Amyotrophic lateral sclerosis (ALS), a fatal progressive neurodegenerative disorder, is the most common motor neuron disease in European populations. Approximately 10% of ALS cases are familial (FALS) and the other patients are considered as sporadic ALS (SALS). Among many ALS causing genes that have been identified, mutations in SOD1 and C9orf72 are the most common genetic causes of the disease. In Iranian patients, it has been shown that SOD1, as compared to C9orf72, plays a much more prominent role. To date, more than 170 mutations have been reported in SOD1. Genotype/phenotype correlation with respect to either different causative genes or different mutations of a specific gene has not been well established.
The inheritance of FALS in many families is atypical with one proband and one or two first/second degree relatives who also have the disease (Valdmanis & Rouleau 2008). The first big breakthrough in the genetics of FALS came in 1993 with the discovery of pathological mutations in the Cu-Zn superoxide dismutase (SOD1) gene in ALS patients (Rosen et al 1993). Since then there has been an explosion of research into the mechanism(s) by which SOD1 mutations cause ALS, however the answer remains elusive. There are now 16 genes associated with Mendelian forms of ALS (Table 1) which have mostly been identified using linkage analysis of rare families with large pedigrees affected by the disease (Lill et al 2011). More recently, studies to identify the proteins found in the ubiquitinated inclusions that are a common neuropathological feature of both ALS and FTD, have identified trans-activation response element (TAR) DNA binding protein of 43kDa (TDP-43) as the major component (Arai et al 2006; Neumann et al 2006). Mutations in the gene encoding TDP-43, TARDBP, were subsequently found as a genetic cause of ALS (Sreedharan et al 2008). The genetics of FALS has moved forward rapidly in recent years, providing invaluable insight into disease pathogenesis and allowing the development of animal models to further study the disease and efficacy of therapeutic compounds.

Autosomal Recessive: Parents do not have to be affected to have an affected child; often skips generations.
For autosomal dominant traits, only ONE copy of the mutated allele is required for an individual to be affected, and this can be inherited from either parent. Individuals with a dominant trait have a 1 in 2 chance of passing that allele, and, therefore, that trait, to each of their children. If a child is affected, one parent must be affected.
Clues to Pedigree Problems

A. When determining if a trait is sex-linked or autosomal…

1. An X-linked trait is usually expressed far greater in males since most are X-linked recessive. If X-linked, we will only consider X-linked recessive traits (none will be X-linked dominant).

2. Y-linked traits are only passed from father to son. Females are never affected. In this assignment, none of the traits are Y-linked.

3. An autosomal trait is expressed approximately equally in males and females.

B. When determining if an autosomal trait is dominant or recessive…

1. Dominant traits only require one allele to outwardly express the trait.

a. Dominant traits are often present in every generation.

b. If a child is affected, at least one parent must be affected.

2. Recessive traits require two alleles to outwardly express the trait.

a. Recessive traits often skip generations.

b. An affected child does not have to have an affected parent.

This is a pedigree from two highly inbred families containing the ALS2 gene, and it displays the autosomal recessive nature of the mutation.

A partial three generation pedigree of the index patient (indicated by the arrow), heterozygous for the p.D90A SOD1 mutation, is shown. The pedigree suggests autosomal dominant inheritance/transmission.