scene Identifying the father in paternity disputes Using DNA from relatives to identify dead bodies Using DNA to confirm relationships (as in the Thomas Jefferson case) GENETIC ENGINEERING: Requires restriction endonucleases, also called RESTRICTION ENZYMES Cutting several pieces of DNA with the same enzyme results in MATCHING "STICKY ENDS" GENETIC ENGINEERING TO PRODUCE A USEFUL GENE PRODUCT (like HUMAN INSULIN or SOMATOSTATIN): 1. Take cells that make a useful product from a human or animal. 2. Cut the DNA with a restriction enzyme into RESTRICTION FRAGMENTS. 3. Isolate the DNA fragment containing the gene. 4. Also isolaste a bacterial PLASMID and cut it with the same restriction enzyme; the plasmid must also have a gene that can be used to select bacteria that have incorporated the plasmid, such as ability to survive on a medium deficient in a particular amino acid or other nutrient. 5. Mix the human DNA fragments with the plasmid; some plasmids will recombine with the human DNA fragments. 6. Allow bacteria to take up the new plasmid, and select for those bacteria that have done so. 7. Test the bacteria for the presence of the human gene; isolate any bacteria possessing the gene. 8. Grow the bacteria in large numbers (called CLONING); allow the bacteria to produce the (medically or commercially useful) protein product of the introduced gene. GENETIC ENGINEERING to cure a human gene defect: 1. Isolate human cells containing the normal version of the gene. 2. Grow these cells in tissue culture; isolate DNA from them. 3. Use a restriction enzyme to cut the DNA into fragments with sticky ends. 4. Isolate DNA from a virus (such as LASN) and cut this DNA with the same restriction enzyme. 5. Mix the DNA fragments and allow new viruses to take up the recombinant DNA. 6. Obtain cells from a patient who is incapable of making an important enzyme because their DNA lacks the normal gene. 7. Combine these cells with the virus containing the recombinant DNA. (The virus thus acts as a VECTOR for inserting the recombinant DNA into human cells.) 8. Grow the human cells in tissue culture and isolate those which make the proper enzyme. 9. Inject the genetically engineered cells back into the patient who donated the cells; hopefully, the cells will proliferate in sufficient numbers to produce adequate amounts of the previously missing enzyme in the patient. OTHER SPIN-OFF TECHNOLOGIES: Making human gene products in other species Growing other species with human genetic traits, in the hope that their tissues, if transplanted into human patients, will not be rejected D. GENETIC INFORMATION CAN BE USED OR MISUSED IN VARIOUS WAYS. GENETIC TESTING AND COUNSELING: Pedigree analysis helps identify dominant and recessive traits; it also distinguishes sex-linked traits from other (autosomal) traits. Identifying a gene (or its gene product): DNA probes for specific genes; enzyme tests for many gene products. Some of these tests are preceded by PCR (polymerase chain reaction) to get enough identical copies of DNA to run the tests. Identifying chromosome abnormalities Newer sampling methods include: AMNIOCENTESIS, CHORIONIC VILLUS SAMPLING Some tests allow us to detect heterozygous carriers of recessive traits; others do not. We can advise testees of future risks for themselves or their children. Most testing requires obtaining INFORMED CONSENT first Genetic testing raises many new ethical issues in medical decision- making ALTERING INDIVIDUAL GENOTYPES: Currently possible only through genetic engineering (RECOMBINANT DNA THERAPY) ALTERING THE GENE POOL OF POPULATIONS: POSITIVE EUGENICS = encouraging certain genotypes to breed in greater numbers NEGATIVE EUGENICS = preventing certain genotypes from breeding (by sterilizing or killing them)-- has led to genocide in the past CLONING (not currently possible with humans) = asexually produced (therefore genetically identical) organisms or cells An ethical question: who decides what is considered a defect? CHANGING THE BALANCE BETWEEN GENETIC AND ENVIRONMENTAL FACTORS: Euphenics: modification of individual phenotypes (producing PHENOCOPIES) Euthenics: providing external assistive devices (like wheelchairs & eyeglasses) Eupsychics: changing other peoples' social attitudes, customs, and laws, or providing special education for handicapped individuals ---------------------------------- Jan., 2001