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Biological Etiology of Stuttering
There are many different ways stuttering aggregates. One's biology and genetics is one way stuttering develops. A person's family history is a big factor because it is believed stuttering is passed down in offspring. Stuttering is coded in one's chromosomes, DNA, and sex.

Phenotypic evidence
The phenotypic features of stuttering also gives plenty evidence to the role of genetics in the disorder. There are phenotypic features that are universal amongst developmental stutterers. Stutterers speak with syllable repetitions, syllable prolongations, and interruptions in the smooth flow of speech known as blocks. If stuttering were more of a learned trait and not an inherited trait, there would be differences in how people stutter and differences between different languages and cultures. Not everyone speaks the same or processes words the same, but yet developmental stutterers have the same traits. This is similar to how even across various cultures, those who suffer from Down syndrome, another genetic disorder, suffer from the same trait.

Familial statistics amongst stutterers
Some cases of stuttering aggregates in families. Approximately half of those who stutter have a family history of stuttering. First-degree relatives have an incidence of about 15%. Biological relatives of developmental stutterers have an approximately 10-fold higher risk than in the general population. Males are more likely to stutter than females, and this is a stronger trend when comparing the prevalence of stuttering in adults. The ratio of males to females who suffer from stuttering goes from approximately 2:1 in early childhood to 4:1 in adulthood. Stuttering is also greatly common amongst highly inbred families and amongst twins.

Etiology
The field of genetics says that one etiology of stuttering has a genetic basis. It is calculated that about 70% of variance for stuttering is attributed to genetic effects. Stuttering is now being acknowledged as a familial disorder.

Studies in families with high histories of stuttering, including twins and inbred families, have shown the potential for links between familial genetics and stuttering. Family histories, DNA, and pedigree were taken and tested from families with stutterers and compared amongst each other to find patterns, similarities, or mutations to find the genetic source of stuttering.

Genetics
Stuttering may be passed down onto offspring from previous generations. Stuttering may be encoded in chromosomes, which hold the genetic information of your DNA. Mutations, deletions, or additions in one’s chromosomes can make and impact on how it is externally represented. A fault in the chromosome that influences speech can cause one to stutter.

Chromosomes that cause stuttering
The DNA extraction comparisons showed connections on a number of chromosomes using logarithm of odds (LOD), which calculates the odds of a chromosome appearing in multiple subjects; the higher the LOD the more likely the multiple appearances is not just a coincidence. Chromosome 9 had a high LOD in ever-stuttering subjects, while chromosome 2 and chromosome 7 had distinctive enough LOD to be evidence for linkage. Chromosome 15 had positive evidence for linkage in persistent-stuttering phenotype with chromosome 13 had modest evidence. Chromosome 12 commonly appeared in highly inbred families in Pakistan, inclining incest plays a factor in stuttering.

Multifactoral Component
The chromosomes may also be multifactoral, interacting with each other to cause stuttering. Chromosome 15 interacts with chromosome 13 and chromosome 20. Chromosome 9 interacts with chromosome 2. Chromosome 7 interacts negatively with chromosome 2. These interactions turn on and off the genotype and phenotype of stuttering.

When Mendelian inheritance was used to look at the genetics of stutterers, the results showed that stuttering is an autosomal dominant locus for stuttering. To trigger stuttering, the genotype for the disease must be on the dominant allele A when paired with the recessive B. Sufferers of stuttering have either an AA or AB genotype. There is a virtually zero risk for those with the genotype BB.

Stuttering may be sex linked
There is a striking difference in sex in the severity and prevalence to those who stutter, inversely afflicting one sex, males, more than the other; mood disorders and autism are similar in its affliction more towards one sex. Males are more likely to suffer from stuttering compared to women. Women are more likely to spontaneously recover from stuttering. Children with a stuttering disorder have far more male relatives with the same disorder than women relatives4. Even though men are afflicted more, women who stutter have a high occurrence of women in their family who have also stuttered. In families where stuttering only affects the women, chromosome 21 has been linked. Chromosome 7 and 20 shows evidence of linkage amongst families with male-only stutterers.

Evidence from twin studies
Monozygotic twins are more concordant for stuttering than dizygotic twins. In monozygotic twins, the concordance for stuttering ranges from 20% to 83%. Dizygotic twins have a concordance rate that has a range of about 4% to 19%. Since monozygotic twins have identical DNA, both twins would have the genetic difference if they both suffered from stuttering. If only one twin from a monozygotic set, it would show that stuttering would more likely come from an environmental factor. If both twins stutter, it is strong evidence that it is a genetic component that causes the suffering. The DNA of dizygotic twins are not identical, but have many similarities. If only one twin stuttered, it could be due to the difference in their genes. This strongly supports that stuttering is a familial aggregation and rules out the explanation of stuttering being a simple imitative learning.