312 Cell a^ Basis of Organic Activity 



has been one of the reasons why many scientists have feared the use 

 of bombs which may expose whole human populations to excessive high 

 energy radiations. 



Lethal Characters. — Occasionally these recessive mutations are 

 lethal in nature; that is, when homozygous they may actually 

 destroy the potential organism. Some may simply make the animal or 

 plant less able to adjust to its environment. An albino animal, for ex- 

 ample, is a form which is able to live, but which is so conspicuous 

 in its environment that its enemies can see and destroy it. Also, the 

 albino animals tend to have less vigor than do normal ones. 



PARENTS F 



YELLOW 



A' 



YELLOW 



A' a 



A' 



Fig. 100. — A cross involving a lethal character. A^ = a dominant factor for yel- 

 low; a = a recessive factor for nonyellow. 



An interesting example of a lethal character is that found among 

 vellow mice. These mice never breed true, matings always producing, 

 on the average, two yellows to one black, brown, or gray. If these heter- 

 ozygous mice followed the normal pattern, there would be three yellows 

 to every black, brown, or gray, that is, one would be homozygous for 

 vellow, two would be heterozygous, and the last would be homozygous 

 for the nonyellow. The actual explanation is that the gene for yellow 

 is a lethal, but kills the animals only when homozygous. It is probable 

 that the homozygous yellows die in a very early embryonic stage. 



LINKAGE 



In the discussion of the dihybrid and multihybrid crosses, each allelo- 

 morphic pair of genes assorted independently. This condition only 

 applies if the different pairs are located on different chromosomes, for 

 it is the chromosomes that assort independently during meiosis. Since 

 there are only a limited number of chromosomes and an exceedingly 

 large number of genes, it is obvious that many genes must be located on 

 each chromosome. Genes located on the same chromosome are said 

 to be linked and are not assorted independently during meiosis. 



