Congenital Defects
,• Approximately 10% of all human malformations are caused by an alteration in a
single gene. Such alterations are transmitted in several ways, of which two are of
special importance.
• First, if the malformation results from autosomal dominant inheritance, the affec
gene generally is inherited from only one parent. The trait usually appears in eve
generation and can be transmitted by the affected parent to statistically half of th
children. Examples of autosomal dominant conditions include
achondroplasia(dwarfism), cleidocranial dysostosis(delayed closure of cranial
sutures,clavical deformity,dental abnormalities), osteogenesis imperfecta(brittle
bone disease), and dentinogenesis imperfecta; the latter two conditions result in
abnormal formation of the dental hard tissues
• Second, when the malformation is a result of autosomal recessive inheritance, th
abnormal gene can express itself only when it is received from both parents.
Examples include chondroectodermal dysplasia, some cases of microcephaly, and
cystic fibrosis(impairment of mucus clearnace from lungs).
, The expression of the genotype is affected by the environment in wh
the embryo develops, and the final outcome of development is term
the phenotype.
Adverse factors in the environment can result in excessive deviation
from a functional and accepted norm; the outcome is described as a
congenital defect. Teratology is the study of such developmental
defects.
,• Approximately 10% of all human malformations are caused by an alteration in a
single gene. Such alterations are transmitted in several ways, of which two are of
special importance.
• First, if the malformation results from autosomal dominant inheritance, the affec
gene generally is inherited from only one parent. The trait usually appears in eve
generation and can be transmitted by the affected parent to statistically half of th
children. Examples of autosomal dominant conditions include
achondroplasia(dwarfism), cleidocranial dysostosis(delayed closure of cranial
sutures,clavical deformity,dental abnormalities), osteogenesis imperfecta(brittle
bone disease), and dentinogenesis imperfecta; the latter two conditions result in
abnormal formation of the dental hard tissues
• Second, when the malformation is a result of autosomal recessive inheritance, th
abnormal gene can express itself only when it is received from both parents.
Examples include chondroectodermal dysplasia, some cases of microcephaly, and
cystic fibrosis(impairment of mucus clearnace from lungs).
, The expression of the genotype is affected by the environment in wh
the embryo develops, and the final outcome of development is term
the phenotype.
Adverse factors in the environment can result in excessive deviation
from a functional and accepted norm; the outcome is described as a
congenital defect. Teratology is the study of such developmental
defects.
