Heredity — Genetics 693 



Nilsson-Ehle found three pairs of genes in certain strains of red 

 wheat. Deepest red is represented by R1R1R2R2R3R3 and white wheat 

 by ririr2r2r3r3. The more genes represented by the capital letters in 

 any individual^ the darker the red. This may be shown by the following: 



Parents Deepest red wheat X White wheat 



(R1R1R2R2R3R3) N^ (ririr2r2r3r3) 



Fi Medium red wheat 



(RiriR2r2R3r3) 



When two of the Fi are crossed, the F2 ratio is: 



1 deepest red (6 red genes) 



6 very deep red (5 red genes) 

 15 deep red (4 red genes) 

 20 medium red (3 red genes) 

 15 pale red (2 red genes) 



6 very pale red (1 red gene) 



1 white (no red genes) 



When two dominant genes located in diflferent pairs of chromosomes 

 interact and supplement each other to produce an altogether new pheno- 

 type, such are called supplementary genes. In the combs of chickens, 

 pea-comb is represented by at least one P and rr, rose-comb by pp and 

 at least one R, single-comb by pprr, walnut-comb by at least one P and 

 at least one R. The following cross shows a homozygous rose-comb 

 crossed with a homozygous pea-comb with the genetic contents and 

 ratios : 



Parents 



Gametes 

 Fi 



When two walnut-combed chickens of the Fi are crossed, the F2 shows: 



9 walnut-combed (at least one P and at least one R) 



3 pea-combed (at least one P and rr) 



3 rose-combed (pp and at least one R) 



1 single-combed (pprr) 



This shows not only a new type in the Fi, but, when intercrossed, a still dif- 

 ferent type, namely, single-comb (pprr), is produced. 



When two dominant genes located in different pairs of chromosomes 

 interact and complement each other (both present to produce a visible 

 effect), such are called complementary genes. When two pure strains 

 of white sweet peas are crossed, they produce only purple-flowered peas 

 in the Fi. Purple flowers are represented by at least one C and at least 



