RECOMBINATION IN SEXUAL ORGANISMS 



85 



dividuals produced. In the corn plant, however, the segregation of one 

 pair of alleles can be observed directly in the gametes. Members of the 

 waxy-starchy gene pair have different effects on the phenotype of pollen; 

 pollen grains carrying the starchy gene form starch and stain blue with 

 iodine, whereas those that possess the waxy gene contain a red staining 

 starch. Pollen produced after the segregation of this pair of alleles shows 

 statistically a 1 : 1 ratio of the two phenotypes (Figure 3. 10). Among micro- 

 organisms, tetrads including all four products of meiosis are analyzable 

 so that Mendel's first law can be directly verified. 



In mating diploid organisms, sperm, pollen, and eggs are necessarily 

 sampled at random. Any two sperm are not likely to be members of the 

 same quartet of meiotic products because of the few progeny studied 

 relative to the tremendous number of sperm produced. In the case of 

 eggs, no two come from the same quartet because only one product of 

 each meiosis survives to form an egg. The union of sperm and eggs is 

 random and does not, in general, depend upon the genotype of the 

 gamete. To illustrate some ratios of diploid genotypes, we will use data 

 on the inheritance of the M and N blood groups in man. Individuals 

 of M blood group produce gametes that are genetically homogeneous and 

 carry a gene for that character. We refer to the gene as M ; fusion of 

 an M sperm with an M egg results in an M /M' individual which 

 can, in turn, produce only M sperm or eggs. The same is true for indi- 

 viduals of N blood group, whose genotype is written M /M' . These 

 conclusions are drawn from the results of the matings which are shown in 

 Table 3.4. The mating M x N, involving fusion of gametes of M com- 

 position with those of M composition, should then give only MN indi- 

 viduals, which in fact, it does. 



Diploid organisms possessing identical alleles at a given chromosomal 

 location {locus) are called homozygotes (e.g., M /M' ) whereas those 

 having different alleles at a locus are called heterozygotes (e.g., M /M ). 



FIGURE 3.10. The segregation of the 

 starchy and waxy alleles among pollen 

 produced by a maize plant heterozygous 

 for this gene pair (after Demerec, in: 

 Sinnott, Dunn, and Dobzhansky, 1958, 

 Principles of Genefics, p. 56, New York, 

 McGraw-Hill). 



