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14-1 Notes Human Heredity Human Chromosomes Biologists can make a karyotype by cutting chromosomes out of photographs. There are 46 total chromosomes in a human body cell 23 from a haploid sperm 23 from a haploid egg Human Chromosomes Two of the 46 chromosomes are called sex chromosomes. Females have 2 X chromosomes Males have 1 X and 1 Y chromosome The remaining 44 chromosomes are called autosomes. Human Traits Pedigree charts are used to show how traits are passed from one generation to the next. Genetic counselors analyze pedigree charts to infer the genotypes of family members Pedigree Chart Human Traits Some of the most obvious human traits are almost impossible to associate with single genes. 2 reasons why: 1.) most traits are polygenic 2.) traits are only partly governed by genetics Human Genes The human genome includes tens of thousands of genes. Some of the first genes to be identified were those that control blood type. Human blood comes in a variety of groups. The Rh blood group is determined by a single gene with two alleles 1.) dominant (+) 2.) recessive (-) Human Genes The ABO group is more complicated. There are 3 alleles for this gene. 1.) IA 2.) IB 3.) i Human Genes IA & IB are codominant and i is recessive. IA & IB produce antigens on blood cells and can give you type A, B, or AB blood. Two ii alleles produce no antigens and can give you type O blood. Human Genes Many human genes have become known through the study of genetic disorders. Most of these disorders are caused by recessive alleles (Albinism, Cystic fibrosis, & PKU) Some are caused by dominant alleles (Huntington’s disease & Achondroplasia) A few are caused by codominant alleles (Sickle cell disease) Cystic Fibrosis Affects chromosome #7 Caused by 3 bases missing from the sequence and an amino acid not being produced. Chloride ions are not able to move across the cell membrane. This results in a build up of thick mucus in airways. Sickle Cell Disease Mainly found in African Americans. Characterized by the bent and twisted shape of red blood cells. Cells get stuck in capillaries and blood stops moving through damaging cells, tissues, and organs. 14-2 Notes Human Chromosomes Human Genes & Chromosomes Only about 2% of the 6 billion base pairs actually function as genes. Average human gene consists of about 3000 base pairs Largest genome has more than 2 million base pairs Human Genes & Chromosomes Chromosomes 21 and 22 are the smallest human autosomes They were the first whose sequences were determined Disorders associated with 22 include leukemia, and neurofibromatosis Disorders associated with 21 include ALS (Lou Gehrig’s disease) Sex-Linked Genes Includes all genes located on the X or Y chromosome More than 100 sex-linked disorders have been found on the X chromosome The Y chromosome is much smaller with few genes Color Blindness Recessive allele located on the X chromosome Found in about 1 of 10 males and 1 of 100 females More common in men since they only have one X chromosome Hemophilia Includes two genes carried on the X chromosome to help control blood clotting A recessive allele for either gene may result in hemophilia Results in a missing protein needed to clot blood Those affected may bleed to death from a single cut X-Chromosome Inactivation Scientists wondered how an extra X in females affected them British geneticist Mary Lyon discovered that one X is “turned off” by a region in the nucleus called a Barr body Also occurs in other mammals such as cats Cat color is determined by the X chromosome Chromosomal Disorders A common error in meiosis is nondisjunction where the homologous chromosomes do not separate This can result in an abnormal number of chromosomes in the gamete and a disorder may occur Down Syndrome Autosomal disorder in which there is a trisomy (3 copies) of chromosome 21 Affects 1 of 800 babies Results in mild to severe mental retardation and often an increased susceptibility to disease Scientists are not sure why an extra copy causes so much trouble Sex Chromosomes Disorders Turner’s Syndrome – female with only one X chromosome Unable to reproduce Klinefelter’s Syndrome – males with an extra X chromosome (XXY) Unable to reproduce The genetic information stored on the Y chromosome determines sex in the developing embryo 14-3 Notes Human Molecular Genetics Human DNA Analysis Humans contain roughly 6 billion base pairs in our DNA. Biologists use these sequences of DNA bases to form the human genome. Biologists can now read, analyze, and even change the molecular code of genes. Testing for Alleles A variety of genetic tests have been developed to spot the differences in DNA sequences. Sometimes these genetic tests use labeled DNA probes. These detect the complementary base sequences found in disease-causing alleles. Testing for Alleles Tests also detect differences between the lengths of normal and abnormal alleles. DNA testing can pinpoint the exact genetic basis of a disorder. Allows for more effective treatment for individuals affected by genetic disease DNA Fingerprinting No individual is exactly like any other genetically; except identical twins. DNA fingerprinting analyzes sections of DNA that have little or no known function, but vary widely. DNA samples can be obtained from blood, sperm, and even hair strands. DNA fingerprinting has been used in the US since the 1980’s Human Genome Project In 1990 scientists in the US and other countries began the Human Genome Project. In 2000 scientists announced that a working copy of the human DNA sequence was essentially complete. To get to this point they first worked on small genomes such as viruses and bacteria. Searching for Genes Only a small part of a human DNA molecule is made up of genes. Biologists continue to search for genes, which they locate in several ways. 1 way-they find DNA sequences that are known to be promoters. binding sites for RNA polymerase indicate the start of a gene Searching for Genes Research groups are looking for genes that may provide useful clues to some of the basic properties of life. This may be useful in developing new drugs and treatments for diseases. Gene Therapy The human genome might be used to cure genetic disorders by gene therapy. In gene therapy, an absent or faulty gene is replaced by a normal, working gene. Thus eliminating the cause of the disorder. The first authorized attempt to cure a human genetic disorder by gene transfer occurred in 1990. Class Discussion-Ethical issues in human genetics It would be great to cure genetic disorders, but what are the costs? Should biologists try to engineer taller people, change eye color, hair, gender, or appearance? What will happen to the human species if we gain the opportunity to design our bodies? What will the consequences be if we develop the ability to clone human beings?
