Inheritance patterns of congenital conditions
Dr Wrishi Raphael
Gregor Johann Mendel, an Augustinian priest and scientist unraveled the mysteries of modern genetics. A scientist and visionary born well ahead of his time, Mendels laws of genetics did not receive any acclaim or recognition till three decades of his death. The Mendelian Inheritance pattern states the most basic rules of heredity whereby cross breeding of certain plants and animals could favor certain desirable traits. His work between 1856 to 1863, not only has enormous implications in the field of agriculture and farming today, but the pattern by which congenital diseases in human beings may progress from one generation to the next also depends largely on Mendels laws of genetics.
Mendel worked with seven characteristics of pea plants: plant height, pod shape and color, seed shape and color, and flower position and color. With seed color, he showed that when a yellow pea and a green pea were bred together their offspring plant was always yellow. However, in the next generation of plants, the green peas reappeared at a ratio of 1:3. To explain this phenomenon, Mendel coined the terms recessive and dominant in reference to certain traits. In the preceding example, green peas are recessive and yellow peas are dominant. He published his work in 1866, demonstrating the actions of invisible factorsnow called genesin providing for visible traits in predictable ways.
Several congenital diseases follow the principle of Mendelian inheritance. Congenital diseases may be defined as diseases which a person may present from birth but the symptoms of the disease may be present from birth or later in life. The five common inheritance patterns are:
1) Autosomal Dominant: As the name suggests, in autosomal dominance the disease carrying gene has dominant features and one must therefore inherit a defective gene from one parent to have the disease.
The parent from whom the gene has been inherited may be suffering from the disease. The parent who has the gene in this mode of inheritance is called the carrier. Only one parent must be a carrier for the child to inherit the disease. Every pregnancy has a 50-50 chance of bearing that abnormal, dominant disease causing gene. Some autosomal dominant disorders include Familial Adenomatous Polyposis, (a disorder where multiple growths or polyps grow all along the colon), hypertrophic obstructive cardiomyopathy (a rare genetic cardiac condition which is characterized by sudden cardiac death in young athletes) and polycystic kidney disease (a kidney problem where multiple fluid filled cavities or cysts grow inside kidneys).
2) Autosomal Recessive: These specific genes which cause recessive disorders are not as powerful as their dominant counterparts. A person may be a carrier of a recessive gene but never exhibit any features of the disease. This pattern of inheritance involves the passing on of defective and diseased genes from both parents. If one parent is a carrier or sufferer of the disease his or her offspring may easily carry the gene but not suffer the disease.
In other words, for a child born to a couple who both carry the gene (but do not have signs of disease), the expected outcome for each pregnancy is:
A 25% chance that the child is born with two normal genes (normal)
A 50% chance that the child is born with one normal and one abnormal gene (carrier, without disease)
A 25% chance that the child is born with two abnormal genes (at risk for the disease)
Thalasaemia, one of the commonest birth defects in our country is an autosomal recessive disease where the offspring develop symptoms like anemia and large abdominal mass from an early age. Cystic fibrosis another disease of similar inheritance pattern produces problems like recurrent respiratory infections, absolute constipation (in new borns) etc.
3) X linked Recessive Inheritance: Since males have only one X chromosome, daughters always inherit their fathers X chromosome as well as one of her mothers two x chromosomes. A son inherits his Y chromosome from his father and one of his mothers two X chromosomes. Accordingly, typical features of X- linked inheritance are the absence of father to son transmission and all daughters of an affected male will be non-disease-affected carriers of the mutant X-linked gene. In simpler terms the male children in a family will have the X linked disease but the girls will be asymptomatic carriers. Haemophylia, a disease where blood fails to clot properly, is the commonest example of X linked recessive inheritance.
4) X linked Dominant Inheritance: This pattern of inheritance is rarer than others. If the father carries the abnormal X gene, all of his daughters will inherit the disease and none of his sons will have the disease.
That is because daughters always inherit their fathers X chromosome. If the mother carries the abnormal X gene, half of all their children (daughters and sons) will inherit the disease tendency. Fragile X Syndrome is one such disorder where children can have mental retardation, abnormal facial features.
All congenital diseases do not fit the Mendelian Inheritance patterns, for example mitochondrial diseases and other disorders which must be explained by sudden changes in genetic makeup. Nonetheless its good for doctors to have a fair idea of genetics while speaking to families considering conception or seeking explanations or predictions about congenital diseases.
The writer can be reached at
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