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.
all chromosomes other than the X and Y chromosomes, which are
designated the sex chromosomes.
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.
position of a gene on a chromosome.
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.
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
Karyogram: a figure
showing the paired chromosomes from a cell
arrayed in a standard sequence
the presence of two copies of each unique chromosome per cell. In
humans, the chromosomes occur in pairs and the diploid (2N) number is
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
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.)
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
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
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
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.)
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.