Incest diagnosis by comparison of alleles of mother and offspring at highly heterozygous loci.

BACKGROUND: Methods that detect a child's homozygosity by examination of allelic products are insensitive for diagnosing incest because, at a given locus, a homozygous state is expected with a frequency of only 0.25 when parents are first-degree relatives. Furthermore, these methods are not specific if the population contains many homozygous individuals or silent alleles that cause apparent homozygosity. STUDY DESIGN AND METHODS: Use of highly heterozygous loci improves specificity, but not sensitivity. Sensitivity may be increased by observing for two kinds of mother-offspring similarities: an offspring of incest tends to be homozygous or heterozygous-identical with respect to its mother's phenotype. At each locus, two conditional probabilities may be calculated for a genetic observation, using allele frequencies expected under a state of incestuous mating versus mating within a specified population. The conditional probabilities at each locus are compared in a likelihood ratio to express a relative probability of incest. RESULTS: In a case of known sibling incest, three likelihood ratios were derived from variable number of tandem repeat phenotypes at five loci. When only offspring homozygosity was observed, the likelihood ratio was 75.3:1. When both homozygous- and heterozygous-identical phenotype similarities of offspring and mother were noted, the likelihood ratio was 130.4:1. When maternal obligatory alleles of the offspring were considered, the likelihood ratio was 262.4:1. CONCLUSION: Comparison of maternal and offspring phenotypes at highly heterozygous loci increases both sensitivity and specificity of genetic tests in cases of suspected incest.

Technical progress in parentage analysis.

At the turn of the 20th century, Mendel's laws were found to be applicable to human blood groups. Within two decades, blood group genetics were applied to problems of parentage. Expansion of immunohematology into leukocyte antigen identification produced the single most informative, expressed polymorphism. About the same time, analysis of a great number of soluble protein polymorphisms followed advances in electric separation methods, enzymology, and immunochemistry. As new, independent loci were discovered, the power to exclude the falsely accused increased, and it became possible to apply Bayesian principles to determined probabilities of biologic relationships. The revolution in nucleic acid technology has dramatically improved analysis and statistical inferences. By the turn of the 21st century, laboratories should be able to determine biologic parentage with virtual certainty.

Two-man and two-sibling paternity cases.

Traditional genetic marker systems rarely fail to resolve paternity disputes when two or more men are accused, except when men are brothers. A sibling of the biologic father may not be excluded by these laboratory tests and sometimes yields calculated odds of paternity that are equal to or higher than the true male parent. Resolved two-brother cases were compared with resolved cases involving two unrelated men. In each case, the residual odds of paternity were determined for each man and the greater was divided by the lesser to produce a paternity fraction. The paternity fraction is a useful indicator of biologic parentage when it exceeds a value of 10 (log10 of-the-odds score greater than or equal to 1). Tests for alleles at highly heterozygous loci are indicated in initial laboratory evaluations of cases involving brothers. Human leukocyte antigen and variable number of tandem repeat polymorphisms appear suitable.

How frequent is heteropaternal superfecundation?

A newly discovered case of heteropaternal superfecundation (HS) is reported. Three HS cases were found in a parentage test database of 39,000 records. The frequency of HS among dizygotic twins whose parents were involved in paternity suits is 2.4%. Although the study population appears similar to the general population with respect to twinning data, inferences about the frequency of HS in other populations should be drawn with caution.

Superfecundation identified by HLA, protein, and VNTR DNA polymorphisms.

Parentage analysis has revealed a high probability that a man accused of paternity is the biological father of a male child. The child in this study, however, was the twin of a female child who could not have been fathered by the accused man. The mother of the children subsequently accused a second, unrelated man, who was excluded from paternity of the boy, but was very probably the biological father of the girl.

Paternity probabilities of biologic fathers and unexcluded, falsely accused men using blood group markers.

A frequent legal argument raised in defense of men accused of paternity, but not excluded by genetic tests, is that the probabilities of paternity of falsely accused men are similar to those of biologic fathers. This assertion was tested in a computer simulation experiment that used a database of 15,000 actual paternity cases to provide red cell and HLA phenotypes of mothers, children, and putative fathers. Tests had a combined probability of exclusion of 97.3 percent. Equal numbers of true and false fathers were generated from the data by computer to achieve a prior probability of paternity of 0.5. True fathers' phenotypes were those of unexcluded men from actual cases (Group A) or of mothers from actual cases (Group B) in which paternity was not excluded. The false father group was created by assigning the phenotypes of racially identical men who were selected at random from among cases other than their own. Probabilities of paternity were calculated for the men in each group and were classified into descriptive intervals. The frequency of men in each group was compared in each interval. The frequency distributions of probabilities of paternity for true fathers and unexcluded, falsely accused men (false fathers) were markedly dissimilar.

HLA in kinship determinations among Haitian immigrants.

Immunogenetic study of alleged first-degree relatives was undertaken among 258 prospective United States immigrants from Haiti. Methods involved serotyping red cells for ABO, Rh, and MN antigens and typing leukocytes for HLA, A, B, and C locus antigens. Kinship was definitely excluded in a relatively low 4.2% of cases involving putative parents and children. Among cases involving alleged siblings, estimates of fraud appeared slightly higher, but the method is suspect because even in true sibships, there may be an absence of obligatory gene markers. Data suggests that some cases involved half-siblings rather than fraud. Of demonstrated exclusions of parent or child, HLA detected the lack of kinship in 87.4% versus 16.9% by red cell typing. However, there were cases in which exclusions were found by red cell methods alone; furthermore, red cell plus HLA typing allows for a calculation of probability of kinship that is analogous to calculations in paternity studies. Together, the red cell and leukocyte systems offer a prior probability of exclusion of parent-child relationships in 91.3%.

Laboratory evidence of unsuspected parental consanguinity among cases of disputed paternity.

A search was conducted to find evidence of possible incestuous unions between the biologic parents of children involved in 2500 paternity cases. Suspicion was raised when either (1) a mother and her child possessed identical HLA phenotypes, or (2) the child appeared to be possibly homozygous for one maternal haplotype (i.e., one of the child's HLA haplotypes was a blank). These mother-child HLA-haplotype dualisms (MHDs) occurred in 5% of all cases. Frequency of exclusion of the accused men in cases demonstrating MHD, was compared with the remaining paternity cases. No significant difference was found in overall exclusion rates between MHD cases and controls when exclusion produced by HLA and red cell antigen systems were observed. However, there was a greater rate of exclusion in MHD cases when comparing exclusions produced by red cell antigen systems regardless of whether HLA tests excluded paternity (p less than 0.025). MHD cases involving teenaged mothers differed from control cases in frequency of exclusion of paternity only on the basis of red cell antigen phenotyping (p less than 0.005). The HLA system's usefulness in paternity testing is diminished when there is MHD; multiple, independently-inherited systems are relatively more useful in these circumstances. The search method detects only half of potential incest cases; proof of incest requires more extensive testing for homozygosity among other polymorphisms. Since calculations of likelihood of paternity are inappropriate in cases involving close consanguinity, detection and follow up studies are important. Data suggest that one-fifth of MHD cases may involve first degree consanguinity and that the incest rate among paternity cases may be as high as 2%.

Utility of HLA and six erythrocyte antigen systems in excluding paternity among 500 disputed cases.

Six erythrocyte antigen systems and the HLA system were evaluated to establish their practical value in 500 cases of disputed paternity. The actual results were very close to predicted values. HLA testing is expected to detect 92% and red cell testing is expected to detect 67% of men falsely accused in paternity suits. The findings of this study show that HLA detected 94% and red cell testing detected 69% of 107 men falsely accused in 500 paternity cases. In order of sensitivity, Rh, MNSs, and ABO were the most useful erythrocyte marker systems. There were six out of 107 cases in which exclusions would have been undetected if red cell typing had not been performed. Five of the six cases involved "common" HLA haplotypes.