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In this article we will discuss about the dominant and recessive types of autosome-linked genetic diseases.
Type # 1. Autosome-Linked Dominant Genetic Diseases:
Experimental genetics strictly defines the dominant mode of Inheritance of genetic diseases as the situation in which a single dose of gene is quite sufficient to allow the trait thus governed full expression (Table 11.1). The heterozygote for such a gene will be phenotypically identical with the homozygote and it is impossible to differentiate between them in terms of physical differences.
The characteristics of the autosomal dominant modes of inheritance are:
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(a) The trait is passed on via one of the parents approximately ½ to their children. This means a 50% inheritance probability for every child of a trait-bearer.
(b) The inheritance is totally independent of the sex of the bearer or the heir. Both sexes are equally affected and the defective gene may come from either father or mother.
(c) It also frequently happens with the diseases of the autosomal-dominant variety that an individual inherits the defective gene and passes it on, while he himself remains phenotypically unaffected. This phenomenon is known as incomplete penetrance.
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(d) Though the diseases of autosomal- dominant mode of inheritance follow generation after generation, irrespective of sex, it may happen that physiological or other reasons make a disease far more dangerous for one sex than for the other. This is known as sex limited effect of the gene.
Basic Rules for the autosomal-dominant mode of inheritance for genetic counselling:
(a) For every potential child of a patient with an autosomal-dominant hereditary disease manifesting complete penetrance, there is a 50% risk. Family members not suffering from the defect cannot pass it on; they are homozygous for the normal allele.
(b) Children resulting from the union of two heterozygotes run a 75% risk of inheriting the anomaly. This figure includes the 25% risk that such a child will be homozygous for the defective gene.
Example of autosomal-dominant genetic diseases:
Huntington’s chorea, Dupuytren’s contracture, Hip dislocation. Hypoplasias of digits, Brachydactyly etc. and some other examples are shown in Table 11.1.
Type # 2. Autosome-linked Recessive Genetic Diseases:
Till now there are approximately 950 known autosomal recessive diseases. Some of them are very important from the viewpoint of their occurrence and severity.
The autosomal recessive genetic diseases showed by their mode of inheritance that an union between the parents who are heterozygous for the defective gene produces:
(a) In the child a 25% chance of inheriting two defective genes and, therefore, being homozygous and sick,
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(b) 50% chance to produce child like the parents, and
(c) 25% chance that a child will be normal and homozygous for normal gene.
The most interesting feature of this type of inheritance is that the pathological gene can be passed on unnoticed from generation to generation in the recessive form and only the accidental union of the two people who are both heterozygous for the same pathological gene raises the 25% probability that a child of theirs will be homozygous for that particular gene and will, therefore, manifest the anomaly.
The rarer a gene in the population, the smaller the probability. A careful examination of the heterozygotes (carrying one pathological gene) regarding the diseases of the autosomal-recessive category will show distinct variation from the normal homozygotes (carrying both normal genes).
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Some of the autosomal-recessive pathological genes containing heterozygotes and their phenotypical defects in comparison to normal homozygotes are given below:
