Genetics in the 21st Century

* Inheritance of Human Genetic diseases

* Sickle Cell Anaemia

* Other causes of Genetic Diseases

* Investigating Genetic Diseases

* Genetic Techniques

Uses of Genetic Engineering

* Genetic Councelling

* Transplant Surgery

*Cloning

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INHERITANCE OF HUMAN GENETIC DISEASE

Gene A length of DNA that codes for the production of a particular polypeptide

Allele 1 of the different forms of a gene which occupy the same locus on homologous chromosomes

Locus The position on a chromosome at which a particular gene is found

Phenotype A persons observable characteristics, resulting from an interaction between its genotype and its environment

Genotype Genetic makeup of an organism. Describes all the alleles that the nucleus of a human cell contains. Can be heterozygous/ homozygous

Dominant Dominant allele always shows its effect on the phenotype

Recessive only shows its effect on the phenotype when the dominant allele is absent

Autosomes all the chromosomes except the sex chromosomes

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GENE MUTATION CAN CAUSE GENETIC DISEASES

* Mutation is an alteration of a DNA sequence/change in number or structure of chromosomes

* Mutation will affect future generations only if it occurs in the gametes

* Very rare

* Phenylketonuria - 1 of the most common inherited disorders/ caused by a mutation on chromosome 12/ Genetic disease caused by a recessive allele( affected individual must have gained a mutant allele from each parent

* The gene codes form phenylalanine hydroxylase which breaks down the AA phenylalanine into tyrosine. When this gene is mutated the shape of phenylalanine hydroxylase changes + its unable to properly break down phenylalanine. Phenylalanine builds up in the blood and tissue fluid and causes severe brain damage.

* Cystic Fibrosis- caused by a mutation in a gene called Transmembrane conductance regulator (CFTR) on chromosome 7

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*2 copies of this gene from mom + dad
mutations are passed from parent to offspring
* CF allele is recessive + is the result of a deletion mutation when nucleotides are lost from normal DNA
* Protein created by CFTR gene spans the outer membrane of cells in the swear glands, lungs, pancreas and other organs + acts as a channel connecting the cytoplasm to the surrounding fluid. Channel is primarily responsible for the movement of chloride ions
* When CFTR protein does no work, sodium chloride balance is upset. Imbalance creates a thick, sticky mucus layer that cannot be removed by cilia + traps bacteria resulting in chronic infection. Recurrent infection scar lungs. Person with CF will need daily therapy to help them cough up mucus
* Mucus blocks secretion of digestive enzymes from pancreas. Lack of these enzymes leads to difficulty absorbing nutrients = malnutrition + poor growth. Patients take pancreatic enzymes orally to help alleviate the problem. Thickened secretions also lead to Liver problems


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HUNTINGTON'S DISEASE

* Brain disorder that affects ability to think talk and move

* Symptoms develop gradually over months/years ( aged 30-50)

* Memory loss, changes in personality, uncontrolled muscle movements.. dementia appears as condition progresses.. Difficulties with speaking and swallowing, weight loss, depression + anxiety

* mutation on chromosome 4

* Triplet repeat mutation when CAG triplet is repeated again + again.. more repeats, earlier symptoms start

* CAG codes for the production of the protein huntington. Huntington protein in people with H.Disease breaks into pieces and clumps together. Protein clumps build up inside nuclei of brain cells which impairs function or kills them

* Mutation inherited as a dominant allele- autosomal dominance. most sufferers heterozygotes- 1 in 2 chance of passing on disease

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SICKLE CELL ANAEMIA

* Codominance = when both alleles affect the phenotype

* SCA result of a mutation in gene producing haemoglobin

* Hb used to represent gene locus for B-polypep chain of haemoglobin.. 2 alleles of this gene; HbA normal + HbS allele for sickle cell.. results in 3 phenotypes

HbA HbA= all normal haemoglobin
HbA HbS= 1/2 normal + 1/2 SC haemoglobin- SC trait. Alleles codominant
HbS HbS= All SC haemoglobin- SCA. RBC have a shorter life in circulation, 15-30 days

* when a couple both have SC trait, each time they are expecting there is a 25% chance child will inherit normal haemoglobin.. 50% chance inherit SC trait... 25% will inherit SCA

* person who inherits SC trait will suffer no ill effects. If get abnormally short of O2 may suffer episodes in which small blood vessels become blocked. Should receive high levels of O2 during general anesthetics

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INHERITANCE OF ABO BLOOD GROUPS

* RBC contain a protein in plasma that determines ABO blood group
* 2 forms of protein, Antigen A + B
* Gene that codes for this protein has 3 alleles
* Gene locus represent by letter I so 3 alleles are : IA, IB + IO
* IA + AB condominant
* IO recessive to both IA + IB so neither antigent produced
* 6 possible genotypes + 4 phenotypes


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INHERITANCE OF SEX LINKED CHARACTERISTICS

* sex chromosomes X and Y are only homologous for part of their length ( part allows them to pair in meiosis)
* women have 2 X + men have 1X + 1Y
* Y chromosome much shorter than X
* X carries larger number of genes, Y very few
* sex linked disease= Haemophilia...blood disease, blood fails to clot adequately.. sufferers may bleed to death from a trivial wound...bleeding occurs into joints + other parts of the body...haemophilia A most common due to single recessive allele that results in body producing less factor VIII
* dominant allele, H, codes for production of factor VIII.. recessive allele, h, results in lack of the factor and causes disease haemophilia A. Because alleles linked to X chromosome they are always shown attached to X chromosome XH + Xh

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GENOTYPE/PHENOTYPE

XH XH/ Normal blood clotting female

XH Xh/ Normal blood clotting female- carrier for haemophilia

Xh Xh/ Haemophiliac female- very uncommon

XH Y/ Normal blood clotting male

Xh Y/ Haemophiliac male


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PREDICTING THE INHERITANCE OF GENETIC DISEASE

* useful way to trace the inheritance of a sex linked disease eg. haemophilia is to collect info about ancestors. Info used to construct a pedigree chart- shows the phenotypes of the various family members. Possible to predict the risks of a particular couple having a child who might suffer from a particular genetic disease.
* pedigree analysis can be used whenever the condition is caused by the allele of a gene that shows a simple inheritance pattern, eg. cystic fibrosis

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AUTOSOMAL LINKAGE

* Genes that have loci close together on autosomes can be linked- tend not to be separated during meiosis

* Eg. inheritance of ABO groups and nail patella syndrome
* NPS rare condition. People affected may have small or absent kneecaps, undeveloped nails, inability to straighten elbows fully + a greater risk of developing kidney disease
* NPS affects males + females equally NOT SEX LINKED
* Autosomal dominant + locus of the gene that causes it found on chromosome 9 very close to ABO blood group gene
* very close + inherited together, because the chances of chiasma forming and crossing over to separate these is less likely


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MAPPING GENE LOCI ON CHROMOSOMES

* In meiosis during prophase 1 the chromatids of a bivalent may break and reconnect to another chromatid resulting in the exchange of gene loci between maternal and paternal chromatids ... CROSSING OVER
* results in diff combinations of alleles in gametes + therefore offspring
* new genotypes called recombinants
* crossover is more likely to occur between genes that are far apart
* frequency of recombinants in the cross over value:

number of recombinant individuals produced/total number of offspring x 100

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DISEASE CAUSED BY TRANSLOCATION AND NON-DISJUNCTIO

* Translocation- another example of chromosome mutation where a piece of chromosome breaks off and is transferred to another chromosome
* can also cause Down's syndrome which occurs when the end of the long arm of chromosome 21 joins another chromosome

* Non-disjunction is an example of chromosome mutation when there is a change in the number of chromosomes
* ND of sex chromosomes during meiosis can result in either a gamete carrying 2 sex chromosomes or no sex chromosomes instead of the normal 1

*Turners syndrome is the most common sex chromosome abnormality of human females.
* 1/2 of the females with TS have X chromosome monosomy - only 1 X chromosome
* most of the other individuals are mosaics ( individuals cells do not all have exactly the same composition of chromosomes). Occurs because sometimes ND happens during early cell division of the embryo resulting in individuals with a mixture of 46, XX cells and 45,X cells. Exact mixture of the 2 cell types depends on when ND occurred.

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THE USE OF KARYOTYPES

* to confirm diagnosis of a disease caused by chromosome mutation a karyotype is used.
* organised profileof the physical appearance of the chromosomes of a single nucleus
* chromosomes are stained in some way, producing banding patterns that help to distinguish members of different pairs of homologous chromosomes

* chromosomes are arranged and numbered by size.. biggest to smallest. Then possible to see if there are any chromosome abnormalities

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Ethics deal with choices we live by.. its about moral judgements and is an expression of attitudes, feelings and preferences about how we ought to live and what decisions we ought to make..

individuals should make their own choices/do good and act in the persons best interests/avoid doing harm/ensure fairness

Down's syndrome most common cause of mental retardation and malformation in a newborn. affects equal number of males/females. as womans age increases the risk of having a down's syndrome baby increases significantly.

Maternal serum screening for DS has been undertaken in the UK since the late 1980's. done in order to inform the mother- she may then make a decision to abort the fetus but it will be a decision based on info.

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KLINEFELTER'S SYNDROME

* affects 1 out of every 500-1000 newborn males
* 4 most common symptoms- sterility, breast development, incomplete masculine body build, social/school learning problems
* most common out the 4 is sterility
* sufferers have normal sexual function but cannot produce enough sperm for conceiving a child
* general comprehension is lacking most KS patients excel in visual and mathematical based tests
* when its discovered prenatally that a mother is carrying a child with KS should she abort the pregnancy since all mothers want a perfect child ?

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KARYOTYPE ANALYSIS

*CELL SAMPLE
*CELLS PLACED IN A CULTURE MEDIUM WHICH STIMULATES MEIOSIS
*INCUBATED FOR 3 DAYS AT 37 DEGREES C..CELLS DIVIDE MANY TIMES
*ADD COLCHICINE WHICH INHIBITS SPINDLE FORMATION SO MITOSIS IS STOPPED AT METAPHASE WHEN CHROMOSOMES ARE EASILY SEEN
* SEPARATE THE CELLS + ADD A SALINE SOLUTION. CELLS SWELL + THE CHROMOSOMES SPREAD OUT
* SPREAD CELLS ON A SLIDE
* STAIN
* PHOTOGRAPH UNDER THE MICROSCOPE
* CUT OUT INDIVIDUAL CHROMOSOMES + PAIR HOMOLOGUES IN ORDER OF DECREASING SIZE

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THE ROLE OF RESTRICTION AND LIGASE ENZYMES

* genetic engineering involves 3 main stages
1. isolating and identifying the gene for the protein you want to make
2. putting the isolated gene into another organism using a vector
3. cloning the organism. produces large numbers of the genetically engineered organism all containing the gene for the protein you want to make

FIRST STAGE... isolate the gene. restriction enzymes will cut the DNA at a particular sequence of bases. some RE cut straight across both chains of DNA forming blunt ends. most useful enzymes make a staggered cut in the 2 strands forming sticky ends.
SECOND STAGE... cut ends are sticky because they have short stretches of single stranded DNA that are complementary to each other. if 2 pieces of DNA have been cut with same enzyme, sticky ends will stick to each other forming hydrogen bonds to complementary base pairs. pieces have annealed. RE highly specific. each RE will cut DNA only at a specific base sequence, 4-8 base pairs long. several recognition sites occur in a length of DNA so fragments of DNA of varying length will be produced... restriction fragments
THIRD STAGE...once the section of DNA has been cut out and annealed with the

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DNA of another organism, the broken DNA needs to be repaired. done by ligase enzymes.
DNA ligase repairs the DNA backbone by forming covalent bonds. it links up the sugar phosphate backbones of the newly paired section,. so restriction enzymes and DNA ligase are used together to join lengths of DNA from different sources.

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TECHNIQUES OF GENETIC ENGINEERING IN MICROORGANISM

Restriction endonucleases used to cut out required gene from DNA of donor organism

prepare a vector molecule to carry this DNA into a host cell- bacterium, bacterial plasmid often used as a vector

vector used to transport the DNA into host cell

same RE used to cut the bacterial plasmid.. leave sticky ends that correspond those of the human gene
sticky ends lined up + gene is attached, annealed by hydrogen bonding between base pairs

DNA ligase joins pieces of DNA together to make a recombinant plasmid.

incorporate engineered plasmid unto bacterium. once inside host bacterium the GM cells will need to multiply and quantities of the human protein can b produced. protein will need to be separated + purified for clinical use



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MARKER GENES

only a small % of cells take up recombinant DNA so its important to know which cells are GM

marker genes allow this

genes that express readily observed characteristics

inserted into the recombinant DNA that will be transferred to the vector

if this DNA is taken up and incorporated into the host cells genome the marker gene will express itself, so it is easy to find later which organisms have been successfully engineered

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ANTIBIOTIC RESISTANCE MARKER GENES AND REPORTER SY

marker gene could code for resistance to a particular antibiotic.
growing them in a medium containing the antibiotic does this

only the GE organisms will be resistant + so only they will grow
antibiotic will kill those that have failed to take up the plasmid. works well but has led to concerns about the spread of these resistance genes

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ANOTHER ROUTE TO ISOLATING A GENE

* complementary DNA (cDNA) is DNA copied from mRNA
* enzyme reverse transcriptase synthesises DNA from an RNA template
* enzyme is found in a group of virus's called retroviruses
* DNA made is single stranded but DNA polymerase can be used to produce a double stranded cDNA gene

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Comments

Andrea

Thanks becky

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