Congenital malformations - Edocr

GLOSSARY OF GENETIC TERMS 373
X
X chromosome: One of the two sex chromosomes,
X and Y.
Xenograft: Tissue or organs from an individual of one
species transplanted into or grafted onto an organism
of another species, genus, or family. A common example
is the use of pig heart valves in humans.
X-inactivation: The repression of one of the two
X-chromosomes in the somatic cells of females as a
method of dosage compensation; at an early embryonic
stage in the normal female, one of the two
X-chromosomes undergoes inactivation, apparently at
random, from this point on all descendent cells will
have the same X-chromosome inactivated as the cell
from which they arose, thus a female is a mosaic composed
of two types of cells, one which expresses only
the paternal X-chromosome, and another which expresses
only the maternal X-chromosome.
X-linked dominant: Describes a dominant trait or
disorder caused by a mutation in a gene on the X chromosome.
The phenotype is expressed in heterozygous
females as well as in hemizygous males (having only
one X chromosome); affected males tend to have a
more severe phenotype than affected females.
X-linked lethal: A disorder caused by a dominant mutation
in a gene on the X chromosome that is observed
almost exclusively in females because it is almost always
lethal in males who inherit the gene mutation.
X-linked recessive: A mode of inheritance in which
a mutation in a gene on the X chromosome causes the
phenotype to be expressed in males who are hemizygous
for the gene mutation (i.e., they have only one
X chromosome) and in females who are homozygous
for the gene mutation (i.e., they have a copy of the
gene mutation on each of their two X chromosomes).
Carrier females who have only one copy of the mutation
do not usually express the phenotype, although differences
in X-chromosome inactivation can lead to varying
degrees of clinical expression in carrier females.
Y
Y chromosome: One of the two sex chromosomes,
X and Y.