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Is it safe to marry my second cousin?

Wednesday, 28 November 2007
Answered by: Dr. Shirish Kumar
Consultant Haematologist,
Sir Ganga Ram Hospital,
New Delhi
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Q. This is further to my previous query regarding my marriage with my second cousin. I would like to know if a consanguineous marriage will always result in defective child? You had mentioned something about genetic disease in your reply. Can you tell me what that means? As far as the medical history is concerned, there have been no known diseases in our family that can be traced. You had also mentioned that there is 2 percent risk for defective children and this risk increases to 3 percent in the case of first cousins. Can you tell about this risk specifically in the case of second cousins?

A.  Genes are structures present in the cells of all living beings (plants and animals) that are the basic physical and functional unit of heredity. Each cell in the human body contains about 20,000 to 25,000 genes. They carry information that helps make what we are and determine traits or characteristics inherited from parents. Thus, parents pass some of their characteristics to their children via genes - physical characteristics like height, colour of hair, eyes, skin and even diseases or propensity to a disease. Each gene contains (encodes) instructions for making proteins which are the ultimate building blocks of everything in our body (both structurally and functionally). A genetic disease is a disorder which is inherited i.e. transmitted from parent to offspring through genes. Genetic diseases arise due to changes (called mutations) in genes which may be due to environmental agents (like ultraviolet light, chemicals etc.) or errors during the process of cell division. The genes in common or shared in double first cousin/uncle-niece are 25%, first cousins 12.5% and second cousins 6.25%. Since relatives share some of their genes by common descent, consanguineous marriage influences the incidence of some inherited disease. The detrimental health effects associated with consanguinity are caused by the expression of rare, recessive genes inherited from a common ancestor(s). The genetic influence in marriages between couples related beyond second cousins differs only slightly from that observed in the general population. The chance of having a child with a serious medical disorder in the general population (non-consanguineous marriage) is about 2% and this is marginally higher (3%) in a consanguineous couple (first cousins). That means the risk is not much higher than in other couples.

A.  Genes are structures present in the cells of all living beings (plants and animals) that are the basic physical and functional unit of heredity. Each cell in the human body contains about 20,000 to 25,000 genes. They carry information that helps make what we are and determine traits or characteristics inherited from parents. Thus, parents pass some of their characteristics to their children via genes - physical characteristics like height, colour of hair, eyes, skin and even diseases or propensity to a disease. Each gene contains (encodes) instructions for making proteins which are the ultimate building blocks of everything in our body (both structurally and functionally). A genetic disease is a disorder which is inherited i.e. transmitted from parent to offspring through genes. Genetic diseases arise due to changes (called mutations) in genes which may be due to environmental agents (like ultraviolet light, chemicals etc.) or errors during the process of cell division. The genes in common or shared in double first cousin/uncle-niece are 25%, first cousins 12.5% and second cousins 6.25%. Since relatives share some of their genes by common descent, consanguineous marriage influences the incidence of some inherited disease. The detrimental health effects associated with consanguinity are caused by the expression of rare, recessive genes inherited from a common ancestor(s). The genetic influence in marriages between couples related beyond second cousins differs only slightly from that observed in the general population. The chance of having a child with a serious medical disorder in the general population (non-consanguineous marriage) is about 2% and this is marginally higher (3%) in a consanguineous couple (first cousins). That means the risk is not much higher than in other couples.

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