THE GENERAL ASPECTS OF HEREDITY ii 



and transmit only the normal condition to all their 

 offspring. In other words, the heterozygous domi- 

 nants will continue to produce both types when 

 mated to normals, while the normals derived from such 

 a cross, being recessive, have entirely lost the brachy- 

 dactylous condition (or rather never had it), and will 

 therefore have only normal offspring even if two such 

 normals from a brachydactylous cross mate together. 

 For a full discussion of brachydactyly, see p. 78. 



Many abnormalities in man are simple dominants 

 and will therefore be inherited in this manner. 



In order to make clearer the nature of Mendelian 

 heredity, let us consider the other types of mating 

 which commonly occur in organisms showing a single 



Heterozygous 

 brachydactyl 

 parent. 



Heterozygous 

 brachydactyl 

 parent. 



Germ cells 



SO X B 



Fi hybrids (zygotes) 

 25 y. B B 



25 y. ..B \75X 'l^/Xcecty, 



25 % B n 



25 y. n. n 25 y^ normal 



Fig. 2. — Results of Cross between Two Heterozygous 



Brachydactyl Parents. 



difference. If tw^o such individual organisms which 

 are pure or homozygous are crossed, the first hybrid 

 generation (written briefly Fj) will show only the 

 dominant character. But if two of these F^ hybrids 

 are intercrossed, their offspring will number on the 

 average three dominants to one recessive. Thus, 

 in a marriage between two heterozygous brachy- 

 dactyls, three-fourths of the children would be ex- 

 pected to be brachydactylous. The reason for this 

 will be understood from the following diagram (Fig. 2). 

 The four possible combinations of the two types 

 of germ cells will occur with equal frequenc}^ and 

 since the factor for brachydactyly is absent from only 



