592 



CONTINUITY OF LIFE 



A study of the inheritance of the ab- 

 normahty, albinism, in man will also dem- 

 onstrate this type of inheritance. An albino 

 is lacking a gene to produce pigmentation 

 in the skin, hair, and even in the eyes. This 

 is inherited as a recessive, hence it must be 

 homozygous (aa) to appear. If, then, a 

 normal man (AA) marries an albino 

 woman (aa), their offspring will be normal, 

 although heterozygous for albinism (Fig. 

 24-7). If this offspring later marries an 

 albino, his children will have a one-to-one 

 chance of being albino, which is similar to 

 a typical test cross and definitely establishes 

 him as heterozygous. If, however, he mar- 

 ries another heterozygote like himself, their 

 children will have one chance in four of 

 being an albino. This is simply an F2 gen- 

 eration of a monohybrid cross. If he marries 

 a nonnal person, that is, a homozygote for 

 normal pigmentation ( AA ) , all of their off- 

 spring will be normal (not shown in Fig. 

 24-7). 



Such test crosses have proven of great 

 value in the establishment of "pure" breeds 

 of domestic animals such as cattle for beef 

 or milk production and chickens for egg 

 laying. Rather tlian pick out the best pheno- 

 types, as was the custom for centuries, 

 breeders now make such test crosses until 

 they establish the genotype for a particu- 

 lar trait. Animals possessing this trait are 

 then used as breeding stock. This has been 

 particularly profitable to farmers who wish 

 to build up their herds in respect to milk 

 production. It is possible through recent 

 techniques in artificial insemination to 

 fertilize thousands of cows with sperms 

 from a single bull that is known to produce 

 offspring with high milk-giving qualities. 

 With the use of progeny selection the out- 

 put of cows is increasing each year until 

 today some record cattle have produced as 

 much as 19,000 quarts of milk a year. This 

 is a far cry from unselected cows of primi- 

 tive people which give no more than 350 

 quarts in a year! 



Dihybrid cross 



So far, we have explained what happens 

 when only one pair of contrasting genes are 

 considered. Now let us attempt to explain 

 what Mendel did not know when he 

 crossed two pairs of contrasting characters. 

 The Fi generations of such crosses are 

 called dihybrids. It sometimes is essential 

 for geneticists to follow three and four or 

 more characters simultaneously; this be- 

 comes increasingly complex with each 

 added character. We shall confine our at- 

 tention to the two-character cross and leave 

 the others for advanced books in genetics. 

 Using normal and vestigial wings in 

 Drosophila again, we can also follow body 

 color. Ebony or dark-colored individuals 

 occasionally occur in stocks of the normal 

 gray-bodied wild type, and these dark indi- 

 viduals are recessive to the gray-bodied 

 wild type (Fig. 24-8). By crossing a gray 

 vestigial female with an ebony normal- 

 winged male the Fi offspring are all gray 

 and normal-winged. Note that each of the 

 parents were recessive and dominant for 

 different traits. We could as well have 

 crossed a gray normal-winged individual 

 with an ebony vestigial fly and obtained tlie 

 same normal-appearing heterozygous off- 

 spring. The sexes could also have been 

 switched and it would have made no dif- 

 ference in the outcome. When these two 

 hybrids are crossed, sixteen combinations 

 are possible in the Fo generation. To follow 

 these possible combinations it is conven- 

 ient to employ the Punnett square again 

 (Fig. 24-8). 



Each of the hybrids can produce four 

 different kinds of gametes during meiosis, 

 each bearing the two different genes. These 

 genes must necessarily be located on differ- 

 ent chromosomes, for if they were on the 

 same chromosome the typical ratios would 

 not occur, for reasons that we shall discuss 

 a little later (linkage, see p. 606). By ar- 

 ranging the four different kinds of sperms 



