Question

In studies of human MZ and DZ twin pairs of the same sex who are reared together, the following concordance values are identified for various traits. Based on the values shown, describe the relative importance of genes versus the influence of environmental factors for each trait.

Trait Concordance

MZ DZ

Blood type 100 65

Chicken pox 89 87

Manic depression 67 13

Schizophrenia 72 12

Diabetes 62 15

Cleft lip 51 6

Club foot 40 4

Answer 1

Monozygotic twins or MZ are twins formed from a single zygote which splits to form two embryos. They are phenotypically and genotypically similar but not exactly the same. They have maximum genetic similarity.

Dizygotic twins are produced when two eggs in the ovary gets fertilized simultaneously by two sperms creating two zygotes which formes two embryos. Dizygotic twins are nothing but normal siblings born together. They do not have as much genetic similarity as MZ twins.

Concordance value indicates the probability that a particular trait is shown by both the individual in a pair of twins.

Now,

For Blood type concordance value for MZ is 100 which shows that both the twins mandetorily will have the same blood type. So this is not related to their environmental factors. In case of DZ it is 65 because blood type is a purely genetic trait and probability of a certain type of blood group will depend upon the genetics of the parents but it is random.

For chicken pox both MZ and DZ have similar values which diminishes the genetic factor and environmental factor takes precedence. As chicken pox is caused by a virus , genetic factors do not have much influence in it.

Manic depression. MZ 67 while DZ is 13. This shows that for manic depression genetic factor is the more influencial factor than environmental factors. This is because both MZ and DZ twins are exoosed to similar environmental condition, but both MZ individuals got high genetic similarity as well as concordance value. But in case of DZ the concordance value is quite less which shows environmental factor is not the predominant cause.

For schizophrenia , concordance value of MZ is much higher than DZ , this shows that occurance of this disease is influenced predominantly by genetics than environmental factors

For Diabetes also the concordance value difference between MZ and DZ is quite high so this trait is also influenced predominantly by genetics than by environmental factors.

Cleft lip, here concordance value difference is also high so I would say this trait is also more genetic than influenced by environment. But as the concordance value is 51 for MZ, the genetic influence is not as predominant as for the above traits. Environmental factors also influences it.

Club foot is not predominantly a genetic disease but it is caused due to defective growth occurance. So in this case also genetic factor is not the predominanat factor as the concordance value for MZ twins is 40. So both genetic and environmental factors are responsible for this almost equally.