الواقع والحياد
22-04-2007, 12:22 AM
Twins
Twin, triplet, or quadruple pregnancies regularly occur in many species of animals. In humans, about 1 in 80 pregnancies results in twins. They can arise froma single fertilized egg (monozygotic twins, MZ) or from two different eggs (dizygotic twins, DZ). Monozygotic twins are genetically identical.
The study of genetic differences between monozygotic (identical) and dizygotic (nonidentical) twins seeks to determine the relative importance of genetic and environmental factors. While twin research has limited value in the search for individual genes, it may shed light on the possible genetic origin of complex traits, such as multifactorial diseases and human behavior.
Comparisons of monozygotic twins who have been raised together with those who were raised apart are especially informative. They allow insight into the role of genetic factors as opposed to environmental influences. In a large study, the Minnesota Twin Study (Bouchard, 1990), monozygotic and dizygotic twins who had been separated since infancy were compared. A series of complex tests demonstrated that monozygotic twins raised apart, compared with those raised together, were very similar in spite of their different environmental influences. About 70% of the variance of mental ability was associated with genetic variation.
A. Types of twins
Monozygotic twins arise by separation at a very early stage of development. They always have a common placenta (monoplacental). They may lie in two amniotic cavities (diamniotic) or in a common amniotic cavity (monoamniotic). Dizygotic twins (DZ) always have their own amniotic cavity (diamniotic). They may each have their own placenta (diplacental), or they may share a common placenta.
http://www.gazzaz.net/wcmn/gazzaz/fld0/200704/twin%2011.jpg
B. Pathological conditions in twins
Incomplete separation (so-called Siamese twins) or connections between the blood circulations lead to pathological conditions in twins.
A relatively frequent form of incomplete separation is thoracopagus (1), in which the twins are joined to various extents at the thoracic region.
The blood circulation of twins can be connected via the common placenta (formation of a shunt) (2). This can lead to one twin receiving less blood or even bleeding to death. Especially severe malformations result from incompletely formed organs, e.g., absence of the heart in one twin (acardius) (3). If one of the twins dies very early in pregnancy, it may disappear by the time the other twin is born (vanishing twin).
http://www.gazzaz.net/wcmn/gazzaz/fld0/200704/twin%202.jpg
C. Concordance or discordance
When twins show the same trait, they are said to be concordant; when they differ, discordant. Comparisons of the rate of concordance allow conclusions about the relative role of genetic factors in the etiology of complex traits such as some malformations, predisposition to diseases, height, blood pressure, etc. However, definitive conclusions about genetic causes can rarely be reached from concordance rates alone. (After Connor and Ferguson-Smith, 1991.)
http://www.gazzaz.net/wcmn/gazzaz/fld0/200704/twin%203.jpg
D. Biochemical differences
Dizygotic and monozygotic twins differ also biochemically. Due to genetic differences, many chemical substances used in therapy are ****bolized or excreted at different rates, owing to different activities of corresponding enzymes. An example is the rate of excretion of phenylbutazone, as studied by E. S. Vesell. This substance is excreted at the same rate in identical twins, whereas the rates of excretion differ between dizygotic twins or siblings.
http://www.gazzaz.net/wcmn/gazzaz/fld0/200704/twin%204.jpg
Twin, triplet, or quadruple pregnancies regularly occur in many species of animals. In humans, about 1 in 80 pregnancies results in twins. They can arise froma single fertilized egg (monozygotic twins, MZ) or from two different eggs (dizygotic twins, DZ). Monozygotic twins are genetically identical.
The study of genetic differences between monozygotic (identical) and dizygotic (nonidentical) twins seeks to determine the relative importance of genetic and environmental factors. While twin research has limited value in the search for individual genes, it may shed light on the possible genetic origin of complex traits, such as multifactorial diseases and human behavior.
Comparisons of monozygotic twins who have been raised together with those who were raised apart are especially informative. They allow insight into the role of genetic factors as opposed to environmental influences. In a large study, the Minnesota Twin Study (Bouchard, 1990), monozygotic and dizygotic twins who had been separated since infancy were compared. A series of complex tests demonstrated that monozygotic twins raised apart, compared with those raised together, were very similar in spite of their different environmental influences. About 70% of the variance of mental ability was associated with genetic variation.
A. Types of twins
Monozygotic twins arise by separation at a very early stage of development. They always have a common placenta (monoplacental). They may lie in two amniotic cavities (diamniotic) or in a common amniotic cavity (monoamniotic). Dizygotic twins (DZ) always have their own amniotic cavity (diamniotic). They may each have their own placenta (diplacental), or they may share a common placenta.
http://www.gazzaz.net/wcmn/gazzaz/fld0/200704/twin%2011.jpg
B. Pathological conditions in twins
Incomplete separation (so-called Siamese twins) or connections between the blood circulations lead to pathological conditions in twins.
A relatively frequent form of incomplete separation is thoracopagus (1), in which the twins are joined to various extents at the thoracic region.
The blood circulation of twins can be connected via the common placenta (formation of a shunt) (2). This can lead to one twin receiving less blood or even bleeding to death. Especially severe malformations result from incompletely formed organs, e.g., absence of the heart in one twin (acardius) (3). If one of the twins dies very early in pregnancy, it may disappear by the time the other twin is born (vanishing twin).
http://www.gazzaz.net/wcmn/gazzaz/fld0/200704/twin%202.jpg
C. Concordance or discordance
When twins show the same trait, they are said to be concordant; when they differ, discordant. Comparisons of the rate of concordance allow conclusions about the relative role of genetic factors in the etiology of complex traits such as some malformations, predisposition to diseases, height, blood pressure, etc. However, definitive conclusions about genetic causes can rarely be reached from concordance rates alone. (After Connor and Ferguson-Smith, 1991.)
http://www.gazzaz.net/wcmn/gazzaz/fld0/200704/twin%203.jpg
D. Biochemical differences
Dizygotic and monozygotic twins differ also biochemically. Due to genetic differences, many chemical substances used in therapy are ****bolized or excreted at different rates, owing to different activities of corresponding enzymes. An example is the rate of excretion of phenylbutazone, as studied by E. S. Vesell. This substance is excreted at the same rate in identical twins, whereas the rates of excretion differ between dizygotic twins or siblings.
http://www.gazzaz.net/wcmn/gazzaz/fld0/200704/twin%204.jpg