Mode of inheritance8/27/2023 ![]() Complex segregation analyses have been developed to allow for more factors to vary and to reduce the restrictions on assumptions to be made for the model tested. ![]() These simple approaches to segregation analyses often encounter problems when different mating types have to be considered and several hypotheses are more or less likely. Different methods for estimating θ have been developed and are easily applied (Singles Method, Weinberg's General Proband Method). As far as the segregation ratio is not significantly different from θ = 0.5, this mode of inheritance is accepted. In the case of a rare disease and an autosomal dominant hypothesis, the segregation ratio θ is assumed to be 0.5 as families segregating for the trait are most likely composed by matings of heterozygous affecteds and homozygous non-carriers. ![]() Arranged matings among animals can be more easily tested for specific modes of inheritance than pedigrees with arbitrary structure, missing data and many inbred animals. Pedigrees used for segregation analysis may be from specifically planned matings or randomly sampled pedigrees with arbitrary structure or sampled through ascertained cases in clinics or veterinary practice. Simple segregation analysis tests the segregation parameter θ under a specified sampling scheme and mating type. In addition, age of onset, sex effects and sampling scheme can be taken into account besides the specific genetic hypothesis under consideration. Modes of inheritance tested in segregation analyses include monogenic (Mendelian), digenic or polygenic models. Segregation analysis is employed to determine whether familial data for particular disorders or other traits are compatible with specific modes of inheritance. The presentation will give an overview on the model components included in estimation of the mode of inheritance based on phenotypic data and further developments for incorporation of molecular genetic data into the analyses. ![]() With increasing molecular genetic data, the type of gene action based on known DNA sequence variation can be characterized by individual genes and the nature of complex genetic traits can be understood much better. General evidence for genetic contribution to a disorder is given when environmental factors can be excluded as the only responsible causes for a disorder and a significant proportion of the phenotypic variation of a disorder can be explained by genetic models. A useful starting point for answering the question whether a disorder is inherited is by drawing pedigrees to provide an initial impression of the distribution of affected and non-affected animals and how frequently the disorder is transmitted from one generation to the next. On the other hand, genetically caused diseases may not necessarily lead to breed differences in incidence but will contribute to variation among families within breeds. The same is often claimed for disorders which show a breed disposition. Familial disorders may have a genetic contribution. Familiarity is assumed for a disorder when families are observed with more than one affected family member. Many disorders in animals are observed more frequently in certain breeds and within breeds more often in the same families.
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