590 



CONTINUITY OF LIFE 



bred) we already know there will be 3 nor- 

 mal and 1 vestigial-winged flies. But why? 



During meiosis of both male and female 

 hybrids, the two alleles are distributed to 

 the sperms and eggs in a random manner. 

 Half of the germ cells of each will contain 

 a gene for vestigial wing and half for nor- 

 mal wing. Upon fertilization these unite 

 according to the laws of chance, and since 

 there are four possibilities, we would ex- 

 pect them to occur in equal numbers for 

 each combination. There is no special at- 

 traction between eggs and sperms of differ- 

 ent or the same kind of gene. One-fourth of 

 the offspring should be homozygous for 

 normal wing (VV), one-fourth should be 

 homozygous for vestigial wing (vv) and 

 the remaining one-half should possess both 

 genes, that is, they should be heterozygous 

 (Vv), and since normal is dominant, they 

 will all have normal wings just like the 

 parents. It does not matter whether we des- 

 ignate the genes as one-fourth Vv and one- 

 four vV, since the end result is one-half 

 heterozygotes Vv, the order of the genes 

 making no difference in the end result. See 

 Table 1, below. 



These germ cells unite at random. This 

 can best be shown by employing the "check- 

 erboard" or Punnett square, named after 

 the British geneticist who first used it. 



Genotypic ratio 



Back crosses 



= 1 homozygous normal 

 wing : 2 heterozygous 

 normal wing : 1 vestigial 

 wing (1:2:1) 



F2 Generation: 



Sperms 



V V 



Eggs 



V 



V 



It is sometimes important to know the 

 genetic constitution of animals showing the 

 dominant trait but suspected of being 

 heterozygotes. This is particularly true in 

 breeding of livestock. In the above F2 flies, 

 three-fourths of them are normal-winged, 

 but the question arises as to which of these 

 are heterozygous and which are homozy- 

 gous. By mating each of them with the 

 homozygous recessive grandparent this can 

 be told. This kind of cross is known as a 

 back cross or test cross. If the normal- 

 winged fly is heterozygous the offspring 

 will be half normal- and half vestigial- 

 winged, but if it is homozygous the off- 

 spring will be normal-winged. This is made 

 clear as follows: 



If the fly is heterozygous for vestigial 

 wings (Vv) the ratio will be: 



Phenotypie ratio = 3 normal wings : 1 ves- 

 tigial wing (3:1) 



In other words, one-half of the offspring 

 would show the recessive trait. 



If the fly is homozygous for the domi- 

 nant trait ( VV) the ratio will be: 



VV X vv (recessive) 

 One kind of 



gamete V v One kind of gamete 



Possible off- 

 spring Vv 



All offspring would show the dominant trait. 



Male 



Parents: 



Normal wings 

 One kind of sperm 



Fi Generation: 



Crossing two hybrids 

 Two kinds of sperms 



TABLE 1 



VV X vv 

 V V 



Vv 



Vv X Vu 

 V &.V V& u 



Female 



Vestigial wings 

 One kind of egg 



Normal wings (Heterozygotes) 

 Two kinds of eggs 



