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    تعريفات في علم الجينات

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    مجاهد ادريس

    المساهمات : 164
    تاريخ التسجيل : 24/02/2010
    العمر : 27
    الموقع : we.ja2009@hotmail.com

    تعريفات في علم الجينات

    مُساهمة من طرف مجاهد ادريس في الجمعة أبريل 09, 2010 4:11 am

    بعض التعريفات المستخدمة في علم الجينيتك

    Gene: a sequence of nucleotides that represents a functional unit of inheritance; a region of DNA that codes for a product, either RNA or protein.

    Chromosome: a highly ordered structure composed of DNA and proteins that carries the genetic information. In humans, there are 46 chromosomes ordered in pairs.

    Autosome: all chromosomes other than the X and Y chromosomes, which are designated the sex chromosomes.

    Homologous chromosomes or homologs: sister chromosomes, the members of a pair of chromosomes in which one is inherited from the mother and the other from the father.

    Locus: the position of a gene on a chromosome.

    Allele: an alternative form of a gene occupying the same locus. An allele may be the result of a mutation. There is a maximum of two alleles per diploid chromosome complement (one allele per chromosome), but multiple alleles may exist within a population.

    Mutation: a permanent heritable change in the sequence of genomic DNA. This may manifest at both the molecular and cytogenetic levels. Not all mutations are negative events. Many are benign (e.g., blue eye color) and some have positive effects (e.g., sickle cell trait in countries with a significant risk of malaria). Individuals with a constitutional mutation (i.e., a mutation present in every cell of the body) may pass that mutation on to their progeny by germ-line transmission. In some cases, notably cancer, an acquired mutation may arise in a single somatic cell, which then divides mitotically, giving rise to a new clone of cells. The mutation will be limited to this clone and will not be transmitted to progeny of the individual. In rare instances of gonadal mosaicism, a de novo acquired mutation may arise in the gonads, resulting in a mixed population of normal and mutant gametes. Progeny receiving the new mutation may display a phenotype not present in either parent.

    Karyotype: the chromosome constitution of an individual.

    Karyogram: a figure showing the paired chromosomes from a cell
    arrayed in a standard sequence

    Diploid: the presence of two copies of each unique chromosome per cell. In humans, the chromosomes occur in pairs and the diploid (2N) number is 46.

    Haploid: one copy of each unique chromosome. In humans, the gametes are haploid (N = 23).

    Homozygous: both alleles at a locus are the same. (In the ABO system, an AA complement represents homozygosity.)

    Heterozygous: the two alleles at a locus are different. (In the ABO system, an AO complement represents heterozygosity.)

    Hemizygous: the presence of only one chromosome or chromosome segment rather than the usual two; applies to males with a single X chromosome.

    Genotype: the genetic constitution of an individual or organism (i.e., what alleles are present). (In the ABO system, AA, AO, BB, BO, AB, and OO are genotypes.)
    Phenotype: the appearance of an individual that results from the interaction of environment and genotype. (In the ABO system, A, B, and O blood types represent the phenotypic expression of the alleles for a given individual.)

    Dominant allele: an allele that is expressed when present in only a single dose (i.e., it ‘dominates' over the other allele present). (In the ABO system, A is dominant over O such that an AO genotype results in an A blood type phenotype. Similarly, the presence of pigment (T) is dominant to the absence of pigment (t) (i.e., albinism), such that Tt results in pigmentation.)

    Recessive allele: in a diploid organism, an allele that is only expressed when homozygous. (In the ABO system, the O blood group is only seen with a OO genotype; O is recessive to A and B. Similarly, albinism (t) is recessive to pigmentation (T), and an albino phenotype only occurs with a tt genotype.)

    Codominant alleles: in a diploid organism, alleles that show no dominance or recessivity to each other but, when present together are both fully expressed. (In the ABO system, A and B are codominant such that an AB genotype expresses both A and B antigens.)

    Independent assortment: random assortment of chromosomes (paternal and maternal) in the gametes; 50:50 chance of inheriting a given chromosome from one parent.

    Linkage: the presence of two or more genes on the same chromosome that tend to be inherited together.

    Crossing over: the physical exchange of genetic material between homologous chromosomes.

    Recombination: the generation of new allelic combinations on chromosomes, usually by crossing over.

    Mitosis: somatic cell division in which the DNA replicates and is evenly distributed to two equal daughter cells.

    Meiosis: cell division in the gonads that produces the gametes. A single DNA replication is followed by two cell divisions which reduces the total DNA content of a cell from 2N to N. Recombination occurs to increase genetic diversity within a population.

    Nondisjunction: failure of chromosomes or chromatids to separate to opposite poles in cell division. Usually results in one too many or one too few chromosomes in a cell.


    Please be advised that all definitions were taken from the Henry's Clinical Diagnosis and Management by Laboratory Methods

      الوقت/التاريخ الآن هو الإثنين يناير 22, 2018 8:57 am