594 



CONTINUITY OF LIFE 



along the top of the checkerboard and the 

 eggs along the left side and placing in each 

 square the gene combinations that result 

 from the union of those particular eggs and 

 sperms, the sixteen possible combinations 

 become obvious. Remembering which are 

 dominant and which recessive, it is clear 

 that there will be 9 gray normal, 3 gray 

 vestigial, 3 ebony normal and 1 ebony ves- 

 tigial. This is the 9:3:3:1 ratio that Mendel 

 verified many times with plants. We see 

 here an explanation for both genotypes 

 and phenotypes. The heterozygotes can be 

 checked for their genes by employing test 

 or back crosses just as was done for the 

 single-pair genes. If, for example, a gray- 

 bodied, long-winged heterozygote (VvEe) 

 is crossed with a double recessive (wee) 

 the offspring will fall into four groups: % 

 long gray, y^ vestigial gray, ^ long ebony, 

 and i<4 vestigial ebony. This is portrayed in 

 Table 2, below. 



A test cross to the dominant parent would 

 yield all long gray offspring and would tell 

 nothing about the genotypes. 



HEREDITARY VARIATION DUE TO 

 INTERACTION OF GENES 



Up to this point we have attempted to 

 explain Mendel's observations and the ex- 

 planation has been essentially simple. With 

 the passing of time a great many genetic 

 variations have been noted that cannot be 

 explained so simply. Several new principles 

 have been discovered which are based on 

 the interaction of alleles as well as genes 

 occupying different loci on the same or 

 other chromosomes. With the addition of 

 these new principles, genetics has become 

 an exceedingly complex science. 



Blending or incomplete dominance 



Occasionally a gene does not demonstrate 

 complete dominance, so that the heterozy- 

 gote shows a mixing or blending of the 

 dominant and recessive traits. This may be 

 due to a cumulative effect where the re- 

 cessive allele is negative and the dominant 

 allele manifests itself according to whether 

 or not it is single or double. The heterozy- 

 gote would then show half the effect that 

 the dominant homozygote would. For ex- 

 ample, in hair color in cattle, a red bull 

 ( WW ) may be mated to a white cow ( imv ) 

 and the hybrid (Ww) will be roan, which 

 is a mixture of red and white (Fig. 24-9). 

 Actually the hairs are still red or white, but 

 they both appear, thus giving the coat its 

 roan color. The appearance of one domi- 

 nant red gene produces only half as many 

 red hairs as the two genes will produce. 

 When the hybrids are mated ( Ww X Ww ) , 

 the Fo generation shows a ratio of 1 red, 

 2 roans, and 1 white. In other words, the 

 genotypes and phenotypes are identical. In 

 such matings it becomes impossible to 

 "breed out" the original stock, because reds 

 and whites will continue to show up in 

 future progeny no matter what selecting 

 procedures are employed. 



Single gene effects 



Not all genes control a single trait; in 

 fact, most of them have multiple effects. 

 One gene in early development may start 

 a series of reactions that will ultimately 

 alter or control several traits. A gene that 

 we usually think of as controlling eye color 

 in Drosophila, for example, may have had 

 its share of influence along the way in the 

 development of a host of other vital proc- 



TABLE 2 



long gray VvEe 

 Four kinds of eggs: VE, Ve, vE, ve 

 Test cross % VvEe, M Vvee, 



(long, (long, 

 gray ) ebony ) 



Ratio: 1 : 1 



