|Year : 1979 | Volume
| Issue : 2 | Page : 35-38
Parental ages, birth order and reproductive fitness in cataracts
T Padma, JS Murty, P Siva Reddy
Department of Genetics, Osmania University, Hyderabad, India
Department of Genetics, Osmania University, Hyderabad
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Padma T, Murty J S, Reddy P S. Parental ages, birth order and reproductive fitness in cataracts. Indian J Ophthalmol 1979;27:35-8
|How to cite this URL:|
Padma T, Murty J S, Reddy P S. Parental ages, birth order and reproductive fitness in cataracts. Indian J Ophthalmol [serial online] 1979 [cited 2020 May 30];27:35-8. Available from: http://www.ijo.in/text.asp?1979/27/2/35/31236
Cataract-a condition representing opacity of the lens is common throughout the world and forms a major cause of blindness. While different types of cataracts are under the control of different and independent genes, either autosomal dominant or recessive or X-linked, factors like drugs and radiation etc., are also known to induce this condition. The association of parental ages, birth rank of the propositii, bilaterality or unilaterality of the condition and such other factors with these cataracts may provide better understanding of their etiology and heterogencity. The role of these factors has been assessed mainly in conditions and diseases caused by rare mutations and chromosomal anomalies but not in the occurrence of common diseases like cataracts.
Another aspect of cataracts, as of any other disease, would be their influence on the fitness of the affected individuals. Senile cataracts are interesting as they occur only among aged individuals and hence may be associated with longevity and genes influencing longevity. One might then expect a higher fitness of these individuals and it would be interesting to examine this aspect of the basis of mortality among the progeny and sibs of the affected individuals.
| Material and Methods|| |
247 patients affected with cataracts (congenital: 87, juvenile: 34 and senile: 126) drawn from the in patient units of Sarojini Devi Eye Hospital, Hyderabad, formed the subjects for the present investigations. There was no abnormal concentration of these patients in any particular caste or ethnic group. All the cases were screened for various genetic markers with a view to find their role and association with the diseases studied. There were 40 familial cases with incidence in first or second degree relatives and 207 isolated cases. Familial incidence was ascertained mostly from the information given by the proband accompanying relatives and other details of methodology are as given in the earlier paper. Unilateral and bilateral distribution of the condition in familial and isolated cases were noted with a view to understand the involvement of genetic component as an ttiological factor leading to the condition. The association of parental ages with bilaterality or unilaterality of the condition was also analysed. Family sizes and mortalities were considered only for complete families. Further, among liveborn, deaths before reproducing age alone were considered for purposes of mortality and fitness estimations.
The association of birth ranks was tested by the method of Haldane and Smith, in which, the sum of birth ranks of affected sibs is worked out in each sibship and the difference between the observed sum and that of expected under the hypothesis of no association is tested as a normal deviate (z).
| Results|| |
In congenital cataracts all familial and 80% of the isolated cases were found to be bilateral whereas in juvenile cataracts 71.4% of the familial and 50.0% of the isolated cases were bilateral. Among senile cases 77.8% of the familial and 12.4% of the isolated types showed bilateral incidence. If the bilateral cases were more genetic in nature than the unilateral isolated cases, a good number of former cases could have resulted due to mutations. Such fresh mutations are usually associated with increased parental ages. However, in the present study, the mean parental ages in unilateral and bilateral cases of familial and isolated types did not show any significant difference. The possibility of genetic differences between bilateral and unilateral cases is therefore ignored and these cases pooled in subsequent analysis.
The average paternal and maternal ages at birth of propositii are presented in [Table - 1]. The fathers at birth of familial senile cataract patients were on the average 5.4 years older than the fathers of isolated senile cataracts. Similarly the mean maternal age at birth of these familial cases differed by 2.7 years. Both these differences were significant (at values for paternal ages and maternal ages being 3.80 and 2.11 respectively, with 115 degrees of freedom). The parental ages did not show any significant differences in other types of cataracts.
The test for association of cataracts with birth ranks revealed a significant association of isolated congenital cataracts with late par (2=4.34, P <0.01) and of isolated senile cataracts with early para (2=2.51,P<0.05). However, the familial cataracts were not associated with para although they were associated with higher parental ages.
The estimates of parameters connected with with reproduction and fitness are given in [Table - 2][Table - 3]. The sibship size on an average was higher in juvenile cataracts than in other cataracts. The sibship mortality was found to be higher in familial juvenile cataracts (31.71%) than in isolated juvenile cataracts (21.48%) and in other types of cataracts. The mortalities in the sibships (24.95%) and progeny- (29.33%) of isolated senile cataracts were higher than in the sibships (16.67%) or progeny (23.08%) of fainisial senile cataracts. None of these - differences were statistically significant on the basis of the usual X 2 test,
If we regard the isolated cases as mostly of non-genetic etiology, a comparison of the average number of progeny of a familial case, with that of an isolated case, provides an estimate of fitness of the genotype for cataract'. The relative fitness of familial cataracts in terms of liveborn progeny was found to be 93% and almost 100% in terms of adults progeny. Similarly the familial cataract patients were 13-16% less fit than the isolated cataract patients, when relative fitness is measured as the ratio of the progeny to sibship size. Both methods of comparison of relative fitness thus indicate a substantial reduction in fitness of hereditary senile cataracts.
| Discussion|| |
It is well known that many rare conditions like chromosomal abnormalities or syndromes are associated with late para and/or increased maternal age. Some rare dominant deleterious conditions like achondroplasia are also known to be associated with increased paternal age. It is however interesting to find a common condition like cataract also to be associated with parental age and birth rank. These associations are also showing heterogenecity between types of cataracts, senile cataracts being associated with increased parental age, both maternal and parental, while the rest are not. The birth ranks in all the hereditary cataracts shcwed negative association while among the isolated cataracts, congenital types were associated with late para_and that of senile with early para. Murty also reported an association of senile cataract with early para, about 50% of the patients being primi para, although he did not differentiate between familial arid isolated cases. These heterogenetics in the associations with parental ages and birth rank do point out a role of some physiological factors which are as yet unidentified in the etiology of cataracts.
Another aspect worth commenting from the present data is on the basis of the argument that senile cataracts as they are expressed only in aged individuals, they should be associated with genes for longevity. If it is so, the families in which senile cataracts occur would have lesser number of segregating lethal and detrimental genes affecting longevity and hence the mortalities in such families would be lower than in families in which senile cataracts do not occur. The finding that the mortalities in the sibships and progeny of hereditary cataracts are lower than in isolated cataracts [Table - 2] provides support to the above argument.
Regarding fitness of cataract patients, senile cataract cases provide a good opportunity to estimate fitness through comparison of progeny and sibship sizes unlike other cataracts i.e. juvenile and congenital types. The present results suggest that while cataracts may be associated with longevity of the affected individuals, the reproductive fitness of such individuals may however be less than normal i.e. by about 15 percent.
| Summary|| |
A total of 247 congenital, juvenile and senile cataracts were studied to ascertain the role of birth rank, unilateral or bilateral distribution of the condition, parental ages at birth and mortality among family members in the expression of the condition in familial and isolated cases.
Negative association of the isolated bilateral cases with parental ages suggests absence of major genetic differences between unilateral and bilateral distribution of the condition. Significant association of parental ages with familial senile cataracts and of late para with congenital cataracts were found suggesting heterogeneity between cataract types and role of physiological factors of unknown etiology. The estimates of relative fitness obtained on the basis of mortality among progeny and sibships, indicate that, while senile cataracts may be associated with enhanced longevity of affected individuals their reproductive fitness may be reduced as compared to normals.
| Acknowledgements|| |
We express our gratitude to Professor O.S. Reddy, Head, Department of Genetics, Osmania University, for providing the necessary facilities. Our thanks are also due to the staff of Sarojini Devi Eye Hospital and Institute of Ophthalmology for their co-operation during the course of the study.
| References|| |
Haldane, J.B.S. and Smith, C.A.B., 1948, Ann. of Eug., 14,117.
Krooth, R.S., 1955, Amer. J. of Hum Genet., 7, 325.
Mc Kusick, V.A., 1975, Mendelian Inheritance In Man, 4th edition., The John Hopkins University Press, New York.
Murty, J.S., 1973, East. Arch. of Ophthal., 1, 1.
Padma, T., Murty, J.S., Lakshmamma, K. and Siva Reddy, P., 1977. East. Arch. Ophthal., 4, 59.
Vogel, F., 1965, Mutations in Man: in Genetics Today. S.J. Geerts, (Ed), 835, Pergaman Press, London.
Wynne Davies, R., 1970, The Genetics of some common congenital malformations: in Modern trends in Human Genetics vol. I A.E.H. Emery, (Ed), 316, Butter Worths, London.
[Table - 1], [Table - 2], [Table - 3]