. Adventures with animals and plants. Biology. 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 16 are more difficult. Try them. T Y •f g Primary sex ceil 1 gametes. Primary s
. Adventures with animals and plants. Biology. 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 16 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 © Male © gametes © © Female gametes ©©©© TT YY tall yellow TT Yg tall yellow Ts YY tall yellow T
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