Genetics · BEGINNER LESSON
Homozygous
Homozygous means having the same allele at a DNA position, or the same state across a genomic region, on both inherited copies. In long genomic regions, this can appear as a run of homozygosity, often reflecting shared ancestry.
What it is
A person, animal, or cell is homozygous at a DNA site when both copies of that site carry the same allele, meaning the two inherited versions match. In genomics, scientists also describe long contiguous stretches where all sites are homozygous; these are called runs of homozygosity, or ROH. ROH are measured by looking at the length and number of uninterrupted homozygous regions in DNA data.
[1] The Influence of Demographic History and Genetic Architecture on Complex Traits via Runs of Homozygosity [2] Clinical significance of regions of homozygosity detection in prenatal ... [3] Genome-wide assessment of runs of homozygosity and ... - PMC
How it happens
Homozygosity can arise when the same haplotype is inherited from both parents because they share ancestry. Recombination breaks chromosomes into smaller pieces over generations, so long ROH usually suggest more recent common ancestry, while shorter ROH suggest older ancestry. ROH can also appear through processes such as parental relatedness, uniparental disomy (UPD, when both chromosome copies come from one parent), or chromosomal events that make long homozygous regions.
[2] Clinical significance of regions of homozygosity detection in prenatal ... [3] Genome-wide assessment of runs of homozygosity and ... - PMC
Why it matters
Homozygous regions matter because recessive alleles can have an effect only when paired with the same allele. Inside an ROH, a rare recessive allele is more likely to meet an identical copy than in a non-ROH region. Studies also use ROH to study trait variation, population history, and inbreeding. In prenatal chromosomal microarray analysis (CMA), ROH can point to possible autosomal-recessive disorders or UPD-related disease, but ROH by itself is not proof of disease.
[1] The Influence of Demographic History and Genetic Architecture on Complex Traits via Runs of Homozygosity [2] Clinical significance of regions of homozygosity detection in prenatal ...
Limitations
How many ROH are detected depends on the analysis rules, such as the minimum segment length and which genetic markers are filtered out. Different methods can also confuse identity by descent (IBD, the same segment inherited from a shared ancestor) with identity by state (IBS, the same alleles without recent shared ancestry). Because of this, homozygosity measures based on ROH can be more direct for recent shared ancestry, but they still depend on the data and thresholds used.
[2] Clinical significance of regions of homozygosity detection in prenatal ... [3] Genome-wide assessment of runs of homozygosity and ... - PMC
SEE THE IDEA
From same allele to detectable homozygous regions
Both inherited DNA copies match at one position.
Many matching sites together form an ROH.
Common ancestry, UPD, or related mechanisms can create the pattern.
The pattern may inform ancestry or disease risk, but it is not a diagnosis by itself.
Matching alleles can cluster into long stretches that scientists measure and interpret in context.
Homozygosity is a basic DNA state; ROH are the longer patterns that can be counted and studied.
TRY IT YOURSELF
Test your understanding
Choose an answer, then check the explanation.
PUT IT TOGETHER
Match each concept to its meaning
Drag a label onto its description, or select a label and then select a description. This also works with a keyboard.
4 pairs to match
Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.