Chapter 9 



LINKAGE AND CROSSING OVER 

 BETWEEN GENES 



T: 



Jhe alleles of a gene pair af- 

 fecting the seed coat of the 

 garden pea were symbolized 

 in Chapter 4 as round (/?) and wrinkled 

 (r). This symbology follows the conven- 

 tion that uses upper and lower case of the 

 first letter (or so) of the phenotype pro- 

 duced by the dominant allele — the one usu- 

 ally found in nature — to represent the domi- 

 nant and recessive alleles, respectively. 



In other conventions (see Figure 9-1), 

 the first letter (or so) of the recessive trait 

 (wrinkled) is used in lower case for the 

 recessive allele (w), and the normally dom- 

 inant allele (round) is given as one of the 

 following: the same symbol in upper case 

 (W); a + symbol as a superscript or base 

 to the lower case symbol (vv+ or +"); or 

 -+- alone. Henceforth in this book we will 

 usually use one form of the -f- system for 

 symbolizing genes. In this system, a mutant 

 gene — Beadex, for example — which is dom- 

 inant to the normal wild-type allele is rep- 

 resented by one (or more) letters of which 

 the first is capitalized (Bx or + "■'') and its 

 wild-type allele is + (or Bx+). The hy- 

 brid + vv can be represented as = or or 

 r WW 



-\-/w to show that these alleles are on dif- 

 ferent members of a pair of homologous 

 chromosomes. 



Each of the first seven pairs of genes 



studied in the garden pea (see Chapter 4, 



p. 48) appeared to segregate independently. 



If this kind of segregation is attributed to 



116 



each gene pair being located in a dilferent 

 one of the seven pairs of chromosomes car- 

 ried by this organism, what result will be 

 obtained when an eighth pair of genes, 

 showing dominance and affecting an unre- 

 lated trait, is included in such a study? 

 When a dihybrid is made of one of the 

 seven gene pairs and the eighth pair men- 

 tioned above, a 9:3:3:1 phenotypic ratio 

 is obtained when the dihybrid is self-ferti- 

 lized, and a 1:1:1:1 phenotypic ratio is 

 obtained when the same dihybrid is test 

 crossed to the double recessive. These two 

 independent tests demonstrate that the two 

 pairs of genes involved are segregating in- 

 dependently. The phenotypic ratios are 

 radically different, however, when a dihy- 

 brid is made with still another of the seven 

 gene pairs — the one affecting seed coat — 



W 

 w 



+ w 

 w + 



w 



w 



+_ 

 w 



— +/ w 

 w 



figure 9-1. Various ways of representing the 

 round-wrinkled hybrid by gene symbols. 



and the same eighth. The other pair of 

 genes involved (the eighth) determines 

 the presence and absence of tendrils — the 

 threadlike structures serving as a means for 

 attachment as the plant climbs. The ten- 

 drilless allele (t) is recessive. When a dou- 

 ble recessive pea plant — wrinkled, tendril- 

 less (w w 1 1) is crossed to a pure double 

 dominant — round, tendrils ( + + + + ), all 

 Fi are round with tendrils (+w +/), as 

 expected. When the F, are self-fertilized 

 (dihybrid by dihybrid), the following re- 

 sults are obtained in F>: 



