Distinguishing full siblings from half-siblings in limited pedigrees.

BACKGROUND: Full siblings were compared with half-siblings to observe how well the two relationships could be distinguished by informative tests. STUDY AND DESIGN METHODS: Parentage analysis ascertained 25 pairs of full siblings and 25 pairs of half-siblings. The pairs were then examined for the sharing of alleles at three independent variable number of tandem repeat (VNTR) loci. A sibling index (SI) and a half-sibling index (HSI) were calculated for each pair, and an SI:HSI ratio was determined. RESULTS: The SI:HSI ratio favored full siblings in 18 of 25 full sibling pairs. The SI:HSI ratio exceeded 100 in 8 of those 25 pairs. Although the ratio favored half-siblings in 23 of 25 half-sibling pairs, as was expected with the use of only 3 loci, it exceeded 0.1 in all 25 pairs. CONCLUSION: Study of more than three highly informative loci is required to improve the identification of full siblings and might well permit the identification of half-siblings.

Determination of sibship in any two persons.

BACKGROUND: Parentage testing laboratories may be asked to provide genetic evidence that two persons are or are not related, when no other relatives are available for study. Simple methods using autosomal, codominant, unlinked genetic systems can determine if two people are blood relatives (e.g., siblings). STUDY DESIGN AND METHODS: The odds ratios (full sibship index) of true sibling pairs were determined from two-child paternity cases and compared with regionally and racially matched control pairs of unrelated children. The sharing of two, one, or no alleles was observed in pairs of children at three independent, polymorphic VNTR (variable number of tandem repeat) sequences loci. The sibship index was calculated as (the chance that an observation would occur if two children were siblings) divided by (the chance that it would occur if the two were unrelated). Sibship indices and the frequencies of shared alleles were determined for 20 sibling pairs and 20 control pairs. RESULTS: Sibship index values were less than 1 in all 20 pairs of unrelated children. Sibship index values were greater than 100 in nine pairs of siblings (45%), between 10 and 100 in five pairs (25%), between 1 and 10 in four pairs (20%), and less than 1 in two pairs (10%). Siblings shared two alleles in 17 of 60 observations (28.3%); controls shared two alleles in 0 of 60 observations (0%). CONCLUSION: The sharing of one allele and the sharing of no alleles at a polymorphic locus of high heterozygosity provide limited information for and against sibship, respectively. The sharing of two alleles produces strong evidence favoring sibship. In a given case, the study of more than three polymorphic loci of high heterozygosity may be needed to develop the evidence that two people are siblings. The general logic and methods used for siblings apply to kinship analyses of other two-person pedigrees.

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.

Parentage analysis by endonuclease shattering of hypervariable DNA.

Single-locus DNA probes for tandem repeat sequences are now used in conjunction with particular endonucleases to characterize heritable restriction fragment lengths in parentage tests. Southern blots of this type, however, demonstrate only two attributes of an allele: its length and the presence of nucleotide sequences that are complementary to the probe. Not all restriction fragments of the same apparent length that react with the same probe are identical. Differences between comigrating fragments can be detected by the selection of a restriction enzyme that recognizes sites in a subset of the repeat sequences, and the information content of these loci is therefore increased. This report describes a paternity case in which two brothers appeared, after DNA phenotyping using Hinf I, to be the father. A second phenotyping using Hae III excluded one of the brothers.

High-frequency elimination of SP02 prophage from Bacillus subtilis by plasmid transformation.

Transformation of competent Bacillus subtilis lysogenic for SP02 with any of three plasmids (pCM194, pUB110, pAM77) generates drug-resistant transformants of which 5 to 20% have lost the infectivity and immunity associated with the SP02 prophage. Such cured derivatives can be again lysogenized with SP02 and again cured by introduction of a different plasmid. Elimination of the SP02 prophage was not detected when plasmids were introduced by PBS1 transduction or by transformation of protoplasts. Similarly, transformants of B. subtilis selected for chromosome markers retained the prophage. The phi 105 prophage was not eliminated from competent B. subtilis transformed with plasmids.

SP02 particles mediating transduction of a plasmid containing SP02 cohesive ends.

SP02 particles that mediate transduction of plasmid pPL1010, a 4.6-megadalton derivative of pUB110 containing an Eco RI endonuclease-generated fragment of SP02 deoxyribonucleic acid that spans the cohesive ends, exhibit three unusual features: the transducing particles have a lower buoyant density than infectious particles; the transduction of pPL1010 occurs at high efficiency; and the transducing activity of the particles is relatively resistant to ultraviolet irradiation when the recipient is recombination proficient. Evidence is presented which indicates that SP02(pPL1010) particles carry the plasmid predominantly as a linear multimer having a molecular mass comparable to that of infectious SP02 deoxyribonucleic acid (ca. 31 megadaltons). The plasmid monomers in the linear multimer appear oriented in the same polarity. The buoyant density difference between infectious and transducing particles appears to be due mainly to the buoyant density difference between pPL1010 (1.699 g/cm3) and SP02 deoxyribonucleic acid (1.702 gm/cm3).