652 GENETICS AND EVOLUTION 



pendent Assortment, may now be given as: the distribution of each pair 

 oi genes into gametes is independent of the distribution of any other 

 pair. 



268. Chromosomal Basis of the Laws of Heredity 



Each cell of every organism of a given species of animal or plant 

 contains a definite number of chromosomes; the constancy of the chrom- 

 osome number is assured by the precise and regular events of mitotic 

 division (p. 39). Many widely different species of animals and plants 

 have the same number of chromosomes. It is not the number of chromo- 

 somes, but the nature of the hereditary factors within them, that dif- 

 ferentiates species. 



The constancy of the chromosome number in successive generations 

 of the same species is assured by the precise separation of the members of 

 the pairs of homologous chromosomes in the meiotic divisions leading 

 to the formation of gametes. The normal number of chromosomes for 

 somatic cells is reconstituted in fertilization when the egg and sperm 

 nuclei fuse. 



The laws of heredity follow directly from the behavior of the 

 chromosomes in mitosis, meiosis and fertilization. W^ithin each chromo- 

 some are numerous hereditary factors, called genes, each of which con- 

 trols the inheritance of one or more characteristics. Each gene is located 

 at a particular point, called a locus (plural, loci), along the chromosome. 

 Since the genes are located in the chromosomes, and each cell has two 

 of each kind of chromosome, it follows that each cell has two of each 

 kind of gene. The chromosomes separate in meiosis and recombine in 

 fertilization and so, of course, do the genes within them. We currently 

 believe that the genes are arranged in a linear order within the chromo- 

 some; the homologous chromosomes have similar genes arranged in a 

 similar order. When the chromosomes undergo synapsis during meiosis 

 (p. 117) the homologous chromosomes become attached point by point 

 and, presumably, gene by gene. 



269. Allelomorphs 



Studies of inheritance are possible only when there are two alter- 

 nate, contrasting conditions, such as Mendel's yellow and green peas or 

 round and wrinkled ones, which are called allelomorphs, or alleles. A 

 pair of alleles are two contrasting traits inherited in such a way that an 

 individual may have one or the other but not both. Thus, curly hair 

 and straight hair are alleles, for a person's hair is one or the other, but 

 curly and blond are not alleles, for hair may be both blond and curly. 



Brown and black coat color are allelomorphic traits in guinea pigs. 

 Each body cell of the guinea pig has a pair of chromosomes which con- 

 tain genes for coat color; since there are two chromosomes, there are 

 two genes per cell. A "pure" black guinea pig (one of a pedigreed strain 

 of black guinea pigs) has two genes for black coat, one in each chromo- 

 some, and a "pure" brown guinea pig has two genes for brown coat. The 



