title: Genotype probabilities of pairs of individuals for X-chromosome markers.
abstract: BACKGROUND
  The usual set of autosomal markers (A-STRs) available in commercial kits is often insufficient to discriminate between close relationships when only two subjects are available for analysis. X-chromosome markers (X-STRs) provide higher statistical power in special cases.
  
  
  METHODS
  Formulas are derived for the probabilities of all possible genotype pairs for X-STRs of any sex combination for seven common relationships. The power of exclusion (PE) of X-STRs in parentage analysis is compared with that of A-STRs of equivalent distribution of allele frequency.
  
  
  RESULTS
  Seventy-three equations were obtained, from which the likelihood ratio of any two alternative hypotheses about the relationship between two individuals can be obtained by division and simplification. For father-daughter and mother-son duos, the PE of X-STRs is almost twice the corresponding value of A-STRs for moderately low values of heterozygosity (0.6-0.75); for alleged pairs of sisters and pairs of half-sisters the PE is equivalent to that of A-STRs in parent-child duos. Considering four real unlinked X-STRs, the cumulative PE for father-daughter and mother-son duos was 99 percent, compared with 94 percent if they were autosomal.
  
  
  CONCLUSIONS
  X-STRs can substantially increase the discrimination capacity of standard A-STRs in parentage analyses involving pairs of individuals. Up to four unlinked X-STRs may be treated as independent loci. When linked loci are included, computer programs that calculate pedigree likelihoods can be used.
authors.affiliation: Unit of Legal Medicine, School of Medicine, University of Pisa, Italy.
authors.forename: Chiara
authors.initials: C
authors.lastname: Toni
authors.forename: Ranieri
authors.initials: R
authors.lastname: Domenici
authors.forename: Silvano
authors.initials: S
authors.lastname: Presciuttini
mesh_terms: D041321:Chromosomes, Human, X; D005260:Female; D005787:Gene Frequency; D005819:Genetic Markers; D005838:Genotype; D006801:Humans; D040582:Inheritance Patterns; D016013:Likelihood Functions; D008297:Male; D008957:Models, Genetic; D010375:Pedigree; D011336:Probability
chemical_list: D005819:Genetic Markers