332 ELEMENTS OF BIOLOGY 



homozygous for that character and cannot transmit White eye to 

 future generations. Now if the heterozygous female is mated with 

 a normal male, the Fi generation consists of phenotypically normal 

 females, one-half of which are genotypically heterozygous, and 

 males, one-half of which are White eye and one-half Red eye. In the 

 two crosses so far given, all the females of the Fi generation are 

 phenotypically Red eye. One can obtain a White-eye female, how- 

 ever. If a female heterozygous for eye color is mated with a White- 

 eye male, as in crossing two from the Fi generation of the preceding 

 example, the resulting offspring will include males that are in equal 

 numbers Red and White eye and females that are in equal num- 

 bers White eye and Red heterozygous. 



A certain type of color blindness in Man, known as daltonism, is, 

 according to the most reliable data, a sex-linked character. If one 

 substitutes in the diagrams given above a symbol for color blindness 

 and another for normal, and then analyzes the table, it is clear why 

 women are rarely color blind and why color blindness is trans- 

 mitted through the mother. To obtain a color-blind female, a mating 

 between a heterozygous normal woman and a color-blind man is 

 necessary and since color blindness is comparatively infrequent in 

 Man, this mating is unusual. A physiological character which is 

 marked by an absence of the blood-clotting process (p. 175), known 

 as HAEMOPHILIA, is also a human sex-linked character and is ap- 

 parently transmitted as is color blindness. 



Linkage. In all that has preceded it has been assumed that in 

 synapsis in gamete maturation the allelomorphic genes of a charac- 

 ter are interchanged between synaptic pairs according to chance. 

 But when the genes for two characters are on the same chromo- 

 some they tend to be inherited together. This is known as linkage. 

 It means that during synapsis these genes, being located near each 

 other on one or the other of the synaptic mates, remain together 

 during the interchange of materials that occurs at this time (Fig. 

 208). In a similar fashion, sometimes the chromosomes do not break 



