466 Orgmiisins Are Products of 



Pure and hybrid organisms. Chromo- 

 somes carry the genes that determine the 

 characters in organisms. Chromosomes 

 and their genes are present in pairs in the 

 fertilized egg and in every other cell 

 (except the sperm and egg cells). One 

 chromosome of a pair comes to the fer- 

 tilized egg with the sperm, the other with 

 the egg. That is, the fertilized egg and 

 every cell of the red-flowered plant 

 (except the gametes) have two genes for 

 redness, making a pair. If, for the mo- 

 ment, we disregard the many other genes 

 and use letters as symbols for the genes 

 for redness we may represent such a 

 plant by RR. The white-flowered plant 

 can be represented by TFir, since every 

 cell has a pair of genes, W and W. Such 

 RR and WW plants are called pure, 

 meaning that the pair of genes being 

 studied consists of two genes that are 

 alike. 



A pink-flowered plant comes from the 

 crossing of a red-flowered one (RR) with 

 a white-flowered one (WW). Because 

 reduction division occurs in the pro- 

 duction of gametes (see p. 460), gametes 

 have only one member of each pair. So, 

 obviously, the pink-flowered plant re- 

 ceives a gene for redness (R) from one 

 parent and a gene for whiteness ( W) 

 from the other parent. It, therefore, has 

 the make-up, RW. The RW plant is not 

 pure; it is known as a hybrid. In a hybrid 

 the two genes of a pair are different. 



Why there is a 1:2:1 ratio. A pink- 

 flowered plant has the pair of genes RW 

 in its primary sex cells and in all its other 

 cells except sperm and egg cells. When 

 the primary sex cells go through matu- 

 ration the two chromosomes that make 

 up the pair part company as indicated in 



Heredity and Enviromnevt unit ix 

 R W 



Fig. 413 Each primary sex cell (i) of the pink- 

 flowered four o'clock forvis two kinds of gam- 

 etes (2). Half the gametes from such a primary 

 sex cell have a gene for redness. Half have a 

 gene for whitejiess. What kind of cell division 

 takes place in primary sex cells? 



Figure 413. The chromosome containing 

 gene R goes into one gamete and the one 

 containing W goes into another gamete. 

 The two are, therefore, not together in 

 one gamete. Gametes (sperms and eggs) 

 are always pure, which means that they 

 contain only one chromosome of a pair 

 and therefore only one gene of a pair. 

 When the many primary sex cells of the 

 RW (pink) plant form gametes, about 

 half will contain the R gene and half the 

 W gene. Under normal conditions if 

 there are large numbers of gametes there 

 will be about the same number of gam- 

 etes containing R as there will be gam- 

 etes containing W. 



You can see better what happens when 

 two pink-flowered plants are crossed if 

 you study the diagram in Figure 414. 

 The two pink-flowered plants which 

 started the experiment are represented as 

 rectangles containing the genes RW. 

 The spemis formed by one plant are of 

 two kinds. They are shown as circles; 

 50 per cent have the gene R and 50 per 

 cent the gene W. The eggs formed by 

 the other parent are likewise shown as 

 circles. 



If now fertilizations occur between the 

 gametes shown in the diagram, there are 

 four possibilities: 



