130 Dihybrids, Trihyhrids, and Poly hybrids 



normal-wing, ebony : 3 curved-wing, gray : 1 curved-wing, ebony. 

 The Fi and F2 w^ould be the same both genotypically and pheno- 

 typically if one parent was homozygous dominant for one gene 

 and recessive for the second while the other parent was reces- 

 sive for the first and homozygous dominant for the second. 

 Thus a homozygous normal-winged, ebony fly {CC ee) mated 

 with a homozygous curved, gray fly {cc EE) would give normal- 

 winged, gray flies in the Fi and would produce an F2 ratio of 9 

 normal, gray : 3 normal, ebony : 3 curved, gray : 1 curved, 

 ebony. As in monohybrids, the results of reciprocal crosses are 

 the same. 



A Dihybrid Human Pedigree. Dihybrid ratios are much less 

 common in human beings because most of the characters that 

 have been discovered and analyzed genetically have been the 

 rarer abnormalities that appear in only a few isolated families. 

 Therefore, the chance that two such traits should appear in one 

 family is much more remote than the chance of getting a di- 

 hybrid in plants or other animals where the individual char- 

 acters that have been discovered are of much wider distribution. 

 An interesting dihybrid pedigree was reported by Beers and 

 Clark. 



In human beings short first toe is inherited as a simple auto- 

 somal dominant. Individuals with the gene for this character 

 have a short first metatarsal bone, and the big toe appears about 

 an eighth to a fourth of an inch shorter than the second toe. 

 Another human character is hemangioma or blood tumors which 

 are inherited in this pedigree as a simple autosomal dominant. 

 They are harmless but produce red spots from one millimeter 

 to several centimeters in size. A woman who had short first 

 toes married a man with hemangioma. The woman apparently 

 was heterozygous as three of their offspring had short first toes 

 and two did not, but the man appears to have been homozygous 

 for all the children had hemangioma. The three short-toed 

 children married people with neither short first toes nor heman- 

 gioma. Four of their children had short first toes and he- 

 mangioma; one had short first toes only and one had neither 

 short first toes nor hemangioma (Fig. 43). 



Incomplete Dominance. With one pair of alleles, when domi- 

 nance is incomplete, the Fi is intermediate and the Fo segregates 



