The probability that a dizygotic twin
with IBS would have a mother with IBS was greater than the chance that the twin would have a cotwin with IBS (Gastroenterology 121:799-804, 2001).
Our findings argue for a hereditary component of pyloric stenosis: (1) pre-dominance in boys; (2) familial aggre-gation in first-, second-, and third-degree relatives; (3) high concordance rate in monozygotic twins; (4) similar degree of aggregation in dizygotic twins
and siblings; (5) difference in risk for siblings vs.
If the similarity is found to be greater among monozygotic than dizygotic twins
, genetic factors play a role in the characteristic under investigation.
Morphological features such as the Carabelli's trait were found to have higher concordance in MZ twins than in dizygotic twins
[Biggerstaff, 1973; Boraas et al.
In studies of male twins, monozygotic twins (single egg) are more likely than are dizygotic twins
(two eggs fertilized by two different sperm) to be concordant for homosexuality.
The investigation involved more than 4,000 monozygotic and dizygotic twins
, aged 22-27 years (average age 25), who were enrolled in a population-based longitudinal twin study.
In fact, they found coronary flow rate was significantly lower in dizygotic twins
with a history of major depression compared to their fraternal twins without major depression, but there was no significant flow rate difference between monozygotic twins discordant for the disorder.
In monozygotic twins, only one placenta is present; consequently, the risk of death of one twin is higher than in dizygotic twins
, where two placentas are present.
The risk of disease in siblings of RA patients compared with that of the general population (Xs) is estimated as 2-17 and the concordance rates for monozygotic twins versus dizygotic twins
range between 15% and 3.
For ASD, probandwise concordance for monozygotic twins was 77% for 45 male pairs and 50% for 9 female pairs; however, concordance rates for dizygotic twins
were 31% for 45 male pairs and 36% for 13 female pairs.
MAKE TWO Non-identical or dizygotic twins
occur when two fertilized eggs are implanted in the uterus wall at the same time.
If the risk for alcoholism, at least in part, results from genetic factors, one would expect monozygotic twins, who have identical genomes, to have a higher concordance rate for alcohol dependence than dizygotic twins
, who on average only share one-half of their genomes.