RECOMBINATION IN SEXUAL ORGANISMS 73 



arate, following their centromeres to opposite poles. As a result, each 

 of the two daughter nuclei that form after telophase has the same two 

 chromosomes as did the original cell. Since every chromosome divides 

 exactly, no matter how many make up each set, or how many sets are 

 present, the daughter nuclei retain the same chromosomal complement 

 as the cell from which they arose. 



The important thing about mitosis is that it preserves the original 

 chromosome number. Therefore, if some genes are located on chromo- 

 somes, mitosis will insure that during asexual vegetative reproduction 

 each daughter cell receives the same genetic complement. Meiosis, on 

 the contrary, is the mechanism for reducing the number of chromosomes 

 in diploid cells by half and thereby segregating the members of homolo- 

 gous pairs into separate cells. The important distinction between mitosis 

 and meiosis is that in mitosis there is one chromosome duplication for 

 every division, whereas in meiosis there are two divisions with only one 

 chromosome duplication. 



Thus far we have considered only one pair of homologous chromo- 

 somes and the alleles they carry. But there are many genes on each 

 chromosome, and many chromosomes. We will now consider the trans- 

 mission of two pairs of alleles in a single cross. Since mitosis during 

 asexual reproduction forms nuclei which have identical chromosome 

 complements, their genetic constitutions are also the same. But at 

 meiosis when the chromosome number is halved, parental combinations 

 of chromosomes and genes are not always maintained. If the two allelic 

 pairs of genes are on nonhomologous chromosomes, they are unlinked 

 and will recombine among the gametes half the time, as shown in Figure 

 3.5. This is because nonhomologous chromosomes behave indepen- 

 dently of one another at the first meiotic division. Thus, despite the 

 fact that each pair of homologous chromosomes and the alleles carried 

 segregate according to Mendel's first law in a 1:1 ratio, parental com- 

 binations and recombinations are produced in equal proportion. In this 

 way the independent behavior of nonhomologous chromosomes accounts 

 for the random assortment of the genes they carry, and, therefore, for 

 Mendel's second law. 



CROSSING OVER AND CHROMOSOME MAPPING 



When two pairs of alleles show less than 50 per cent recombination, 

 they are said to be linked. If the physical basis of linkage were the 

 chromosome, one might expect either complete linkage or none at all. 

 What is commonly found, however, is partial linkage or restricted recom- 



