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

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

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

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?

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

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

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?

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

X-Chromosome Inactivation 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

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

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

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?

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

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