Figure 12.4 Percentage ofTwins with Similar Measures of Cognitive Ability. The data are from a Swedish study of twins at least eighty years old. As expected, identical, or monozygotic, twins share a higher percentage of all cognitive abilities than do fraternal, or dizygotic, twins. Similar to other polygenic traits, cognitive ability is complex, and some measures, like spatial ability, show less heritability than speed, for example.
mapped on chromosome 1, with a possible intervention of genes located on chromosome 17. They have a rough estimate of the number of genes involved and the rough location of a few of these genes on chromosomes. Because the human genome is huge and we can not do experimental crosses of humans, it may be years before the genes for autism are identified. However, the completion of the Human Genome Project will help to identify this and other genes involved in polygenic traits in humans.
To further illustrate the complexity of diseases influenced by many genes, it is believed that alcoholism, schizophrenia, and manic-depressive disorder are also at least partially controlled by genes. However, in spite of various claims, we still do not know how many genes are involved or where these genes are located in the human genome.
The situation is slightly different with agricultural plants and animals. Another name given to genes involved in multigenic traits is "quantitative trait loci," or QTLs. QTLs are responsible for phenotypes in plants, such as fruit weight, acidity, content of soluble solids, and so forth. Many QTLs have been mapped and some even cloned. The main reason for this success is that, unlike with humans, controlled crosses can be done with plants. Crosses tell us what kinds of pheno-types are obtained in the offspring, and the genes corresponding to these phenotypes can be mapped by using DNA markers and linkage analysis as discussed in chapter 9. Then individuals originating from various crosses are analyzed for polymorphisms at the DNA level, also as discussed in chapter 9. To use the example of the tomato, more than three hundred polymorphic loci have been identified in this plant, with six affecting fruit weight and five affecting acidity. Agronomists now conduct this type of research in a wide variety of edible plants in order to find the genes involved in many polygenic traits that affect the quality of our food.
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