dizygosity

Dizygosity is a term used in genetics to describe the origin of a pair of individuals from two separate fertilization events. It is most commonly applied to dizygotic, or fraternal, twins, who develop from two independently fertilized ova and are therefore genetically distinct. Dizygotic twins typically share about 50 percent of their segregating genes, similar to ordinary siblings.

In contrast to dizygotic twins, monozygotic twins arise from a single fertilized egg that splits during development, producing two individuals with nearly identical genetic material. The distinction between dizygosity and monozygosity is important in twin studies, where the degree of genetic relatedness influences estimates of heritability for traits and diseases.

Dizygosity can be determined after birth by comparing genetic markers in the twins, or inferred during pregnancy through ultrasound and, later, through DNA testing if needed. In research settings, knowing whether twins are dizygotic or monozygotic is essential for properly interpreting concordance rates for various traits.

Beyond its use in describing twin pairs, zygosity fundamentally refers to the genetic similarity at a locus within an individual: homozygous individuals carry two identical alleles, while heterozygous individuals carry two different alleles. The term dizygosity is primarily encountered in the context of twins; within an individual, the broader concept of zygosity applies to the allelic composition at specific genetic loci.