580 GENETICS AND EVOLUTION 



case. Creeper fowl have wings and legs which are shorter than normal. 

 When two creeper towl are bred, the ratio of creepers to normal in the 

 offspring is 2:1. One quarter of the eggs, those homozygous for the 

 creeper factor, have marked abnormalities of the whole skeletal system, 

 especially of the vertebrae, and die without hatching. 



These lethal genes produce a visible phenotypic expression when 

 heterozygous and thus are dominant to the normal allele. Many— perhaps 

 most-lethal genes have no effect when heterozygous, but result in the 

 death of the organism when homozygous. These recessive lethals can be 

 detected only by special genetic techniques. When such techniques have 

 been applied to wild populations of the fruit fly, Drosophila, the pres- 

 ence of many recessive lethals has been revealed and it is believed that 

 similar lethals occur in most wild populations. 



284. Penetrance and Expressivity of Genes 



Genetic research on the mode of inheritance of certain traits is com- 

 plicated by the fact that these genes do not always produce the expected 

 phenotype. In the examples presented so far, recessive genes always pro- 

 duce their phenotype when homozygous and dominant genes always 

 produce their phenotype when homo- or heterozygous. Such genes are 

 said to have complete or 100 per cent penetrance. With certain other 

 genes only a fraction of the inclividuals homozygous for a recessive gene 

 actually show the expected phenotype. Such genes are said to show in- 

 complete penetrance; the percentage of penetrance is calculated from 

 the number of individuals that actually show the phenotype in every 

 hundred individuals that would be expected to show it. Penetrance is 

 essentially a statistical concept of the regularity with which a gene pro- 

 duces its effect when present in the requisite homozygous (or hetero- 

 zygous) state. The percentage penetrance of many genes may be altered 

 by changing the environmental conditions— temperature, nutrition, 

 moisture, etc.— under which the organism develops. 



Certain inbred stocks homozygous for a particular gene show wide 

 variations in the phenotype. For example, fruit flies of a stock homo- 

 zygous for a gene which produces shortening and scalloping of the wings 

 may show wide variations in the degree of shortening and scallopmg 

 in the wings of any individual fly. Such differences are known as varia- 

 tions in the expression or expressivity of the gene; they may also be 

 altered by changing the environmental conditions during the organism's 

 development. 



285. Inbreeding and Outbreeding 



It is commonly believed that the mating of two closely related indi- 

 viduals—brother and sister or father and daughter— is harmful and leads 

 to the production of monstrosities. Even the marriage of first cousins is 

 forbidden by law in some states. Carefully controlled experiments, car- 

 ried out over many generations and with many different kinds of plants 

 and animals, have shown that there is nothing harmful in the process 



