Embryonic lethals hava been described in mice, cattle and pigs, and, when chromosomal deficiencies are involved, are common in humans. In pigs a recessive lethal in the segment homoglous to that including the myotonic dystrophy locus on chromosome 19 has been defined from disturbed segregation. Apart from recessive lethals similar disturbed segregation would follow any gametic preference for allelic difference, an option with major advantages in conserving heterogeneity by imposing 'incest aversion' at the chromosomal level by selectiong for difference, a feature widely exploited in plants. Losses from recesive embryonic lethals are likely to be far commoner than even the commonest common disorder. The increase in power will usually be about fourfold.
Common sense can usually restrict studies to individuals with common grandparental origins.
Loci with similar effects are likely to be too close to be distinguished or randomly distributed over the genome. This additivity has allowed almost all the mapping of Mendelian phenotypes in humans using Morton's z-scores in the sixties and early seventies and, later, Ott's computer program allowing generalisation to untidy families.