CHAPTER THIRTEEN 



Gene Mutation 



The unit of Mendelian heredity is the gene. The genes are parts of the 

 chromosomes, and hence they ordinarily exist in pairs, just as the chromo- 

 somes do. Whenever the two genes of a pair are identical, there can be 

 no doubt as to what trait they will determine. Whenever the two genes 

 of a pair are unlike, however, several possibilities arise. It may be that the 

 effect of one gene shows up to the exclusion of the other, in which case 

 the one which is expressed is referred to as dominant, while its allele 

 (alternative gene) is referred to as recessive. Dominant and recessive 

 genes are symbolized by capital and small letters respectively. Or the two 

 genes may collaborate to cause a trait intermediate between the two pure 

 types. Such intermediates are called compounds. Lastly, the two unlike 

 genes could collaborate to produce a character unlike either pure type. 



ELEMENTARY MENDELIAN CONCEPTS 



The Mendelian Laws. These genes are inherited in a statistically pre- 

 dictable manner (Figure 71). When gametes are formed, the two genes 

 of a pair are separated into sister gametes, so that each gamete contains 

 only one gene for each character ( Mendel's first law— the Law of Segre- 

 gation ) . As a result, whenever an organism is heterozygous for a particular 

 gene (that is, the two members of the pair are unlike), two types of 

 gametes will be formed in equal numbers. These unlike genes are not in 

 any way diluted or modified in the direction of an intermediate because 

 of their association in the hybrid. If either gene again becomes homozy- 

 gous ( both members of the pair alike ) in a zygote, the original character 

 will reappear unmodified. This is Mendel's second law, the Law of the 

 Purity of Gametes. With the exception of certain special cases, when 

 hybrid organisms interbreed, any type of sperm or pollen has an equal 

 probability of fertilizing any type of egg or ovule ( the principle of random 

 fertilization). As a result, the offspring of two hybrids (Afl) include 25 

 per cent homozygous dominant {AA), 25 per cent homozygous recessive 

 {aa), and 50 per cent heterozygous ( Art ) . As the heterozygotes show the 

 dominant character, this results in 75 per cent dominant and 25 per cent 

 recessive offspring, the famous 3 to 1 ratio. Such ratios are usually not 

 obtained exactly, but rather, because they depend upon the laws of 



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