682 General and Applied Biology 



chromosomes and genes are known as homologues or homologous chro- 

 mosomes. In each homologous chromosme there is a gene at a particu- 

 lar place or locus which affects a certain trait, although in some traits 

 several genes may be required. Two genes at the same locus on homolo- 

 gous chromosomes but producing somewhat different effects on the indi- 

 vidual are called alleles or allelomorphs (al-el') (Gr. allelon, one an- 

 other). For example, tall and dwarf traits in peas are due to alleles 

 located at the same locus in homolooous chromosomes. When one allele 



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(gene) expresses itself to the exclusion of its "partner" allele, the former 

 is called a dominant gene and the trait is known as a dominant trait. 

 The allele whose effect is not visibly expressed is called a recessive gene, 

 and the trait is known as a recessive trait. An organism having two 

 identical genes at one locus is homozygous for that gene or is said to 

 possess homozygous genes. When an organism possesses a dominant 

 allele (gene) and a recessive allele (gene) at the same time, it is hetero- 

 zygous for that gene or is said to possess heterozygous genes. For ex- 

 ample, TT and tt are homozygous, while Tt is heterozygous (T repre- 

 sents tall; t represents dwarf pea plants) . 



It is known that genes in the nucleus of cells control cellular metabo- 

 lism, the synthesis of various biochemical compounds, and the inherit- 

 ance of certain traits. How do these genes act? How can the genes 

 (contributed by both parents) in the fertilized ^gg determine the various 

 structures and functions of the embryo and eventually the adult? As a 

 matter of fact, the nucleus with its genes and the surrounding cytoplasm 

 constitute a complex system or unit whose interactions are responsible 

 for the phenomena suggested. It is not well known how these reactions 

 function but it is theorized that the genes in the nucleus interact with 

 certain specific substances also in the nucleus to form the products of 

 genie action. The latter products may interact with other newly formed 

 products in a sort of chain reaction so that numerous products may be 

 formed in the nucleus. It is thought that eventually some of the origi- 

 nal gene products as well as some of the newly formed gene products 

 pass into the cytoplasm. In the latter, the various gene products may 

 react still further with each other or with certain products of the cyto- 

 plasm. It is probable that some of the cytoplasmic products may dif- 

 fuse back into the nucleus so that the entire process is one in which the 

 great numbers of genes interact in a great variety of ways to lay the 

 basis for cellular phenomena. 



The fertilized egg by repeated mitoses develops into a multicellular 

 embryo whose cells quite early show slight inequalities, some being 



