. Biology; the story of living things. 450 THE MAINTENANCE OF SPECIES In actual practice, if a combination of three or more characters is desired, one character at a time in either pure dominant or recessive form is obtained. By this method, since the expectation of either a pure dominant or a pure recessive in a monohybrid is one out of four, early reahzation of the desired combination is likely. Unit Characters and Factors A great deal has been learned about heredity through the experi- mental breeding of plants and animals since Mendel's laws became available. Many of the facts gained, howe
. Biology; the story of living things. 450 THE MAINTENANCE OF SPECIES In actual practice, if a combination of three or more characters is desired, one character at a time in either pure dominant or recessive form is obtained. By this method, since the expectation of either a pure dominant or a pure recessive in a monohybrid is one out of four, early reahzation of the desired combination is likely. Unit Characters and Factors A great deal has been learned about heredity through the experi- mental breeding of plants and animals since Mendel's laws became available. Many of the facts gained, however, are at first sight in apparent contradiction to these laws, but the value of the fundamental concepts of dominance, independent assortment, and segregation in the ^ ,. interpretation of inherit- nSreQIiary OOmailU wcice remains unques- Determiners Characters tioned. Any adequate \_) .«...___^ consideration of the ap- parent departures from the clear-cut conclusions of Mendelism would re- quire many more pages than are available in this book. For one thing, Men- del's experiments led him to the idea of Unit Characters, each spon- sored by a single germinal determiner. There is now abundant evidence that whatever it is in the germplasm that, under suitable environmental conditions, becomes eventually expressed as a single character, it is often made up of more than one unit. This discovery has led to the development of the factor hypothesis, which implies that there is usually, if not always, an in- terplay between different hereditary factors in determining the con- tribution which inheritance furnishes to the formation of a character in an individual. Moreover, a constellation of interacting hereditary factors may be responsible, in certain instances, for the expression of more than one visible character. Modified Ratios. The existence of factors, or fractional rather. B Diagram of the relation between hereditary determiners and resulting somatic characters. A, three or m
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