698 General and Applied Biology 



called crossing over (Fig. 342). The controlling mechanism is unknown, 

 but the greater the distance between the two loci and any two given 

 genes, the greater the chance that crossing over will take place between 

 them. Likewise, the smaller the distance between the loci of any two 

 genes, the less the chance that crossing over will occur. New combina- 

 tions of linked genes within the pair of chromosomes involved result 

 from the crossing-over process. 



From data secured from crossing-over experiments it is possible to 

 locate approximately the genes (loci of genes) within the chromosome. 

 Such an approximate location of genes is called a chromosome map (Fig. 

 343 ) . The approximate locations of genes are determined experimen- 

 tally by recording the percentage of crossing over between them. Let us 

 take a theoretical example. If the experiments show that in a certain 

 chromosome crossing over between its genes A and B occurs 8 per cent 

 of the time, then these two genes are considered to be eight "units" of 

 distance apart in that chromosome. If crossing over between genes B 

 and C occurs 3 per cent of the time, they are three "units" apart. This 

 could be interpreted as meaning that the sequence is A: : : : : : :B: :C. 

 However, G might be between A and B. If the genes A and C cross 

 over 5 per cent of the time, then the sequence is A: : : :C: :B. In a simi- 

 lar manner other genes may be located within this chromosome. 



Linkage and crossing over have been observed in many types of plants 

 and animals, including human beings. 



12. Sex Determination and the Sex Ratio. — From scientific cytologic 

 studies of the fruit flv it has been found that each somatic cell contains 

 three pairs of chromosomes known as autosomes and one pair of sex 

 chromosomes (Fig. 344). The female has three pairs of autosomes and 

 a pair of sex chromosomes called X chromosomes, while the male has 

 three pairs of autosomes and one X chromosome and one Y chromosome. 

 When female gametes (sex cells) are produced, each contains three 

 autosomes and one X chromosome. When male gametes are produced, 

 one type contains three autosomes and one X chromosome, while the 

 other type contains three autosomes and one Y chromosome. Since the 

 two types of male gametes are produced in equal numbers and each type 

 is thought to have somewhat equal chances for fertilizing an egg, the 

 ratio of male and female offspring is approximately 50:50. 



It is known that somatic cells of human beings contain twenty-three 

 pairs of autosomes and a pair of sex chromosomes as described for the 

 fruit fly. The distribution of sex chromosomes in man is similar to that 

 in the fruit fly. 



