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Aggregation
The clustering of disease within a family, typical
of a complex disease such as diabetes. Because such
diseases do no follow patterns of inheritance characteristic
of single-gene disorders (AD, AR, XLR), prediction
of recurrence risk or disease severity is difficult.
Alleles
Different versions of the same gene. (Each person
has two versions, or two alleles, of each gene. There
is an exception: because males have only one X chromosome,
they have only one allele of each gene located on
the X chromosome.)
Blue sclerae
Sclera is a fibrous tissue that helps form the outer
layer of the eyeball. A blue-gray tint to the "whites"
of the eyes, or blue sclerae, may be apparent in several
genetic conditions, including osteogenesis imperfecta.
Chromosomes
Thread-like structures in the nucleus of a cell that
are made of DNA and structural proteins. Human cells
other than egg and sperm normally have 46 chromosomes
(23 pairs). Egg and sperm cells have 23 chromosomes.
Gene
A segment of DNA that contains instructions for making
a specific protein or proteins required by the body.
Genes are found in succession along the length of
chromosomes. Human beings have about 30,000 to 35,000
genes.
Genetics
The branch of science concerned with the means and
consequences of biologic inheritance.
Genome
All of the DNA of a given organism. One speaks, for
example, of the human genome or the mouse genome.
Genomics
the study of whole genomes, usually focusing on extensive
DNA sequences.
Independent assortment
The principle that different genes are distributed
independently into egg and sperm. For example, the
distribution of the genes associated with DI has no
effect on the distribution of genes associated with
eye color. Independent assortment explains why it
is possible to look like a certain family member,
but not to have the same medical conditions or traits,
and vice versa.
Isolated
A medical trait that appears to occur alone, without
accompanying co-findings.
Karyotype
A picture, or organized graphic representation of
the chromosomes in a single cell. Normal human karyotypes
show 23 pairs of chromosomes, numbered from larger
to smaller. The 23rd pair is the sex chromosomes (XX
= female; XY = male) for a total of 46.
Mutations
Changes in the DNA code. Mutations can be beneficial,
neutral, or harmful
(for example, disease-causing).
Pedigree
A diagram showing the genetic relationships between
members of a family that is annotated with relevant
medical information. Pedigrees are used to visualize
inheritance patterns and to aid in diagnosis and risk
assessment.
Proband
The person who has brought the condition in question
to medical attention.
Variable expressivity – The different ways the
same disease or disease-causing mutation can affect
individuals, even within the same family. Most genetic
conditions have several recognizable traits, but persons
with the same disease may show different combinations
of these traits, which can be less or more severe.
Segregation
The distribution of genes during the formation of
an egg or sperm. Each person has two versions (called
alleles) of each gene, but can only contribute one
version (allele) to each egg or sperm cell. In
complex diseases, chromosomes, genes, and mutations
in genes still obey the laws of inheritance (i.e.,
they still segregate). However, the disease itself
does not segregate in the predictable manner of a
single-gene disorder. The failure of the disease to
appear in a regular pattern presumably results from
the effects of multiple genes and multiple environmental
influences.
Syndrome
Multiple signs and symptoms that, together, constitute
the picture of a disease.
Variable expressivity
The different ways the same disease or disease-causing
mutation can affect individuals, even within the same
family. Most genetic conditions have several recognizable
traits, but persons with the same disease may show
different combinations of these traits, which can
be less or more severe.
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