Congenital hearing loss is one of the most common birth defects and affects about 3 in every 1000 babies. Roughly 60 percent of babies born with hearing loss is inherited. There are a two different types of hearing losses. Non-syndromic hearing impairment is the most common inherited hearing loss which accounts for about 70 percent of hearing impaired babies. Syndromic hearing impairment is when the deafness is due to other clinical abnormalities. This type of hearing impairment accounts for about 15 to 30 percent of hearing impaired babies. "Over 400 syndromes are known to include hearing impairment and can be classified as: syndromes due to cyotgenetic or chromosomal anomalies, syndromes transmitted in classical monogenic or Mendelian inheritance, or syndromes due to multi-factorial influences, and finally, syndromes due to a combination of genetic and environmental factors." Hearing impairments can be inherited in four ways: autosomal dominant inheritance, autosomal recessive inheritance, X-linked inheritance, and mitochondrial inheritance. A child has a 50 percent chance of being hearing impaired through the autosomal inheritance. Through the autosomal recessive inheritance, a child has a 25 percent chance of developing a hearing disorder. With the X-linked inheritance, females with a defective gene still have a possibility to hear because the other X-chromosome will compensate. Males, on the other hand, only have one X-chromosome, so any defective gene will be more likely to develop the hearing impairment. With the mitochondrial inheritance, only the mothers can pass on the defective mitochondria that can lead to deafness.