Genetics Exam 1 Terms And Def.

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Probability

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The chance or possibility that a specific event will occur

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Cross

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Mating of two organisms.

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Offspring

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- Results of a cross; "young" produced by cross.

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Gene

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DNA sequence , Section of a chromosome that carries the information for a specific trait.

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Allele

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Alternate forms of a gene

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Chromosomes

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Threadlike strand found in the nucleus made up of a series of genes; carries genetic information, DNA

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Dominant

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Gene/trait that appears or expresses itself; shown with a capital letter (e.g. Tall = T, Brown = B, etc.).

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Recessive

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Gene/trait that is hidden in the presence of a dominant; shown with a small, or lower-case, letter (e.g. short = t, blue = b, etc.).

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Genotype

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Genetic makeup of an organism (e.g. TT, Tt, tt).

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Phenotype

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Physical appearance of an organism (e.g. tall, short). Usually the appearance of the dominant gene, or the recessive if it is a purebred trait.

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Hybrid

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Organism with different alleles/genes for a trait (see heterozygous) .

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Purebred

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- Organism with different alleles/genes for a trait (see heterozygous) .

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Sex chromosomes

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The chromosomes that determine the sex or gender of an organism. An organism with two X chromosomes (XX) is a female. An organism with one X chromosome and one Y chromosome (XY) is a male.

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ACROCENTRIC

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Acrocentric chromosomes are those with the centromere very close to one end, giving the chromosome a 'V' shaped appearance. The acrocentric chromosomes are numbers 13, 14, 15, 21 and 22. The short p arms are very short and usually have small round appendages on stalks, known as 'satellites'.

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ALLELE

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There are usually two copies of a gene. These two copies are called alleles. In some cases, one or both, alleles will be mutated or altered in some way.

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AMNIOCENTESIS

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A procedure for obtaining amniotic fluid for prenatal diagnosis. Using a sterile needle, a sample of amniotic fluid is removed from the uterus; the amniotic fluid contains cells from the fetus which can be analysed to determine if the fetus has a specific disorder. The test is usually carried out in the 14th - 18th week of pregnancy (see Genetics Fact Sheet 15: Prenatal Testing and Screening for Fetal Abnormalities).

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AUTOSOMAL GENE

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Any gene which is located on an autosome.

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AUTOSOMAL DOMINANT MUTATION

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A dominant mutation in a gene which is carried on an autosome.

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AUTOSOMAL RECESSIVE MUTATION

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A recessive mutation in a gene which is carried on an autosome.

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AUTOSOME

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Any chromosome that is not a sex chromosome (that is not an X or Y chromosome). In humans, the autosomes are the numbered chromosomes and are given the numbers 1 - 22. Chromosome 1 is the largest and 22 is the smallest.

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BALANCED TRANSLOCATION (RECIPROCAL TRANSLOCATION):

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A rearrangement of the chromosomes with no apparent loss or gain of chromosomal material. A person with this rearrangement is not affected in any way. When a translocation chromosome results in the gain or loss of genetic material, it is said to be unbalanced and may cause a problem in health, growth or development.

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CELL

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The basic structural unit of all living organisms. While some organisms are made up of only one or several cells, humans are composed of millions of cells. Each cell is enclosed by a membrane and has a nucleus which contains the genetic material (DNA) in the form of chromosomes. Mitochondria are also found randomly scattered throughout the cell.

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CELL CULTURE

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A method for growing cells in the laboratory.

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CELL DIVISION

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The mechanism by which cells multiply during the growth of tissues or organs. The type of cell division involved in the growth of the body is known as mitosis. The cell division which produces sperm or ova in the testis or ovary is known as meiosis.

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CENTROMERE

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The constricted part of the chromosomes which separates it into its two arms. The short arm is called the 'p' arm (for 'petite'); the long arm is called the 'q' arm (because q follows p in the alphabet).

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CHIMERA

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A special kind of mosaicism in which an individual or tissue contains a mix of cells derived from two genetically different individuals. The blood of a patient who had a transfusion from another individual would be chimeric for example.

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CHORION

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The chorion develops into the placenta. Chorionic cells have the same genetic composition as cells of the fetus. Cells of the chorion are sampled during a prenatal diagnostic test called CVS (chorionic villus sampling).

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CHORIONIC VILLUS SAMPLING (CVS):

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A procedure for obtaining cells of the chorion to enable testing of the fetus for specific abnormalities. Samples of the cells may be taken through the vagina or through the abdomen of the pregnant mother: it is usually carried out in the 10th - 12th week of pregnancy (see Genetics Fact Sheet 15: Prenatal Testing and Screening for Fetal Abnormalities).

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CHROMOSOME

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A threadlike structure found in the nucleus of all the body cells (except red blood cells) consisting of DNA and proteins. Each chromosome can be thought of as a string of beads where every bead represents a gene

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CODOMINANCE

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The equal expression of both copies of a gene in an individual eg. presence of both haemoglobin A and S on electrophoresis in an individual heterozygous for sickle-cell disease.

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CONSANGUINITY

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Relationship between two individuals with a common ancestor, for example, cousins

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CROSSING OVER

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When chromosome pairs join together during meiosis (the division process which produces the egg and sperm cells), the two chromosomes may exchange material: part of one chromosome 'crosses over' and exchanges places with the corresponding part on its partner chromosome.

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CYTOGENETICS

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The microscopic study of chromosomes and how changes in chromosome structure and number affect individuals

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DIPLOID NUMBER

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This is the number of chromosomes in the somatic (body) cells. There are two copies. Opposite to concordant.

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DISOMY

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Meaning 'two bodies'. Where there are two copies of each chromosome. This is the normal chromosome complement.

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DIZYGOTIC TWINS

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Nonidentical twins, arising from two different eggs fertilised by two different sperm; such twins are also referred to as fraternal twins

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DOMINANT

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Every cell contains two copies of each gene. Where only one of the gene copies or allele is mutated, and the other allele is 'correct', but the person is affected by a disorder due to that mutation, the mutation is described as dominant. The mutated gene is said to be dominant over the other 'correct' copy of the gene. A disorder or characteristic caused by a dominant gene mutation only requires one of the genes to be mutated for the person to be affected.

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GAMETE

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Refers to the sperm cells in males and the egg cells in females.

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GENE

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The basic unit of heredity; a segment of DNA which contains the information for a specific characteristic or function.

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GENE CLONING

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Isolating a gene and then making multiple copies of it by inserting it into a bacterial cell or another organism

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GENE MAPPING

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Determining the relative locations of different genes on chromosomes.

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ISOCHROMOSOME

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A chromosome in which the arms ('p' and 'q') are of equal length and the information in each of the two arms is genetically identical.

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KARYOTYPE

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The term used to describe an individual's chromosomes that have been photographed through the microscope and then arranged according to a standard classification based on their group and size.

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LINKAGE

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The tendency for genes or segments of DNA which are located close together on the same chromosome to be inherited together.

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LOCUS

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The position on a chromosome of a segment of a gene.

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MARKER CHROMOSOME

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A chromosome, or part of a chromosome, usually small, of unknown origin.

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MEIOSIS

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The special cell division which only takes place in reproductive cells and results in egg and sperm cells which contain 23 chromosomes (the haploid number).

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MENDELIAN INHERITANCE

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This refers to the inheritance of single genes and follows specific patterns: autosomal dominant, autosomal recessive and X linked inheritance

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METACENTRIC

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Refers to a chromosome which has its centromere in the middle and the short (p) and long (q) arms are of equal length.

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MITOCHONDRIA

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These compartments or organelles in the cell are the main energy source: they are often called the powerhouse of the cell. The mitochondria also contain their own DNA and therefore genes; mitochondrial genes follow maternal inheritance

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MITOSIS

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The process of cell division in all cells except the reproductive cells. Mitosis results in 'daughter' cells which are genetically identical to the parent cells.

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MONOSOMY

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Where one chromosome is represented once only instead of twice; for example, girls with Turner syndrome have only one X chromosome instead of the usual two copies (Monosomy X).

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MOSAIC

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An individual who has some cells with an abnormal or unusual genetic or chromosomal makeup while the rest of the cells in the body have the usual genetic or chromosomal constitution. For example, a person who is mosaic for trisomy 21 would have some cells which have 47 chromosomes with an extra chromosome number 21 and other body cells which have the usual 46 chromosome complement. The number of cells with abnormal genetic or chromosomal content will determine the level of severity of the disorder.

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MUTATION

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A permanent change in a gene. If the mutation occurs in the germ line cells, it is then able to be inherited. Mutations in somatic cells cannot be inherited. Mutations can occur naturally and spontaneously or they may be due to exposure to mutagens.

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NONDISJUNCTION

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Where the chromosome pairs fail to separate correctly in meiosis, resulting in sperm or egg cells which have missing or extra chromosomes e.g. if chromosome number 21 fails to separate in the formation of an egg (or sperm), one egg (or sperm) will contain an extra copy of chromosome 21 (24 chromosomes) while the other egg (or sperm) will contain only 22 chromosomes

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PEDIGREE

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A diagrammatic representation of a family health history.

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PENETRANCE

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The probability of detecting the presence or clinical expression of a gene or combination of genes when they are present. If the penetrance of a particular disorder is less than 100%, not all individuals who carry a mutation in the gene or genes responsible for the disorder will develop symptoms. Such a disorder is said to have reduced or incomplete penetrance.

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PHENOTYPE

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The physical and/or biochemical characteristics of a person, an animal or other organism which are determined by their genetic makeup and/or environment.

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RECESSIVE

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Every cell contains two copies of each gene. Each gene contains the information for a particular gene product, such as a protein. If a gene is mutated, the gene no longer codes for the gene product. Where an individual has one gene copy or allele mutated and the other copy 'correct', the cell will only be producing half the amount of gene product. If this does not result in any disorder for the individual, the mutation is described as being hidden or 'recessive' to the correct copy of the gene. An individual with this genetic constitution is said to be a 'carrier' of a recessive gene mutation. For a recessive gene mutation to result in a particular characteristic or a disorder, both copies of the genes must be mutated

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RECURRENCE RISK

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The risk that an inherited disorder will occur again in a family.

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RFLPs

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(RESTRICTION FRAGMENT LENGTH POLYMORPHISMS):
The fragments of DNA which result when it is cut by special enzymes called restriction enzymes. The patterns of these fragment lengths are used to indicate the presence or absence of mutations in particular genes.

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RING CHROMOSOME

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This occurs as a result of the fusion of the two ends of the same chromosome; there is a consequent loss of genetic material.

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ROBERTSONIAN TRANSLOCATION

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A type of translocation exclusive to the acrocentric chromosomes (13, 14, 15, 21 and 22) in which two of these chromosomes join at or near their centromeres. This is effectively a fusion between two whole chromosomes

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SEX CHROMOSOME

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An X or a Y chromosome which are different from the 22 autosomes.

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SEX INFLUENCED

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A disorder or characteristic whose expression or severity differs between the sexes. That is, it occurs more frequently in either males or females.

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SEX LIMITED

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A disorder or characteristic that is expressed in only one sex. That is, it occurs only in males or in females.

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SEX LINKED

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A disorder or characteristic which is determined by genes carried on the X chromosome.

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SOMATIC CELLS:

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All the cells of the body except the reproductive cells (sex cells)

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TELOMERE

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The terminal or end segment of each chromosome arm.

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TETRAPLOIDY

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Four copies of every chromosome in a cell which results in 92 chromosomes in the cell, instead of the usual 46.

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TETRASOMY

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Four copies of a particular chromosome present in a cell, resulting in 48 chromosomes in the cell instead of the usual 46

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TRANSLOCATION

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This occurs when a piece of one chromosome breaks off and attaches to another, different chromosome. When no material is lost or gained the translocation is said to be 'balanced' and the individual is not affected. An 'unbalanced' translocation results

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TRIPLOIDY

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Having three copies of every chromosome resulting in 69 chromosomes in a cell instead of the usual 46.

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TRISOMY

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Three copies of a particular chromosome are present in a cell resulting in 47 chromosomes instead of the usual 46.

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UNIPARENTAL DISOMY

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Where both members of a chromosome pair are contributed by one parent rather than one from each parent. Uniparental disomy may be maternal or paternal.

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X LINKED GENE

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Any gene which is located on the X chromosome.

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X LINKED RECESSIVE MUTATION:

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A recessive mutation in a gene carried on the X chromosome.

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X LINKED DOMINANT MUTATION

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A dominant mutation in a gene carried on the X chromosome.

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ZYGOTE

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The single cell with 46 chromosomes resulting from the fertilization of an egg (23) by a sperm (23). Through cell division (mitosis), the zygote develops into a multicellular embryo and then into a fetus.

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Autosome

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A chromosome not involved in sex determination. The diploid human genome consists of 46 chromosomes, 22 pairs of autosomes, and 1 pair of sex chromosomes (the X and Y chromosomes).

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Biotechnology

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A set of biological techniques developed through basic research and now applied to research and product development. In particular, the use by industry of recombinant DNA, cell fusion, and new bioprocessing techniques.

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Centimorgan

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Centimorgan (cM): A unit of measure of recombination frequency. One centimorgan is equal to a 1% chance that a marker at one genetic locus will be separated from a marker at a second locus due to crossing over in a single generation. In human beings, 1 centimorgan is equivalent, on average, to 1 million base pairs.

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Centromere

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A specialized chromosome region to which spindle fibers attach during cell division.

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Chromosomes

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The self- replicating genetic structures of cells containing the cellular DNA that bears in its nucleotide sequence the linear array of genes. In prokaryotes, chromosomal DNA is circular, and the entire genome is carried on one chromosome. Eukaryotic genomes consist of a number of chromosomes whose DNA is associated with different kinds of proteins.

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Crossing over

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The breaking during meiosis of one maternal and one paternal chromosome, the exchange of corresponding sections of DNA, and the rejoining of the chromosomes. This process can result in an exchange of alleles between chromosomes. Compare recombination.

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Diploid

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A full set of genetic material, consisting of paired chromosomes one chromosome from each parental set. Most animal cells except the gametes have a diploid.

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Eukaryote

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Cell or organism with membrane- bound, structurally discrete nucleus and other well- developed subcellular compartments. Eukaryotes include all organisms except viruses, bacteria, and blue- green algae. Compare prokaryote. See chromosomes.

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FISH

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(fluorescence in situ hybridization): A physical mapping approach that uses fluorescein tags to detect hybridization of probes with metaphase chromosomes and with the less- condensed somatic interphase chromatin.

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Flow cytometry

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Analysis of biological material by detection of the light- absorbing or fluorescing properties of cells or subcellular fractions (i.e., chromosomes) passing in a narrow stream through a laser beam. An absorbance or fluorescence profile of the sample is produced. Automated sorting devices, used to fractionate samples, sort successive droplets of the analyzed stream into different fractions depending on the fluorescence emitted by each droplet.

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Flow karyotyping

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: Use of flow cytometry to analyze and/or separate chromosomes on the basis of their DNA content.

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Gamete

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Mature male or female reproductive cell (sperm or ovum) with a haploid set of chromosomes (23 for humans).

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Gene

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The fundamental physical and functional unit of heredity. A gene is an ordered sequence of nucleotides located in a particular position on a particular chromosome that encodes a specific functional product (i.e., a protein or RNA molecule). See gene expression.

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Gene expression

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The process by which a genes coded information is converted into the structures present and operating in the cell. Expressed genes include those that are transcribed into mRNA and then translated into protein and those that are transcribed into RNA but not translated into protein (e.g., transfer and ribosomal RNAs).

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Gene families

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Groups of closely related genes that make similar products.

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Gene mapping

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Determination of the relative positions of genes on a DNA molecule (chromosome or plasmid) and of the distance, in linkage units or physical units, between them.

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Gene product

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The biochemical material, either RNA or protein, resulting from expression of a gene. The amount of gene product is used to measure how active a gene is; abnormal amounts can be correlated with disease- causing alleles.

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Genetics

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The study of the patterns of inheritance of specific traits.

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Genome

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All the genetic material in the chromosomes of a particular organism; its size is generally given as its total number of base pairs.

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Haploid

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A single set of chromosomes (half the full set of genetic material), present in the egg and sperm cells of animals and in the egg and pollen cells of plants. Human beings have 23 chromosomes in their reproductive cells. Compare diploid.

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Heterozygosity

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The presence of different alleles at one or more loci on homologous chromosomes.

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Homologous chromosomes

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A pair of chromosomes containing the same linear gene sequences, each derived from one parent.

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Human Genome Initiative

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Collective name for several projects begun in 1986 by DOE to (1) create an ordered set of DNA segments from known chromosomal locations, (2) develop new computational methods for analyzing genetic map and DNA sequence data, and (3) develop new techniques and instruments for detecting and analyzing DNA. This DOE initiative is now known as the Human Genome Program. The national effort, led by DOE and NIH, is known as the Human Genome Project.

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In situ hybridization

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Use of a DNA or RNA probe to detect the presence of the complementary DNA sequence in cloned bacterial or cultured eukaryotic cells.

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Interphase

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The period in the cell cycle when DNA is replicated in the nucleus; followed by mitosis.

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Karyotype

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: A photomicrograph of an individuals chromosomes arranged in a standard format showing the number, size, and shape of each chromosome type; used in low- resolution physical mapping to correlate gross chromosomal abnormalities with the characteristics of specific diseases.

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Linkage

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The proximity of two or more markers (e.g., genes, RFLP markers) on a chromosome; the closer together the markers are, the lower the probability that they will be separated during DNA repair or replication processes (binary fission in prokaryotes, mitosis or meiosis in eukaryotes), and hence the greater the probability that they will be inherited together.

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Linkage map

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A map of the relative positions of genetic loci on a chromosome, determined on the basis of how often the loci are inherited together. Distance is measured in centimorgans (cM).

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Localize

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Determination of the original position (locus) of a gene or other marker on a chromosome.

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Locus

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The position on a chromosome of a gene or other chromosome marker; also, the DNA at that position. The use of locus is sometimes restricted to mean regions of DNA that are expressed. See gene expression.

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Metaphase

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A stage in mitosis or meiosis during which the chromosomes are aligned along the equatorial plane of the cell.

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Mitosis

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The process of nuclear division in cells that produces daughter cells that are genetically identical to each other and to the parent cell.

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Recombination

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The process by which progeny derive a combination of genes different from that of either parent. In higher organisms, this can occur by crossing over.

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Restriction fragment length polymorphism

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polymorphism (RFLP): Variation between individuals in DNA fragment sizes cut by specific restriction enzymes; polymorphic sequences that result in RFLPs are used as markers on both physical maps and genetic linkage

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Sex chromosomes

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The X and Y chromosomes in human beings that determine the sex of an individual. Females have two X chromosomes in diploid cells; males have an X and a Y chromosome. The sex chromosomes comprise the 23rd chromosome pair in a karyotype. Compare autosome.

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Single- gene disorder

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Hereditary disorder caused by a mutant allele of a single gene (e.g., Duchenne muscular dystrophy, retinoblastoma, sickle cell disease). Compare polygenic disorders.

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Somatic cells

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Any cell in the body except gametes and their precursors

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Telomere

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The ends of chromosomes. These specialized structures are involved in the replication and stability of linear DNA molecules. See DNA replication.