Testcross 85 



c gene. The phenotypic ratio when the Fi is backcrossed to the 

 recessive parent is 1 wild type : 1 curved. The 1 : 1 ratio will 

 be obtained whenever any heterozygote is crossed with a homozy- 

 gous recessive. ''Backcross" is literally appropriate only when 

 an Fi animal or plant is crossed with one of its own parents. At 

 other times ^'testcross" is more appropriate, although the terms 

 are frequently used indiscriminately. 



Testcross ' 



The cross of a heterozygote with a recessive will give a 1 : 1 

 ratio whereas a cross between a homozygous dominant and a 

 recessive will produce only dominants. Use can be made of these 

 facts to test whether a dominant plant of unknown ancestry is 

 homozygous or heterozygous. In certain varieties of lupines, 

 red flowers are dominant over white. If a commercial seed house 

 wishes to market seeds of a red variety of lupine, claiming that 

 only red-flowered plants will be produced, and if they have a 

 number of red-flowered plants from which to obtain their seeds, 

 they must know the genotypes of the plants before they can 

 market seeds from them with a guarantee that all will yield 

 plants with red flowers. If the red-flowered plants used for seed 

 are homozygous, all the seed from them will produce red- 

 flowered plants; but if some are heterozygous, one-fourth of 

 the seed from those plants will produce plants with white flowers. 

 If the company guarantees the seeds to produce only red-flowered 

 plants, it must know which of the plants are heterozygous and 

 which will breed true for red flowers. One of the most widely 

 used methods of testing them is to cross them with recessive, 

 white-flowered plants. Those plants which produce only red- 

 flowered offspring, when mated with recessives, are the homozy- 

 gotes and are used to produce the red-flowered seed for the 

 market. Those, on the other hand, that give approximately equal 

 numbers of red- and white-flowered plants when crossed with 

 the recessive are heterozygotes and worthless for this particular 

 purpose. A cross of a dominant of unknown genotype with the 

 recessive is a widely used method of determining the genotype 

 of phenotypically dominant plants and animals. 



Practical Considerations in Using the Testcross. It is obvious 

 that the method of the testcross is not the only way of determin- 

 ing whether a plant is homozygous or heterozygous. Self-fertiliz- 



