CYTOLOGY AND MENDELIAN HEREDITY 191 



such recessives, while males can have only one. The F-chromosome 

 has no influence on the character. When a male has the recessive gene, 

 he is color-blind, for no normal allele is present to dominate it (first part 

 of diagram). His offspring by a normal female all have normal vision: 

 the daughters are heterozygous, carrying one recessive gene dominated 

 by a normal one, while the sons have onlj^ a normal gene. If one of the 

 daughters mates -with any normal male, the probabilities are that half of 

 her daughters ^^ill resemble her in carrying the hidden defect, while half 

 of her sons will be color-bUnd. 



A female is color-blind only when she carries the recessive genes in 

 both of her A^-chromosomes (second part of diagram) . When mated to a 

 normal male, her daughters all have normal vision but carry the hidden 

 defect, while all her sons are color-blind. When one of these heterozygous 

 daughters mates Avith a color-blind male, the probabihties are that half 

 of the offspring of both sexes will be color-bUnd. What happens when a 

 normal male is involved has been shown in the preceding paragraph. 



When a color-blind female mates with a color-blind male, the results 

 are even more serious (third part of diagi-am). In such a case every 

 A-chromosome carries the recessive gene, and all the offspring are 

 color-bHnd. This is the fifth of six possible crosses, the sixth being normal 

 b}^ normal. Fortunately the fifth type occurs very rarely. 



Among organisms that have been extensively studied cytogenetically 

 many such sex-linked characters have been identified. 



Summary and Conclusions. — The characters exhibited by an organism 

 are determined by the constitution of the protoplasm and by the environ- 

 mental conditions, external and internal, in cooperation with which the 

 protoplasm undergoes development. Character relationships of succes- 

 sive generations, when attributable to protoplasmic composition, con- 

 stitute organic heredity. 



The data on the inheritance of many different characters show the 

 dependence of those characters upon constitutional factors located in the 

 nucleus, and they suggest that the factors, or genes, are individualized 

 organic units or semi-independent portions of the chromosomes. That 

 a given gene is present is indicated when it exists in two forms producing 

 two effects. When both forms are present together, one commonly 

 dominates the other in character production. A given gene affects 

 several characters, while a given character results from the action of 

 numerous genes acting at the same or different stages of ontogeny. 



The Mendelian rules according to which characters are inherited have 

 their cause in the distribution of the chromosomes through successive 

 life cycles. The association of chromosomes beginning at syngamy 

 results in a combination of parental characters in the offspring. The 

 segregation of their controlling genes pair by pair when si)ores or gametes 



