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| Behavioral genetic studies To understand the genetics of a behavioral trait, Olson's study has been the main research in reading processes. This study used DeFries twin studies to select RD groups to test for phonological and orthographic coding scores. The results of this study showed 47% of the twins had deficits in phonological coding (i.e. phonics) but only 28% had orthographic coding deficits (i.e. spelling). The problem with this study was the suggestion that these groups represented the entire population but other studies followed. Another study was done by Plomin et al. with the additional consideration of segmental language skills (i.e. word recognition) for DeFries cotwins. The results for this study were 93% phonological genetic covariance to -16% orthographic coding. These twins had 54% word recognition and phonological coding deficits and only 28% had orthographic deficits. The low orthographic coding score showed a low heritability and genetic covariance suggestive of environmental factors. One study that has great implications for future research was Cardon's study11 that found families with RDs had a QTL on C6. This finding suggests that C6 may be an area causing phonological coding deficits for families with this type of SLD. Yet, there is more reset to be done for understanding the other QTLs, C15 and Chromosome 1+2 (C1,2), for those families that may inherit this trait. Also, it is important not to exclude the possibility that a potential C6 parent may have a C15 child who may have a C1,2 child. Name Study Area Specifics Findings DeFries, Olson family twins-id./frat. cot. 73% id. cot. herit.47% frat. cot. herit. DeFries, Alarcon family twins-id/frat 68% id. cot. herit.38% frat. cot. herit. Rabin, Feldman family sib. pair QTL C15, C1 Froster, Schulte-Korne family sib. pair QTL C1+2 Hallgren (in DeFries) linkage Mendelian autosomal dom. Finucci linkage modes of transmithomog. or heterog. autosomal rec. or dom.multifactoral Stevenson linkage modes of transmithomog. or heterog. autosomal dom. or QTL C15 Pennington linkage Mendelian autosomal dom. 20% Bisgaard linkage heterog. QTL C15->homog.>aut. dom. Smith linkage heterog. QTL C6 Olson (with DeFries) behavioral coding deficits 47% twins phonological28% twins orthographic Plomin, McClearn behavioral segmental language 54% twins phonological28% twings orthographic Gayan, Cardon behavioral QTL and coding deficits QTL C6+phonological family id. : identicalfrat. : fraternalcot. : cotwinsherit. : heritabilitysib. : siblingdom. : dominantrec. : recessivehomog. : homogeousheterog. : heterogeous |
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