Question

5. Concordance of monozygotic twins for epilepsy is 85% and dizy 20%. Determine what affects th genotype or environment. gotic e development of epilepsy in this case 6. Concordance of monozygotic twins for diabetes is 78% and dizygotic 18.5%. Determine, what affects diabetes in this case environment. - genotype or 7. A total 98 pairs of twins were examined for the manic-depressive syndrome. Among them were 45 monozygotic pairs and 53 pairs - dizygotic. Concordant for this trait were 41 pairs of monozygotic twins and 11 pairs of dizygotic twins. Calculate concordance on the manifestation of manic- depressive syndrome for each group of twins. Determine what affects the development of manic-depressive syndrome in this case, heredity or environment. 8, Concordance for myocardial infarction in monozygotic twins is 33%, and in dizygotic twins-2790. Is heritability coefficient determined by heredity or environment? 9. Concordance for thyrotoxicosis in monozygotic twins is 47%, while dizygotic twins is 3.1%. Is heritability coefficient determined by heredity or environment? 10. Concordance for gastric ulcer in monozygotic twins is 25%, and dizygotic twins in 10.7%. Is heritability coefficient determined by heredity or environment? 11.Concordance for rheumatic fever in monozygotic twins is 33%, and dizygotic twins in 10%. What determines the heritability coefficient: heredity or environment?

Answer 1

In genetics twin studies are done to determine whether a particular trait is determined by genotype or environment. For this concordance rate, ie the probability of two individuals having the same trait, is compared between identical or monozygotic twins and fraternal or dizygotic twins. If the trait is determined by genotype, monozygotic twins being almost genetically identical, will have higher probability of sharing the trait under consideration, hence giving very high concordance rate while fraternal twins having different genetic make up will have much lower concordance rate. On the other hand, if the trait is determined by environment or some other developmental factor, both monozygotic and dizygotic twins will have much lower and almost similar concordance rate, ie there won't be much difference in the concordance rate, in this case, between the two types of twins.

Now let us try to solve the given set of questions:

5. The development of epilepsy in this case is definitely genetic as the concordance in monozygotic twins for epilepsy is 80% which is very high whereas that for dizygotic twins is only 20%. Clearly there is a huge difference between the concordance rates in the two types of twins.

6. Due to the same reason as given above, diabetes is also determined by genotype.

7. Concordance is calculated as follows:

Number of twins concordant for a trait ×100 / (total number of twins under observation)

​​​​​​Now here we have out of 98 pairs of twins, 45 are monozygotic pairs and 53 are dizygotic pairs. Out of 45 monozygotic pairs, 41 pairs show concordance and hence concordance rate for manic depressive syndrome in monozygotic twins is (41×100)/45 = 91%

Out of 53 dizygotic twin pairs, only 11 pairs show concordance and hence concordance rate in this case is (11×100)/53 = 20%.

Clearly the trait of manic depressive syndrome is determined by heredity.

8. The concordance rate for myocardial infarction in monozygotic twins (33%) and that in dizygotic twins (27%) is very close, clearly suggesting that the trait is not determined by heredity but is environmental.

9. The concordance for thyrotoxicosis between the two types of twins has huge difference, that in monozygotic ones is 47% and in dizygotic twins is only 3.1%. Clearly this trait is determined by heredity and the low concordance in even monozygites might be due to the involvement of not one but two or more genes in the development of the trait.

10. Although there is quite a difference between concordance in monozygotic twins and dizygotic twins, the concordance in monozygotic twins is quite low (only 25%). This suggests gastric ulcer might be determined by both heredity and environment. Heredity might be making an individual susceptible to the disease when exposed to certain environmental factors.

11. Concordance data for rheumatic fever suggests that just like gastric ulcer, it ia also determined by both heredity and environment, heredity making an individual prone to the disease.