Ch 14- Human Genome

1. Ch 14- Human Genome • How many chromosomes does a human cell have? • Karyotype- photograph of chromosomes grouped in order in pairs • Sex chromosomes- 2 chromosomes that determine the sex of the individual • Females have two copies of a large X chromosome • Males have one X and one small Y chromosome • Autosomes- the other 44 chromosomes • Males and females are born 50:50 ratio because sex chromosomes segregate during meiosis • Half of sperm cells carry X chromosome and other half carry Y chromosome

3. Human Traits • Biologists must identify if an inherited trait is controlled by a single gene • Pedigree- chart that shows the relationships within a family that can be used to help identify inherited traits • Genetic counselors analyze pedigrees • So can you look at a pedigree and figure out which family the good looks came from? • Shape of eyes and ears are actually polygenic traits • Many traits influenced by environmental factors such as nutrition and exercise

5. Human Genes • Human genome- complete set of genetic information- includes tens of thousands of genes • DNA sequences on these genes carry information for specifying many characteristics- eye color, protein structure in cells • Scientists spent years studying the genetics of our species- able to identify genes that directly control a single human trait • Some of first genes identified were those that control blood type

6. Blood Types • Number of genes responsible for human blood groups- ABO and Rh blood groups • Rh blood group determined by a single gene with alleles- positive and negative • Positive allele is dominant • ABO blood group has three alleles for this gene, I^A, I^B, and I • A and B are codominant, blood type AB • I allele is recessive, I^AI^A and I^Ai is blood type A • I^BI^B and I^Bi is blood type B • People who are homozygous for I allele (ii) are blood type O • Why is knowing the blood type important?

7. Autosomal Disorders in Humans • Human genes have become known through study of genetic disorders • Phenylketonuria (PKU)- recessive allele, people lack the enzyme that is needed to break down phenylalanine, may build up in tissues during child’s first year and cause mental retardation • Tay- Sachs- recessive allele, lipid accumulation in brain cells, blindness, death early in childhood • Achondroplasia- dominant allele disorder, dwarfism • Huntington’s disease- dominant allele, mental deterioration and uncontrollable movements, symptoms appear in middle age

8. How do the actual DNA sequences in genes affect phenotype so profoundly? • Many genetic disorders are still being worked on, but in both cystic fibrosis and sickle cell disease there is a small change in the DNA of a single gene that affects the structure of a protein, causing the disorder • Cystic fibrosis- recessive allele disorder, excess mucus in lungs, digestive tract, liver. More likely to get infections • Sickle cell disease- characterized by bent and twisted shape of red blood cells, tend to get stuck in capillaries and cause damage to cells, tissues, organs • Hemoglobin in person has different amino acid sequence- just one DNA base is changed • Common among African Americans

9. Sec 2- Human Chromosomes • Chromosomes 21 and 22- smallest human autosomes • First chromosomes whose sequences were determined • Long stretches of repetitive DNA are unstable sites where rearrangements occur • Form of leukemia and neurofibromatosis found on chromosome 22 • Gene associated with Lou Gehrig’s disease found on chromosome 21 • Genes located together on same chromosome tend to be inherited together • Is there a special pattern of inheritance for genes located on the X chromosome or the Y chromosome?

10. Sex Linked Genes • Sex linked genes- gene located on the X or Y chromosome • Many sex linked genes found on X chromosome • Y chromosome is much smaller than X, contains only a few genes • Colorblindness- an inability to distinguish certain colors, most common is red-green, more common in males • Sex linked disorders are more common in men • X-linked alleles are always expressed in males, because males have only one X chromosome

11. Hemophillia- another sex linked disorder, a protein necessary for normal blood clotting is missing • Duchenne Muscular Dystrophy- sex linked disorder, results in progressive weakening and loss of skeletal muscle • X-Chromosome Inactivation • Females have 2 X chromosomes, one X chromosome is enough • How does the cell “adjust” to the extra X chromosome in female cells? • In female cells, one X chromosome is randomly switched off-this forms a Barr body • Female cats

12. Chromosomal Disorders • Nondisjunction- homologous chromosomes fail to separate during meisois • May involve autosomes, sex chromosomes, homologous chromosomes • If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes, and a disorder of chromosome numbers may result • Trisomy- when two copies of autosomal chromosomes fail to separate during meiosis, individual is born with three copies of chromosomes • Down snydrome- most common form of trisomy, involves three copies of chromosome 21 • Produces mild to severe mental retardation, and higher frequency of some birth defects

13. Sex Chromosome Disorders • Turner’s syndrome- females only inherit one X chromosome • Are sterile- unable to reproduce • Sex organs do not develop at puberty • Klinefelter’s syndrome- males inherit an extra X chromosome • Prevent individuals from reproducing • No reported instances of babies born without an X chromosome • Contains genes vital for survival and development of embryo

14. Sec 3- Human Molecular Genetics • Roughly 6 billion base pairs in your DNA • Genetic tests are now available for hundreds of disorders • DNA fingerprinting- form of DNA analysis that analyzes sections of DNA that have little or no known function that vary widely from individual to individual • Used to identify individuals • Used since 1980’s in United States • How has DNA fingerprinting been beneficial?

15. Human Genome Project • Human Genome Project is the ongoing effort in analyze the human DNA sequence • Scientists have analyzed smaller genomes, such as those of viruses and bacteria • Why is this an important breakthrough? • Provides useful clues to some of basic properties of life • May be useful in developing new drugs and treatments for diseases

16. Gene Therapy • The process of changing the gene that causes a genetic disorder • In gene therapy, an absent or faulty gene is replaced by a normal, working gene • Both positive and negative results • What potential ethical problems might occur if humans gain the opportunity to design our bodies? • High School Biology Resources