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Student Handout

Identifying Autism Genes by Tracking Gene Mutations

Caption: The diagram is a pedigree of a mother (AU-3104), father (AU-3103), daughter (AU-3102) and son with autism (AU-3101). Females are represented by circles and males by squares. Unaffected individuals are open symbols and the autistic son is a shaded symbol. The double line connecting the two parents indicates that they are related. Each horizontal bar represents a map of single-nucleotide polymorphisms (SNPs) along one arm of chromosome 3 of the family members. Red and blue vertical stripes indicate homozygous SNPs for either one of two alleles, yellow stripes represent heterozygous SNPs, and white gaps represent genetic deletions. The horizontal black line demarcates a region with a pattern of homozygosity not found in the boy with autism’s parents or sister.


Autism is a disorder that affects brain development and is usually characterized by repetitive behaviors and

interests, and by difficulties in communicating and interacting with others in social situations. Autism has a

strong genetic component; however, it has been difficult to identify which specific genes contribute to autism.

It’s likely that many different genes are involved.

When parents share ancestry (for example, if cousins have a child together), their children are at increased risk

of conditions caused by recessive genetic mutations. This is because relatives are more likely to have the same

recessive mutations, which may be rare in the population as a whole, and they can pass these mutations on to

their children, who will then be homozygous for the mutations. In these families, scientists use a technique

called “homozygosity mapping” to identify the locations of mutations associated with a recessive genetic

disorder. The technique basically involves finding regions in the genome where the DNA of an individual with the

recessive disorder is homozygous. Scientists can then further study those regions to determine which genes and

mutations are there.

To identify regions of homozygosity, scientists map single-nucleotide polymorphisms (SNPs), which are

variations found throughout the human genome at known locations. For each SNP, researchers determine

whether an individual is heterozygous or homozygous at that location. If an individual is homozygous for two

adjacent SNPs, which are typically mapped an average of 6,000 base pairs apart, then one can reasonably

assume that the entire stretch of DNA between the two SNPs will also be homozygous. Scientists then

determine which regions of homozygosity are in the affected person’s genome but not in the genomes of their

unaffected relatives. Such regions are likely to hold the genes responsible for the individual’s disorder.

Dr. Eric Morrow and colleagues mapped SNPs on chromosome 3 in a family in which the parents shared ancestry

and had a son with autism and a daughter without the disorder.

Genetics Revised September 2017 Page 1 of 1

  • Identifying Autism Genes by Tracking Gene Mutations

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