تعريفات مهمة في علم الgenetic engineering
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.
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Chromosome:
a highly ordered structure composed of DNA and
proteins that carries the genetic information. In humans,
there are 46 chromosomes ordered in pairs.
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Autosome:
all chromosomes other than the X and Y chromosomes,
which are designated the sex chromosomes.
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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.
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Locus:
the position of a gene on a chromosome.
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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.
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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.
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Karyotype:
the chromosome constitution of an individual.
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Karyogram:
a figure showing the paired chromosomes from a cell
arrayed in a standard sequence
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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.
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Haploid:
one copy of each unique chromosome. In humans, the
gametes are haploid (N = 23).
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Homozygous:
both alleles at a locus are the same. (In the ABO
system, an AA complement represents homozygosity.)
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Heterozygous:
the two alleles at a locus are different. (In the ABO
system, an AO complement represents heterozygosity.)
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Hemizygous:
the presence of only one chromosome or chromosome
segment rather than the usual two; applies to males with
a single X chromosome.
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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.)
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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.)
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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.)
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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.)
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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.)
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Independent assortment:
random assortment of chromosomes (paternal and
maternal) in the gametes; 50:50 chance of inheriting a
given chromosome from one parent.
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Linkage:
the presence of two or more genes on the same
chromosome that tend to be inherited together.
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Crossing over:
the physical exchange of genetic material between
homologous chromosomes.
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Recombination:
the generation of new allelic combinations on
chromosomes, usually by crossing over.
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Mitosis:
somatic cell division in which the DNA replicates and is
evenly distributed to two equal daughter cells.
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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.
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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.
المصدر كتاب :
McPherson & Pincus: Henry's Clinical Diagnosis and Management by Laboratory Methods