Hereditary diseases

Hereditary diseases are diseases that follow inheritance patterns within families. Since there is inherent difficulty in studies of heredity in humans (due to the impossibility of controlling crosses, long generation time and small number of offspring), the study of hereditary diseases is done through the construction and analysis of an ‘heredogram’. Alternatively, studies are also carried out on monozygotic twins (commonly called true twins) and dizygotic twins (commonly called false twins). Through these studies it is possible to associate hereditary diseases with different inheritance patterns: autosomal recessive, autosomal dominant, X-linked recessive, X-linked dominant, Y-linked and mitochondrial.

Autosomal recessive

Diseases within this group are determined by the presence of a recessive allele, i.e., in a family the disease:

skips generations (can pass several generations without any individual being affected);
has equal frequency in both men and women;
unaffected parents have affected children;
in the event that both parents are affected individuals, offspring are always affected.

Recessive autosomal diseases are more common in cases of consanguinity. There are many known examples of this group of disorders: Tay-Sachs disease (characterized by accumulation of the lipid ganglioside GM2), phenylketonuria (which is characterized by the defect in the enzyme phenylalanine hydroxylase) and cystic fibrosis (which affects mostly the lungs).

Autosomal dominant

The allele responsible for diseases of this group is the dominant and, in a family, disease phenotype is characterized by:

is present in all generations (affected parent has affected offspring);
has equal frequency in both men and women;
affected parents have healthy offspring.

An exception to these features concerns de novo mutations. Examples of autosomal dominant diseases are polydactyly (supernumerary fingers), familial hypercholesterolemia (characterized by a high concentration of cholesterol in the blood due to a defect in a protein, LDL receptor), Marfan Syndrome (which is characterized by defects in connective tissue) and Huntington’s disease (which, being a neurodegenerative disease, affects motor coordination).

X-linked recessive

While the disorders referred to above relate to mutations in autosomal chromosomes, these disorders are related to mutations in sex chromosomes (in this case on the X chromosome) and have different characteristics:

can skip generations;
men are most affected (since they only have one sex chromosome X, they only need to inherit a copy of the mutated allele to be affected);
unaffected women (but carriers of the mutated allele) have affected male offspring;
affected women have all male children affected;
affected father don’t have affected sons;
all daughters of an affected father will be carriers (they may or may not be affected).

Examples of diseases with this type of inheritance pattern are haemophilia (deficiency in important proteins for blood clotting), color blindness (inability to distinguish green from red), and Duchenne muscular dystrophy (muscular degeneration and atrophy leading to premature death).

X-linked dominant

The diseases of this group have the following characteristics:

is present in all generations (affected offspring has affected parents);
appears in both men and women (most often in women);
affected men have healthy children and affected daughters;
affected women pass the mutated allele to half of the sons and daughters.

Examples of such diseases are hypophosphatemia (which is characterized by a low level of phosphate in the blood).

Y-linked

For these diseases, which are associated with the Y chromosome, the characteristics are:

is present in all generations;
only men are affected;
affected men pass the mutated allele to all male offspring.

The diseases of this group often cause infertility to the affected man.

Mitochondrial

Since that in fertilization the mitochondria that remain in the egg or zygote come from the ovum (that is, from the mother), these diseases are characterized by being passed from mother to daughter only.

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References:

Alberts B., Johnson A., Lewis J., Raff M., Keith R., Walter P. (2007). Molecular Biology of the Cell (5th edition). Garland Science, New York.

Griffiths A.J.F., Miller J.H., Suzuki D.T., Lewontin R.C., Gelbart W.M. (2000). An Introduction to Genetic Analysis (7th edition). W. H. Freeman, New York.

Hereditary diseases