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Interindividual differences in the Learning ability are also known from mice, dogs and other animals, and the achievements of Pure Strains can be improved by Breeding . In such a way also Behaviour Genetics is contributing to our knowlegde on the inheritance of mental traits. There is an open question to which degree differences of animal behaviour have any meaning for differences in human intelligence.

The main interest of the broader public is focused on the question, which role play Nature Versus Nurture in the development of IQ. At the contrary, the science of genetics is trying to discover the genes contributing to IQ differences.


METHODS AND RESULTS



The similarity of relatives with respect to their intelligence


In the case of the s should be 1.00, practically the upper bound of this correlation is given by the Reliability of the test.
Sibs and dizygotic twins share half of their genes and the correlation of their scores should be 0.50, half sibs 0.25 and is halved by one additional decreasing degree of genetic relationship.

Bouchard and McGue (1981) have reviewed such correlations reported in 111 original studies. The mean correlation of IQ scores between monozygotic twins was 0.86, between sibs 0.47, between halfsibs 0.31, between cousins 0.15. From such data the Heritability of IQ can be estimated, varying between 0.40 and 0.80.

In 1869 Francis Galton in his book "Hereditary Genius" replaced mere speculations by statistical data:






Highly Gifted Men and the Percentage of their Highly Gifted Male Relatives


(classified by Occupation and Achievement )































































































































 GaltonTermanBrimhallWeiss 
 %%%%n (Weiss)
Probands10084+10097+1972: 1329


1994:   357
Fathers26412940346
Brothers47-4949220
Sons60- 5577
Grandfathers14-99681
Uncles16-1314615
Nephews23--2276
Grandchildren14----
Greatgrandfathers0--41290
Uncles of the parents5--51996
Cousins16-9#18570
Greatgrandchildren7----
Cousins of parents---112250
"+": classified by occupation; 100 %, if classified by test


  • ": classified only by IQ; classification by occupation gives about 55%; n = 820.


"#": some cousins were still to young and did not have full opportunity to become distinguished

"-": no data



Sources:
  • Francis Galton: Hereditary Genius. London 1869. {Link without Title} .

    100 famous Famous men (n = 43) of science and the percentage of their famous male relatives.
  • M. H. Oden: The fullfillment of promise: 40-year follow-up of the Terman gifted group.

    Genetical Psychology Monographs 77 (1968) 3-93.

    The mean IQ (transformed to 100;15) of the Sample of probands was 146 (n = 724); the cut-off score IQ 137.
  • Dean R. Brimhall: Family resemblances among American men of science.


    The American Naturalist 56 (1922) 504-547; 57 (1923) 74-88, 137-152, and 326-344.

    In 1915 questionnaires were filled in by 956 distinguished American men of science and their relatives.
  • Volkmar Weiss: Mathematical Giftedness and family relationship. European Journal for High Ability 5 (1994) 58-67. {Link without Title}

    Highly gifted males (mean IQ 135 +/- 9) and their relatives in professions, typically associated with an IQ above 123.


    		•


    In view of the differences in methods and societies there is a remarkable parallelism in the published statistics. The ITO-method by Li and Sacks (1954) allows from this set of data the estimation of the underlying number of genes and their allele frequencies.


    The inheritance of cognitive deficits


    There is no doubt that genes are in many cases the cause of an IQ below 100. The number of such genes already known is some hundred. For example, a .

    There are number of cases known, where the homozygotes have severe cognitive deficits and the heterozygotes show a small decrease of IQ. In such metabolic pathways further SNP ’s of such genes are investigated whether they are influencing IQ or not. For example, one SNP of the the gene ALDH5A1 causes an IQ difference of 1.5 points more or less [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=14981524&query_hl=8&itool=pubmed_docsum .

    In 2005 the discovery of genes underlying Dyslexia has proven that also complex intellectual achievements can be traced to the effects of genes.


    Mendelian segregation within the sibships of families


    Since Mendel the classical method to infer the existence of genes is the segregation of traits within families. Wilhelm Peters (1915) was the first who showed that in families where both parents have an IQ above 124 all their children also have an IQ above 124, of course, under the condition of normal education and environment and without any serious accidents. Both parents having an IQ below 105, all their children have an IQ below 105. Both parents having an IQ between 104 and 125, 50% of their children fall into the same range, 25% have an IQ above 124, 25% below 105. Such segregation in accordance with the Mendelian Laws can only be caused by a polymorphic gene.


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