Each gene, like a word, contains patterns of
nucleotides. Chromosomes, like sentences and
paragraphs, present strings of genes that convey
integrated and coordinated sets of instructions for
specific structures and functions throughout the
body. Collectively these genetic instructions are
the pages, written in code, that form an individ-
ual’s GENOTYPE. The outcome, the individual’s out-
ward presentation of his or her genetic code from
appearance to health, is the PHENOTYPE.
Decoding the messages: the cells The cells
decode, interpret, and implement an individual’s
genotype. Each gene carries an encoded message
that it transcribes to RNA(ribonucleic acid), a carrier
molecule within the cell. The RNA conveys the
gene’s message to the cell’s ribosomes. Ribosomes
are organelles (defined structures with specific
functions) within the cell. The job of the ribosome
is to translate the gene’s message into a specific pro-
tein. The protein then carries the message to its tar-
get within the body, which is usually molecular.
Transmitting the code: inheritance patterns
The function of conveying a genotype is as much
one of mathematics as biology. INHERITANCE PAT-
TERNS—the ways in which genes reorganize into
new pairs at CONCEPTION—are the patterns of statis-
tics. A geneticist can calculate with astonishing
accuracy the likelihood of certain traits passing
from parents to offspring. Such calculations
accommodate the potential combinations that can
arise from each parent’s genotype.
Health and Disorders of Genetics
In some respects what is perhaps most remarkable
about human genetics is the precision and consis-
tency with which myriad, intricate, and complex
biochemical actions take place not only to produce
a new human being but also to choreograph its
functions for eight decades or longer. Though
everyone’s genotype contains some mutations,
researchers believe most mutations have no con-
sequence for the body’s structure or function.
However, understanding of the complex interac-
tions among genes continues to evolve as geneti-
cists engage in further research.
It is a common misperception that there are
genes that cause disease, such that there are spe-
cific genes for HEMOPHILIAor CYSTIC FIBROSISin the
same fashion as there are certain genes for brown
HAIRor green eyes. There are not really “disease”
genes, however. There are instead flaws and errors
in the structures of certain genes (mutations) that
cause them to give the wrong instructions for syn-
thesizing their specific proteins. The consequence
is a gap, expansion, or rearrangement in the infor-
mation. In some situations a gene, or more com-
monly a segment of or an entire chromosome, is
missing—as if pages or chapters are torn from the
genetic book of life. In other situations the gene
may have extra material or its material is
rearranged—as if pages or chapters are inserted
into the book. The resulting errors in structure or
function can be quite significant.IDENTIFIED GENETIC AND MOLECULAR DISORDERS
ALPORT SYNDROME CLEFT PALATE/CLEFT PALATE AND
CONGENITAL HEART DISEASE LIP
CYSTIC FIBROSIS DOWN SYNDROME
EDWARDS SYNDROME EPIDERMOLYSIS BULLOSA
FAMILIAL ADENOMATOUS FAMILIALMEDITERRANEAN FEVER
POLYPOSIS(FAP)FANCONI’S SYNDROME
FRAGILE X SYNDROME G6PDDEFICIENCY
HEMOCHROMATOSIS HEMOPHILIA
HEREDITARY NONPOLYPOSIS HUNTINGTON’S DISEASE
COLORECTAL CANCER(HNPCC) hypertrophic
KERATOCONUS CARDIOMYOPATHY
KLINEFELTER’S SYNDROME LONG QT SYNDROME(LQTS)
MARFAN SYNDROME MUSCULAR DYSTROPHY
MYOPATHY myotonia congenita
NEURAL TUBE DEFECTS PATAU SYNDROME
PHENYLKETONURIA(PKU) POLYDACTYLY
PORPHYRIA PROGERIA
RETINOBLASTOMA SICKLE CELL DISEASE
SYNDACTYLY TAY-SACHS DISEASE
THALASSEMIA TRIPLEX SYNDROME
TURNER’S SYNDROME VACTERL
WILSON’S DISEASE VONWILLEBRAND’S DISEASE
WOLFF-PARKINSON-WHITE
SYNDROMEResearchers have identified more than 6,000
monogenic (single gene) mutations that result in
health disorders, affecting 1 child in every 200
born. Among them are CYSTIC FIBROSIS,SICKLE CELL
DISEASE, MARFAN SYNDROME, HUNTINGTON’S DISEASE,
andHEMOCHROMATOSIS. Other disorders, such as
CLEFT PALATE/CLEFT PALATE AND LIP, result from poly-
genic (multiple gene) mutations or CHROMOSOMAL110 Genetics and Molecular Medicine