In many marine species, high levels of gene flow ensure that the genetic signal from population differentiation is weak. As a consequence, various errors associated with estimating population genetic parameters that might normally be safely Ignored assume a relatively greater importance. This fact has important implications for the use of genetic data to address two common questions in fishery conservation and management: (1) How many stocks of a given species are there? and (2) How much gene flow occurs among stocks? This article discusses strategies to maximize the signal:nolse ratio in genetic studies of marine species and suggests a quantitative method to correct for bias due to a common sampling problem. For many marine species, however, genetic methods alone cannot fully resolve these key management questions because the amount of migration necessary to eliminate most genetic evidence of stock structure (only a handful of Individuals per generation) will generally be Inconsequential as a force for rebuilding depleted populations on a time scale of interest to humans. These limitations emphasize the importance of understanding the biology and life history of the target speciesfirst, to guide design of the sampling program, and second, so that additional information can be used to supplement Indirect estimates of migration rates based on genetic data.

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Separating the wheat from the chaff: patterns of genetic differentiation in high gene flow species

Here, we give a historical overview of the search for genetic variants that influence the susceptibility of an individual to a chronic disease, from RA Fisher's seminal work to the current excitement of whole-genome association studies (WGAS). We then discuss the concepts behind the identification of common variants as disease causal factors and contrast them to the basic ideas that underlie the rare variant hypothesis. The identification of rare variants involves the careful selection of candidate genes to examine, the availability of highly efficient resequencing techniques and the appropriate assessment of the functional consequences of the implicated variant. We believe that this strategy can be successfully applied at present in order to unravel the contribution of rare variants to the multifactorial inheritance of common diseases, which could lead to the implementation of much needed preventative screening schemes.

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Common and rare variants in multifactorial susceptibility to common diseases

In his book "Analysis of Human Genetic Linkage" Jurg Ott laid out the principles by which geneticists perform studies of genetic linkage in humans. Here, in the "Handbook of Human Genetic Linkage", Joseph Terwilliger and Jurg Ott provide a companion piece to the previous work, showing how its principles may be applied in practice. "Handbook of Human Genetic Linkage" contains detailed instructions on how to carry out linkage analysis, guiding the reader through each process on a step-by-step basis. The chapters are grouped into three main sections: two-point linkage analysis, multi-point linkage analysis, and such advanced topics as mutation rates, gene frequencies, linkage disequilibrium, complex diseases and computer simulation. Each chapter ends with exercises that can be performed on an IBM PC-compatible computer. The solutions are supplied at the end of each section.

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Handbook of Human Genetic Linkage

Island populations are much more prone to extinction than mainland populations. The reasons for this remain controversial. If inbreeding and loss of genetic variation are involved, then genetic variation must be lower on average in island than mainland populations. Published data on levels of genetic variation for allozymes, nuclear DNA markers, mitochondrial DNA, inversions and quantitative characters in island and mainland populations were analysed. A large and highly significant majority of island populations have less allozyme genetic variation than their mainland counterparts (165 of 202 comparisons), the average reduction being 29 per cent. The magnitude of differences was related to dispersal ability. There were related differences for all the other measures. Island endemic species showed lower genetic variation than related mainland species in 34 of 38 cases. The proportionate reduction in genetic variation was significantly greater in island endemic than in nonendemic island populations in mammals and birds, but not in insects. Genetic factors cannot be discounted as a cause of higher extinction rates of island than mainland populations.

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Do island populations have less genetic variation than mainland populations

The prevalence of Charcot-Marie-Tooth's disease (CMT) was studied in Western Norway, an area with several isolated districts with a population of 725,000 (1968). Three hereditary types were distinguished in the area: autosomal dominant CMT with an estimated prevalence of 36/100,000; X-linked recessive CMT with a prevalence of 3.6/100,000; and autosomal recessive CMT with a prevalence of 1.4/100,00. Gene frequencies were 3 · 7. 10-4, 1 · 9. 10-4, and 4 · 8. 10-4 in autosomal dominant, X-linked, and autosomal recessive CMT, respectively, while the corresponding mutation rates were 13 · 0, 5 · 5, and 3 · 5 per million gametes per generation. The penetrance was almost complete for all three variants of CMT.



Strict diagnostic criteria were followed in the selection of the 37 index cases. A family investigation was carried out with 238 subjects, during which 69 secondary cases were detected. Another 57 subjects had unspecific neuropathy (Un), which did not fit a diagnosis of CMT or other neurological disease. In the diagnosis of Un, a score system was used, with age and sex corrections based on findings in a normal population.



Generally, the most severe disease course was found in the recessive CMT types, but there was also more clinical variation, suggesting CNS involvement in some cases (upper motor neuron affection, cerebellar signs). Scoliosis and spinal ataxia were not infrequent, even in cases with autosomal dominant CMT.



The prevalence cf Un was highest in the relatives of recessive CMT cases, with a ratio of affected to normal in sibs compatible with a hypothesis of several cases of heterozygous manifestation. In the relatives of autosomal dominant CMT cases, Un prevalence was also higher than in the population, but lower in 2nd degree relatives than in 1st degree; the ratios fitted a hypothesis of polygenic Un inheritance. Significant differences were found in the score patterns of Un in the recessive CMT families and in the autosomal dominant families, suggesting their difference of origin. The reason for clustering of Un cases in autosomal dominant CMT families is obscure, since it can be only partly attributed to early manifestation of CMT. It is suggested that Un and CMT, mainly in autosomal dominant CMT, interact to form a spectrum of differing phenotypes, so explaining the problem of “transitional forms” between CMT and other hereditary nervous disorders. Recessive CMT, being a more generalized nervous disease, attains, through differing expressivity, phenotypes which vary between individual cases.

Genetic and clinical aspects of Charcot‐Marie‐Tooth's disease

Publisher Summary This chapter presents the genetics of human population. The primary point of departure in experimental genetics is to study the hereditary factors that determine certain characters. The chapter discusses three types of characters— namely: (1) hereditary characters in the limited sense; (2) characters due to the environment; and (3) characters depending both on heredity, environment, and constellational. There is also the fact that characters may emerge at different ages. The nature of the population determines to some extent the type under which a character is to be classified. To illustrate this, the chapter discusses a character normally contingent on both heredity and environment– that is, to say a constellational character where the hereditary factors necessary for its occurrence are found only in a part of the population, and where the same is true of the environmental factor or factors that are similarly necessary. In a population where the necessary environmental factor is present in practically all individuals, the character will be determined by heredity; conversely, in a population where the hereditary factor is present in practically everyone the character will be determined by the environment.

Gunnar Dahlberg

Papers

Genetics of human populations.