472 Organisms Are Products of 



dihybrid TsYg go through reduction di- 

 vision, as diagramed in Figure 423. The 

 gametes may be TY, Tg, sY, sg. Try it 

 on paper. This is what is meant by inde- 

 pendent assortment of genes. The pair 

 of genes for one character will act inde- 

 pendently of the pair of genes for the 

 other character. Figure 424 illustrates all 

 the possible offspring when these dihy- 

 brids are crossed. You can demonstrate 

 this by doing Exercise 14. Exercises 15 

 and 1 6 are more difficult. Try them. 



T Y 



•f g 



Primary sex ceil 1 gametes 



Primary sex cell 2 



gametes 



Fig. 423 Diagram of two pr'miary sex cells of 

 a plant that is hybrid for size and for seed 

 color. How nia?iy different kinds of ga?netes 

 may a dihybrid produce? 



Heredity and Efivirofiment unit ix 



(Optional) Linkage. Research workers 

 soon found that genes did not always 

 assort independently. When they hap- 

 pened to study the inheritance of two 

 genes which lay in the same chromosome, 

 the results were different. In the fruit 

 fly the gene for black body lies in the 

 same chromosome as the gene for short 

 (vestigial) wing. When reduction divi- 

 sion occurs these two genes naturally 

 remain together. They are said to be 

 linked. Since there are genes for many 

 different characters in one pair of chro- 

 mosomes, there are many cases of linkage. 

 The characters detemiined by such genes 

 are not inherited independently. Mendel 

 happened to study only characters whose 

 genes were in different pairs of chromo- 

 somes and for this reason he believed 

 that every character is inherited as a unit, 

 independently of the others. Exercises 17 

 and 18 are difficult. Try them. 



(Optional) Sex is inherited like other 

 characters. Long ago it was noticed that 

 the male and female organisms of a 

 species differed in one pair of chromo- 

 somes. In many animals the male had one 



Fi cress 



Ts Yg 



X 



TsYg 



Fig. 424 Crossing two dihy- 

 brids. The parents were hy- 

 brids for both size and seed 

 color. See also Figure 42^. 

 How many of the sixteen 

 kinds of crossings result in 

 tall yellow plants? In short 

 yellow? In tall green? In 

 short green? What ratio does 

 this give when you are deal- 

 ing with large mivibers? 

 How many different kinds 

 of gene coynbinations are ob- 

 tained in this ():y.y.i ratio? 



