The parents of a child with an autosomal recessive disorder are usually not affected by the disorder, but are carriers of one copy of the defective gene. If both parents are carriers for the biotinidase deficiency, there is a 25% chance that their child will be born with it, a 50% chance the child will be a carrier, and a 25% chance the child will be unaffected.The chromosomal locus is at 3p25. The BTD gene has 4 exons of lengths 79 bp, 265 bp, 150 bp and 1502 bp, respectively. There are at least 21 different mutations that have been found to lead to biotinidase deficiency. The most common mutations in severe biotinidase deficiency (<10% normal enzyme activity) are: p. Cys33PhefsX36, p.Gln456His, p.Arg538Cys, p.Asp444His, and p.[Ala171Thr;Asp444His]. Almost all individuals with partial biotinidase deficiency (10-30% enzyme activity) have the mutation p.Asp444His in one allele of the BTD gene in combination with a second allele. Pathophysiology
Symptoms of the deficiency are caused by the inability to reuse biotin molecules that are needed for cell growth, production of fatty acids and the metabolism of fats and amino acids. If left untreated, the symptoms can lead to later problems such as comas or death. Unless treatment is administered on a regular basis, symptoms can return at any point during the lifespan. Diagnosis
Biotinidase deficiency can be found by genetic testing. This is often done at birth as part of newborn screening in several states throughout the United States.
Results are found through testing a small amount of blood gathered through a heel prick of the infant. As not all states require that this test be done, it is often skipped in those where such testing is not required. Biotinidase deficiency can also be found by sequencing the BTD gene, particularly in those with a family history or known familial gene mutation. Treatment
Treatment is possible but unless continued daily, problems may arise. Currently, this is done through supplementation of 5–10 mg of oral biotin a day. If symptoms have begun to show, standard treatments can take care of them, such as hearing aids for poor hearing. Epidemiology
Based on the results of worldwide screening of biotinidase deficiency in 1991, the incidence of the disorder is:
5 in 137,401 for profound biotinidase deficiency

One in 109,921 for partial biotinidase deficiency
One in 61,067 for the combined incidence of profound and partial biotinidase deficiency
Carrier frequency in the general population is approximately one in 120. See also
Biotin
Biotin deficiency
Multiple carboxylase deficiency
Holocarboxylase synthetase deficiency
3-Methylcrotonyl-CoA carboxylase deficiency

References
Further reading
Dobrowolski, Steven F.; Angeletti, Janine; Banas, Richard A.; Naylor, Edwin W. (2003). "Real time PCR assays to detect common mutations in the biotinidase gene and application of mutational analysis to newborn screening for biotinidase deficiency". Molecular Genetics and Metabolism. 78 (2): 100–7. doi:10.1016/S1096-7192(02)00231-7. PMID 12618081. McMahon, Robert J. (2002). "Biotin in metabolism and molecular biology". Annual Review of Nutrition. 22: 221–39. doi:10.1146/annurev.nutr.22.121101.112819. PMID 12055344. C. Neto, E.; Schulte, J.; Rubim, R.; Lewis, E.; Demari, J.; Castilhos, C.; Brites, A.; Giugliani, R.;  et al. (2004).