324 ELEMENTS OF BIOLOGY 



is spoken o£ as the genotype; in the Fi generation in the example, 

 the genotype is Tall dominant with Dwarf recessive. It is customary 

 to represent the dominant of an alternative pair of genes by a capital 

 letter and the recessive by a small letter. Thus the genotype of Fi 

 is written as Tt (Fig. 204) . 



When the germ cells of the Fi generation mature, a gamete, 



vS 



having the — number of chromosomes, can have either M chromo- 

 somes or P chromosomes, not both; hence only the gene for Tall 

 or the gene for Dwarf, not both, since the other allelomorph goes 

 either to the other secondary spermatocyte or to the first polocyte 

 at the time of the reductional division, as a part of the synaptic 

 mate. Thus when maturation is completed as far as the distribution 

 of allelomorphic genes, there are two types of sperms and two types 

 of eggs. When fertilization takes place, the union between the eggs 

 and sperms occurs at random and the fertilized tgg which devel- 

 ops into the F2 generation may contain one of three possible com- 

 binations of genes. If the tgg with the chromosome that contains 

 the gene T unites with a sperm containing T, then there will be 

 no gene for Dwarf ness present; such individuals are Tall and con- 

 tinue to breed true for Tallness throughout succeeding generations. 

 If the egg with the chromosome that contains the gene for Dwarf- 

 ness unites with a sperm containing the Dwarfness gene, then the 

 new individual will be pure for that character and all its future 

 progeny will be Dwarf. But if the egg contains the gene T and the 

 sperm the gene t, or the egg contains t and the sperm the gene T, 

 then the synaptic mates constituting a pair of chromosomes in the 

 new individual will contain different genes, T and its alternative 

 for Dwarfness, t. Since T is dominant, the phenotype will be Tall. 

 But such an individual is capable of producing a Dwarf; the geno- 

 type is exactly that of the Fi generation. Individuals which con- 

 tain genes for both of the alternative characters are known as 

 heterozygous; when both genes are for the same quality of char- 



