Allied Chapter 24 Genetics

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Genetic information

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* genetics - the scientific study of inheritance and human disease
* Genetic information is used to detect and treat diseases, improve crop and livestock production, predict predisposition to illness, and even determine guilt or innocence in the criminal justice system

The science of genetics attempts to explain how biological characteristics are inherited.
* DNA is tightly wound into threadlike structures called chromosomes that are found in the nucleus of most cells in the body.
* Gene segments of the DNA strand carry information for a particular trait.

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human genome

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* refers to the entire set of human chromosomes and the collection of genetic material in each typical cell of the human body
* You might consider your genome as your "book of life" because instructions for your body characteristics, structure, and function are contained within your genome
* An ideogram is often used in genomics to show the physical structure of a chromosome
* the typical human genome includes 46 individual nuclear chromosomes and one mitochondrial chromosome.

* One of the biggest science news stories of the new century was the announcement that the Human Genome Project had completed its task of sequencing the human genome in early 2001. Scientists are hoping that this "map" of the genetic makeup of humans will allow us to identify specific genes associated with diseases and develop treatments specific to the disease.
* The total number of human genes identified was approximately 32,000. It is felt that the final total will reach 40,000, which is far less than the estimated number of 100,000 genes.

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codominance

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* two different dominant genes occurring together

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gamete

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* a male or female reproductive (sex) cell (sperm or egg) which contain only 23 chromosomes each (or half the usual number of nuclear chromosomes (the haploid number)).
* Females have two copies of the X chromosome (XX), while males have one X and one Y chromosome (XY).
* Meiosis of the spermatogonium and oogonium involves the production of gametes,
* female gametes are homogametic - only X
* male gamete is heterogametic - X & Y

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gene

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* a specific segment of base pairs in a chromosome
* one of many segments of a chromosome (DNA molecule)
* a distinct code within a DNA molecule
* may be actively transcribed in the chromatin form of DNA but not in the chromosome form

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mutagens

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* agents that cause mutations

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chromosome or chromatin strand

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* DNA molecule (an individual DNA molecule, not a group of DNA molecules)
* NOT a segment of a gene
* gametes contain 23 chromosomes each
* every cell in the human body, both male and female contains 46 chromosomes, EXCEPT gametes. (22 pairs of autosomes and one pair of sex chromosomes)
* each human cell contains 22 pairs of autosomal chromosomes and one pair of sex chromosomes
* sex chromosomes = eggs = X , sperm = X & Y

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albinism

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* condition that results from a lack of melanin
* a recessive trait that is present only in those with two recessive genes, two aa genes will result in albinism

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Mechanisms of Gene Function

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* Our bodies are formed from 50 to 100 trillion cells. * Each cell contains all of the body's genetic instructions stored as DNA.
* However, each cell uses only the instructions from the part of the DNA that it needs.

For example, a muscle cell uses only the DNA that provides the instructions for the muscle apparatus, whereas a nerve cell uses only DNA that provides the instructions for the nervous system.
* Each DNA molecule can be viewed as a set of shorter DNA sequences, or genes.
* Each gene is uniquely coded and ultimately directs a trait, structure, or function of a particular segment of the body.

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nucleotides

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* are arranged in a twisted ladder structure called a double-helix to form DNA

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complementary base pairing

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* describes the way in which two strands of DNA are linked together by the bases
* the sides of the DNA ladder are composed of the sugar-phosphate molecules and the rungs of the ladder are composed of the DNA bases, which are arranged in pairs on the ladder according to strict rules.
* Adenine can only pair with thymine (A-T)
* Cytosine can only pair with guanine (C-G)
* this system of bonding is called complementary base pairing.

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base sequencing

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* describes the sequence, or the order of the bases along a single strand of DNA
* it is the sequence of base pairs in each gene of each chromosome that determines heredity.
* although the types of base pairs in all chromosomes are the same, the order of the "sequence" of base pairs is not the same.

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Distribution of Chromosomes to Offspring

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* Within the cell, each DNA molecule is a tightly wound chromosome.
* A human cell contains 46 chromosomes arranged in pairs: 23 inherited from the mother and 23 inherited from the father:
a. Twenty-two of these pairs are called autosomes
b. The twenty-third pair, the sex chromosomes, differs between males and females. Meiosis of the spermatogonium and oogonium involves the production of gametes,

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autosomes

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* they appear identical to each other in both males and females.
* Each autosome is numbered by size and contains all 46 chromosomes (23 pairs)

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proteomics

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* the study or the analysis of the proteins encoded by the human genome.
* the ultimate goal is to understand the role of each protein in the body
* proteomics represents an effort to establish the identities, quantities, structures, and biochemical and cellular functions of all proteins in an organism, organ, or organelle, and how these properties vary in space, time, and physiological state

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humane proteome

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* The entire group of proteins encoded by the human genome

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independent assortment

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* of chromosomes ensures that each offspring from a single set of parents is likely to be genetically unique
* applies to individual genes or groups of genes

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Genetic Variation

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* aka. genetic recombination
* the independent assortment of chromosomes that ensures that each offspring from a single set of parents is likely to be genetically unique.
* The exact combination of genes we inherit is sometimes due to a process our chromosomes undergo, known as genetic recombination
* During meiosis, chromosomes become paired. Sometimes during this process, the maternal and paternal chromosomes get mixed up and redistribute themselves independently of the other chromosome pairs, which creates a genetically unique individual or genetic variation
* While the chromosomes are pressed together, they may also break and each may swap a portion of its genetic material for the matching portion from its mate. This form of recombination is called crossing-over.
* When the chromosomes glue themselves back together and separate, each has picked up new genetic material from the other.
* The information each carries is now different from before crossing-over and offers additional possibilities for genetic variation among offspring of a set of parents.

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crossing-over

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* when genes from a particular location cross over to the same location on the matching gene
* one phase in meiosis
* when the chromosomes are pressed together, they may also break and each may swap a portion of its genetic material for the matching portion from its mate.

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Hereditary Traits

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* one characteristic can have many different forms.
* An example of this might be human blood types. Each of these—type A, B, AB, and O—is different and is created by variations in the gene for that characteristic. The different variations of a gene are called alleles.

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Alleles

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* an alternative form of a gene, a variation of a gene

Alleles can be inherited in different ways:
* A dominant trait is expressed when one allele of a gene masks the expression of its partner in the pair or when two dominant alleles are inherited from each parent.
* An allele that is camouflaged by a dominant partner is referred to as the recessive allele. For a recessive trait to be expressed, both recessive alleles must be inherited: one from each parent.
* Sometimes, each allele in the gene pair is expressed equally and is said to be codominant. The blood type AB is an example of codominant genes. Traits inherited this way will show up as a combined physical characteristic.
ie. hair color and blood type

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dominant gene

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* one whose effects are seen and whose effects are capable of masking the effects of a recessive gene for the same trait
* are represented by uppercase letters and recessive genes by lower case letters
* if a person has a dominant and recessive gene such as Aa, he is said to be a genetic "carrier" of a trait.

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genetic "carrier"

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* has the ability to pass on that trait to future offspring
* a person has a dominant and recessive gene such as Aa

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Sex Determination

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The sex of a child is predetermined by the male gamete (sperm).
* The female gamete (ova) is said to be homogametic because the cell can possess only one sex chromosome, the X chromosome.
* The male gamete (sperm) is called heterogametic because approximately half of them contain the X chromosome and the remainder possess the Y chromosome.
* When fertilization occurs, two chromosome combinations are possible for the sex chromosomes: XX and XY.
* Since the female can only contribute an X chromosome to the pair, it is the male partner that determines the sex of an offspring by contributing either an X or Y gamete.

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Sex-Linked Traits (inheritance)

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* Any hereditary factor (nonsexual traits) that are carried on a sex chromosome
* also called X-linked traits
* In addition to determining the gender of a child, sex chromosomes also account for the expression of features that are carried on the X and Y chromosomes
* The X chromosome has more genetic information and is larger than the Y chromosome, so most sex-linked traits are carried on the female X chromosome and are often referred to as X-linked traits.
* There are no matching genes in the Y chromosome to mask recessive genes on the X chromosome. For this reason, X-linked recessive traits or sex-linked traits appear much more frequently in males than in females
* a person must have two dominant genes to get the condition for that gene. ie. sickle-cell, hemophilia, baldness, color-blindness
* only females with two recessive X-linked genes can exhibit the recessive trait.
* Because males inherit only one X chromosome—from the mother—the presence of one recessive X-linked gene is enough to show the recessive trait

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color-blindness

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* occurs more frequently in males, females can only inherit this form if her father is color-blind and her mother is either color-blind or a carrier

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codominance

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* when two different genes have equal effects
ie. blood group AB, combines the dominant trait of A with the dominant trait of B

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Genetic Mutations

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* a permanent change in the DNA sequence that makes up a gene
* Mutations range in size from one DNA base to a large segment of a chromosome
* Each cell depends on thousands of proteins to do their jobs in the right places at the right times. A mutation can cause a protein to malfunction or to be missing.
* When a mutation alters a protein that plays a critical role in the body, a medical condition called a genetic disorder can result
* Some mutations actually have a beneficial effect

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genetic disorder

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* When a mutation alters a protein that plays a critical role in the body
* Genes do not cause disease—genetic disorders are caused by mutations that make a gene function improperly

* More than 4,000 diseases stem from altered genes inherited from one's mother and/or father. Common disorders such as heart disease and many cancers arise from a complex interplay among multiple genes that result in mutations and between genes and factors in the environment.

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Beneficial Effect

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* An example of a beneficial mutation is one that could result in a protein that would protect the organism from a new strain of bacteria or virus.
* Certain mutations can produce new versions of proteins that help an organism and its future generations better adapt to changes in their environment.

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Genetic diseases

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* are medical conditions caused by mutations in a gene or a set of genes
* Mutations are changes in the DNA sequence of a gene, which cause an abnormality in the genetic code. * This can occur at any time in the life span
* In some disorders, entire chromosomes, or large segments of them, are missing, duplicated, or otherwise altered. Examples of these kinds of disorders are Down syndrome, Turner syndrome, and Klinefelter syndrome.
* Other disorders result when a mutation causes the protein product of a single gene to be altered or missing. Examples of these kinds of disorders are hemophilia, cystic fibrosis, and phenylketonuria.
* Even some cancers are thought to be caused, at least in part, by abnormal genes called oncogenes. However, it is uncertain how these genes become activated and produce cancer

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punnett square

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* a grid used to determine the probability of inheriting genetic traits
* a chart that demonstrates all possible gene combinations in parents whose gene types are known. * Punnett squares are named for an English geneticist, Reginald Punnett, who discovered some basic principles of genetics, including sex linkage and sex determination.
* useful for attempting to predict genetic traits, only predict probability, not actual outcomes.

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karyotype

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* an organized profile of a person's chromosomes
* disorders involving trisomy, monosomy, and broken chromosomes can be detected after this is produced
* Chromosomes are arranged and numbered by size, from largest to smallest, to help scientists quickly identify chromosomal alterations that may result in a genetic disorder.
* scientist take a picture of someone's chromosomes, cut them out and match them up using size, banding pattern, and centromere position as guides.
* cells can be collected from the lining of the cheek, blood sample, or by amniocentesis if done on a fetus.
*

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monosomy

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* abnormal genetic condition resulting from the presence of only one autosome instead of a pair
* may also result from nondisjunction.
* may be life threatening
* congenital condition that may also be called chromosomal genetic diseases

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gene therapy

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* treatment protocol begun in 1990 for children having adenosine deaminase deficiency, or severe combined immune deficiency

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pedigree

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* chart that illustrates genetic relationships in a family over several generations
* The chart is produced using medical records and family histories to gain insight into the probability of producing offspring with certain genetic disorders.
* Pedigrees chart the genetic history of family lines. Squares represent males, and circles represent females. Fully shaded symbols indicate affected individuals, partly shaded symbols indicate carriers, and unshaded symbols indicate unaffected noncarriers.

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trisomy

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* an abnormal genetic condition resulting from the presence of a triplet of autosomes rather than a pair.
* If the abnormal gamete takes part in a fertilization event, its two chromosomal copies are united with one chromosomal copy from the other gamete. This results in an embryo that has three copies of one particular chromosome, a condition called trisomy.
* Usually, trisomy of any autosome is fatal, but if it occurs in autosome pair 13, 15, 18, 21, or 22, a person can survive, but usually with profound developmental effects.
* congenital condition that may also be called chromosomal genetic diseases

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Mechanisms of Genetic Disease

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* An abnormal number of chromosomes result when a mistake called nondisjunction occurs in the production of ova or sperm

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nondisjunction

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* a mistake in meiosis that occurs when a pair of chromosomes fails to separate properly during gamete production
* An abnormal number of chromosomes result when a mistake called nondisjunction occurs in the production of ova or sperm
* the paired copies of a particular chromosome do not separate at cell division, and therefore, two copies of a chromosome—instead of one—are placed in a gamete

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Single-Gene Diseases

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* any genetic disorder caused by a change affecting only one gene
* the mutation of a single gene is both necessary and sufficient to produce the disease. Whether or not one develops
* the disease depends heavily on other factors such as inheritance patterns.
* Since single-gene diseases usually follow the genetic patterns of inheritance (autosomal dominant and recessive, or sex-linked), genetic counseling can often determine genetic risk for offspring developing the disease.

Single-gene diseases include:
* Cystic Fibrosis
* Phenylketonuria (PKU)
* Hemophilia
* Tay-Sachs disease (TSD)

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Cystic Fibrosis

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* This is a condition characterized by excessive secretion of thick mucus and concentrated sweat,
* often causing obstruction of the GI or respiratory tracts.
* It is caused by a recessive gene in chromosome 7.

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Phenylketonuria (PKU)

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* This condition is caused by a recessive gene that fails to produce an enzyme (phenylalanine hydroxylase) needed to convert phenylalanine into the amino acid tyrosine.
* Phenylalanine accumulates in the blood and destroys brain tissue. Phenyl ketones spill into the urine, causing phenylketonuria (PKU), which is where the disorder got its name.
* babies born with this condition are at a risk of progressive mental retardation and perhaps death

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Hemophilia

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* Caused by a recessive X-linked gene, this is a group of blood clotting disorders caused by a failure to form clotting factors VII, IX, or XI.

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Chromosomal Diseases

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Chromosomal diseases occur when the entire chromosome, or large segments of a chromosome, is missing, duplicated, or otherwise altered.
* Down Syndrome is a prominent example of a chromosomal abnormality.
* Other chromosomal diseases that result from nondisjunction of chromosomes include:
a. Klinefelter Syndrome
b. Turner Syndrome

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Turner Syndrome

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* Turner syndrome occurs in females.
* It is caused by the complete or partial absence of one of the two X chromosomes normally found in women. (XO syndrome)
* The syndrome is named after Dr. Henry Turner, who was among the first to describe its features in the 1930s.
* Characteristics of the disease include failure of the ovaries and sex organs to mature, cardiovascular defects, dwarfism, webbed neck, and possible learning disorders.

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Klinefelter Syndrome

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* Klinefelter syndrome affects male children and is the result of an extra X chromosome (XXY). * Characteristics of the syndrome include long legs, enlarged breasts, low intelligence, small testes, sterility, and chronic pulmonary disease.

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Down Syndrome

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* Down syndrome is a genetic disorder caused by three copies of chromosome 21.
* For this reason, Down syndrome is also called Trisomy 21.
* In 90 percent of Down syndrome individuals, the additional chromosome comes from the mother's ova.
* occurs in only 1 out of 600 or so live births
* after age 35 chances increase dramatically, by age 40 it occurs in 1 in 80 births.

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severe combined immune deficiency (SCID)

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* failure of lymphocytes to develop properly
* aka. Alymphocytosis
* characteristic is usually a severe defect in both the T- & B-lymphocyte systems.
* This usually results in the onset of one or more serious infections within the first few months of life.
* These infections are usually serious, and may even be life threatening, they may include pneumonia, meningitis or bloodstream infections.

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sickle-cell anemia

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* abnormal hemoglobin is produced

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Genetic Counseling

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* is available to families concerned about the risk of producing children with genetic diseases.
* Highly trained professionals have several tools and resources to assist parents regarding family planning. * A genetic counselor may also evaluate whether current family members have a disorder and may offer resources to ensure the highest quality of life.

some evaluative tools used by genetic counselors:
1. Karyotype: an organized profile of a person's chromosomes
2. Punnett square: a chart that demonstrates all possible gene combinations in parents whose gene types are known
3. Pedigree: a chart that illustrates genetic relationships in a family over several generations

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Treating Genetic Diseases

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1. Gene therapy is a technique for correcting defective genes responsible for disease development
2. gene augmentation therapy, normal genes are introduced with the hope they will augment, or add to , the production of the needed protein
* bacterial DNA rings called plasmids that have been altered by recombinant DNA techniques to carry the therapeutic genes
3. Gene replacement therapy - does not actually replace the defective genes - it instead inserts normal genes so that normal proteins can "replace" abnormal or missing proteins in the body's metabolic pathways.
4. Human engineered chromosome (HEC)

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Gene therapy

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* Gene therapy is a technique for correcting defective genes responsible for disease development
* In most gene therapy studies, a "normal" gene is inserted into the genome to replace an "abnormal," disease-causing gene.
* A major setback to gene therapy came in January 2003 when the FDA placed a moratorium on all gene therapy trials using retroviral vectors in blood stem cells
* This action was taken after the FDA learned that a second child treated in a French gene therapy trial had developed a leukemia-like condition. Both this child and another who had developed a similar condition in August 2002 had been successfully treated by gene therapy for X-linked severe combined immunodeficiency disease (X-SCID), also known as "bubble baby syndrome."

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gene augmentation therapy

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* normal genes are introduced with the hope they will augment, or add to, the production of the needed protein
* first step, white blood cells are collected from the patient and cultured,
* next they are infected with viruses carrying the therapeutic gene. these genetically altered cells are cultured until they have multiplied up to 1000-fold.
* finally, the genetically altered white blood cells are injected into the bloodstream, thereby reducing the immunity-inhibiting effects of the disease
* attempts to add genetically altered cells to the body rather than to change existing body cells as in gene replacement therapy.
* another type of gene augmentation therapy uses plasmids that have been altered to carry therapeutic genes

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bacterial DNA rings

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* a type of gene therapy
* bacterial DNA rings called plasmids that have been altered by recombinant DNA techniques to carry the therapeutic genes
* the gene is spliced into a plasmid
* next the plasmids are allowed to reproduce.
* finally the plasmids containing the therapeutic gene are delivered to the lung tissues by means of a common inhaler

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DNA

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* DNA is in a threadlike form called chromatin, during cell division each replicated strand of chromatin coils to form a compact mass called a chromosome
* less than 2% of the DNA carries genes, the rest called "junk DNA" either is not used or is edited out of mRNA before it is used to make proteins.

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pseudogenes

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* some of the noncoding DNA or "junk DNA" may actually be made up of broken bits of genes that are no longer functional - remnants of our evolutionary past
* these formerly functioning genes are like genetic fossils that have begun to reveal an interesting history of our genetic past

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genomics

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* the study of or analysis of the genome's code

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ideogram

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* a simple cartoon of a chromosome to show the overall physical structure of a chromosome
* the constriction of the diagram shows the relative position of the chromosome's centromere
* the shorter segment of the chromosome is called the p-arm, and the longer segment is called the q-arm.

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gene linkage

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* when a whole group stays together and crosses over as a single unit

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genetic predisposition

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* when certain diseases are not solely caused by genetic mechanisms, such conditions are not genetic diseases in the usual sense of the word, they are instead said to involve a genetic predisposition

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Tay-Sachs disease (TSD)

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* a recessive condition involving failure to make a subunit of an essential lipid-processing enzyme, hexosaminidase.
* abnormal lipids accumulate in the brain tissue of Tay-Sachs victims, causing severe retardation and death by 4 years of age
* most prevalent among Jewish populations
* higher than average incidents in French Canadians in southeastern Quebec and Cajun French families in southern Louisiana

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chorionic villus sampling

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* a procedure in which cells from chorionic villi that surround a younger embryo are collected through the opening of the cervix
* used in Karyotype analysis

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Human engineered chromosome (HEC)

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* a set of therapeutic genes is incorporated into a separate strand of DNA that is inserted into a cell's nucleus, thus acting like an extra or 47th chromosome
* a treatment for genetic disease

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adenosine deaminase (ADA) deficiency

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* a rare recessive disorder, that the gene for producing the enzyme ADA is missing from both autosomes in pair 20. Deficiency of ADA results in severe combined immune deficiency (SCID) making victims highly susceptible to infection

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electrophoresis

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* automated machines that can chemically analyze chromosomes and "read" their sequence of nucleotides - the genetic code
* DNA fragments are chemically processed, then placed in a thick fluid or gel. an electric field in the gel causes the DNA fragments to separate into groups according to their relative sizes
* the pattern that results represents the sequence of codons in the DNA fragment

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RNA interference (RNAi)

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* may also become a weapon against genetic disorders in an approach called RNAi therapy.
* RNAi is a method of silencing particular genes
* RNAi can turn off one gene at a time - greatly increasing the chances of figuring out which protein is encoded by that gene and what the function of that protein is.

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enzyme

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* the end product of protein synthesis
* helps regulate the biochemistry of the body

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Gregor Mendel

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* father of genetics

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amniocentesis

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* provides fetal cells that can be tested to determine probabilities of birth defects of fetal disease
* the fluid can detect disorders that involve trisomy, monosomy, and broken chromosomes