Mechanisms of cellular and tissue dysfunction in genetic diseases
are as varied as the organs they affect. To some extent, these mechanisms
are similar to those that occur in nonheritable disorders. For example,
a fracture resulting from decreased bone density in osteoporosis
heals in much the same way as one caused by a defective collagengene in osteogenesis imperfecta, and the response to coronary atherosclerosis
in most individuals does not depend on whether they have inherited
a defective low-density lipoprotein (LDL) receptor. Thus, the pathophysiologic
principles that distinguish genetic disease focus not so much on
the affected organ system as on the mechanisms of mutation, inheritance,
and molecular pathways from genotype to phenotype.
This chapter begins with a discussion of the terminology used
to describe inherited conditions, the prevalence of genetic disease,
and some major principles and considerations in medical genetics.
Important terms and keywords used throughout the chapter are defined
in Table 2–1.
Table 2–1 Glossary of Terms and Keywords.
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Table 2–1 Glossary of Terms and Keywords.
|Acrocentric||Refers to the terminal location of the centromere on chromosomes
13, 14, 15, 21, and 22.|
|Allelic heterogeneity||The situation in which multiple alleles at a single locus
can produce one or more disease phenotypes.|
|Amorphic||Refers to mutations that cause a complete loss of function
for the respective gene and, therefore, yield the same phenotype
as a complete gene deletion.|
|Aneuploidy||A general term used to denote any unbalanced chromosome complement.|
|Antimorphic||Refers to mutations that, when present in heterozygous form
opposite a nonmutant allele, will result in a phenotype similar
to homozygosity for loss-of-function alleles.|
|Ascertainment bias||The situation in which individuals or families in a genetic
study are not representative of the general population because of
the way in which they are identified.|
|Autosomal||Located on chromosomes 1–22 rather than X or Y.|
|CpG island||A segment of DNA that contains a relatively high density
of 5′-CG-3′ dinucleotides.
Such segments are frequently unmethylated and located close to ubiquitously
|Dictyotene||The end of prophase during female meiosis I in which fetal
oocytes are arrested prior to ovulation.|
|Dominant||A pattern of inheritance or mechanism of gene action in which
the effects of a variant allele can be observed in the presence
of a nonmutant allele.|
|Dominant negative||A type of pathophysiologic mechanism that occurs when a mutant
allele interferes with the normal function of the nonmutant gene
|Dosage compensation||Mechanism by which a difference in gene dosage between two
cells is equalized. For XX cells in mammals, decreased expression
from one of the two X chromosomes results in a concentration of
gene product similar to an XY cell.|
|End-product deficiency||A pathologic mechanism in which absence or reduction in the
product of a particular enzymatic reaction leads to disease.|
|Epigenetic||Refers to a phenotypic effect that is heritable, through
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