380 INTRODUCTION TO EVOLUTION 



known, Mendel himself experimented with garden peas. During the course 

 of his experiments he produced about 20,000 individuals as offspring of 

 heterozygous parents. The ratio he obtained from them was 2.996:1.004, 

 certainly a near approach to the ideal 3:1. 



During some years of teaching an elementary course in heredity the 

 present author has utilized the coin model mentioned above. Students are 

 asked to toss two coins and record the number of times both coins come 

 up heads; the number of times one coin comes heads, the other tails; and 

 the number of times both come up tails. To date, a cumulative total of over 

 57,000 such tosses has been amassed. Adding together the tosses in which 

 there is ot least one head (as in combining the MM and Mm groups 

 above) we obtain a ratio of 2.987:1. 



Thus we see that the distribution of genes from parents to their offspring 

 is dependent upon the operation of the laws of probability (chance). The 

 1:2:1 ratio (and its modification the 3:1 ratio) is the ideal ratio ap- 

 proached when two equally numerous kinds of sperms fertilize two equally 

 numerous kinds of ova. 



In our examples so far we have noted that genes occur in pairs in the 

 genotypes of parents, while in germ cells produced by those parents the 

 members of pairs are separated so that each germ cell receives but one 

 gene of each pair. This means that while genes occur in pairs in the body 

 (somatic) cells of animals, they occur singly in the mature reproductive 

 cells. What is the explanation? To answer this question we must learn 

 something of the behavior of the chromosomes which contain the genes. 



Meiosis 



As a specific example, let us take the heterozygous black, male hamster 

 just mentioned. His body cells contain chromosomes arranged in pairs; 

 one member of each pair came from his mother, one from his father. 

 Early in his embryonic development certain cells were set aside to form 

 the sperm cells which he would require when he reached sexual maturity. 

 These primordial germ cells are called spermatogonia. They contain 

 chromosomes in pairs just as do the body cells. At the top of Fig. 17.2 

 we see spermatogonia containing a long pair of chromosomes and a short 

 pair. (For sake of simplicity only two pairs are shown.) One member of 

 each pair is white, one is shaded. The white member may be thought of as 

 the one derived from the mother, the maternal one, the shaded member 

 as the one derived from the father, the paternal one. Somewhere on each 

 chromosome there is a centromere or spindle fiber attachment. This is 

 represented as a large dot in the diagram. 



