Autism is a neurodevelopmental condition with a wide spectrum of characteristics affecting social interaction, communication, and behavior. The established role of genetics in autism leads many to ask about the genetic makeup of those with the diagnosis, specifically whether they have a different number of chromosomes.
Human Chromosomes Explained
Every human cell contains a nucleus, which houses our genetic blueprint in structures called chromosomes. These thread-like structures are composed of DNA and proteins, organized into genes that carry inherited instructions for all bodily functions and traits.
A human cell has 46 chromosomes organized into 23 pairs. Of these, 22 pairs are autosomes carrying genetic information for the body’s general functions. The final pair consists of sex chromosomes, which determine biological sex; females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
Autism and Chromosome Count
The vast majority of people with an autism diagnosis have the standard 46 chromosomes. Their karyotype, or the organized profile of their chromosomes, shows the typical 23 pairs. Autism is not defined by aneuploidy, which is the presence of an abnormal number of chromosomes.
This distinguishes it from conditions like Down syndrome, caused by an extra copy of chromosome 21. While chromosomal anomalies can be associated with autism, these are specific cases and not characteristic of the autistic population as a whole.
Genetic Influences in Autism
While the chromosome count is standard, genetics play a part in the development of autism. The condition is highly heritable, and its genetic basis is complex, involving the interplay of many genes rather than a single one. This polygenic nature means numerous genetic variations each contribute a small amount to the likelihood of developing the condition.
These genetic factors include different types of variations, such as small changes in a single gene’s DNA sequence, known as mutations. Another type is a Copy Number Variation (CNV), where segments of DNA on a chromosome are deleted or duplicated. These are different from having an entire extra chromosome, as they involve smaller pieces of genetic material. For example, research has identified CNVs on chromosomes 15 and 16 that are present in about 1% of children with autism.
Associated Chromosomal Conditions
Although autism is not caused by an atypical number of chromosomes, certain genetic syndromes that involve chromosomal abnormalities are associated with a higher incidence of autism. In these cases, autism is a co-occurring condition, not the defining feature of the chromosomal disorder.
For example, Fragile X syndrome, caused by a mutation on the X chromosome, is a known single-gene cause of inherited intellectual disability and autism. Other conditions like Down syndrome (Trisomy 21) and specific microdeletion syndromes, such as 22q11.2 deletion syndrome, are also linked with a greater likelihood of an individual having ASD.