Genetic Counselling for Couples for Chance of Having a Child with Congenital Defect,Heterozygotes,Cystic Fibrosis,Enzyme Activity,Chromosome Structure

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Genetic Counselling for Couples for Chance of Having a Child with Congenital Defect
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	Genetic Counselling for Couples for Chance of Having a Child with Congenital Defect - Genetic counselling for couples who believe that there may be a risk of producing a child with a congenital disease, has now become routine aspect of medical practice, particularly in the developed countries. These parents may either voluntarily abstain from having any child or may undergo selective abortions on suspicion or after ascertaining it through antenatal diagnosis as discussed earlier.
A genetic counsellor should first be able to identify the disease and therefore should be first a clinician and then a geneticist. The hereditary or, developmental defects are tested through ammiocentesis. At about fifteenth week of pregnancy, a sample of amniotic fluid containing fetal cells is removed and cultured in vitro.
The cultured cells are then used for karyotype analysis and biochemical enzyme tests. The simplest cases asking for genetic counselling will be those having a family history of disease and the parents may like to know the chances of having a child free of that disease. In the simplest case, a couple may have one abnormal child (carrying disease) and would like to know the chances of having a normal child on the next pregnancy. In such a case, if the defect is controlled by single recessive gene and both parents are normal, the chance is three in four of having a normal child, although even a normal child will have a two third chance of being a carrier.
The parents may like to give birth to such a child who may be a carrier, because the chance of his or her spouse also being a carrier will be remote. However in such cases, even the possibility of having the abnormal grand child can be worked out, if the frequency of heterozygotes in the population available for marriage of the child is known. For instance, in case of fibrocystic disease (cystic fibrosis) of the pancreas, the frequency of heterozygotes in general population (in UK) is J/22. The chance of the normal child to be a carrier being 1122, the' chance of the grand child to be abnormal should be 2/3 x 1/22 x 1/4 = 1/132 (since 114 is the chance that a child born to both heterozygote parents will be defective).
	A risk of one in 132 mayor may not be worth taking depending upon temperament of the parents and the circumstances. More detailed calculation in these simple cases and also in cases of polygenic nature and variable (B.C.-22) penetrance can be done. The readers are advised to consult a book on human genetics for further details.In some cases, detection of a heterozygote may also be useful and possible. Following are the situations where it is possible. (i) When heterozygote, though phenotypically normal, produces a particular enzyme activity intermediate to those found in two homozygotes, the heterozygote can be identified. In, such cases if biochemical laboratory is available, deficiency like HGPRT (lesch-nyhan syndrome) in heterozygous condition can be detected from a blood sample or some skin cells.

(ii) When the mutant produces an altered form of gene product, the heterozygote may produce two different forms of protein that can be separated by electrophoresis to enable the identification of a heterozygote.

(iii) If the defect is associated with a chromosome structure, then with the availability of a cytogenetics laboratory, such an abnormality in heterozygous condition can be identified.