Question

Pure line breed refers to
A. Heterozygosity only
B. Homozygosity only
C. Heterozygosity and linkage
D. Homozygosity and self assortment

Answer 1

Hint: Pure line breed represents a group of identical individuals which when intercrossed, always produce offspring of the same phenotype. It is the method of breeding in which both pairing breeds are of pure lines who only produce one type of offspring, i.e. they breed true to their genotype and phenotype.

Complete answer:
First, we should know about plant breeding to answer this question. To increase the value and utility for human welfare, plant breeding is a method of altering the plant's genetic pattern. Plant breeding is a manipulation of plants to produce desired plant types that are best suited for cultivation, deliver good yield and are disease resistant. Based on the studies with the beans (Phaseolus vulgaris) variety named Princess, Johannsen proposed the concept of pure line. He got the seeds from the market and noted that the lot consists mainly of a mixture of seeds of both larger and smaller sizes. There were variations in the size of the seeds. Johannsen selected seeds of various sizes and individually cultivated them. Larger seed progenies produced larger seeds and smaller seed progenies produced only small seeds.

Now, let us find the solution from the options-
-The condition of having two separate alleles at a locus is heterozygosity. The study of genetic variation in populations is central to it. All pure-line genotypes are homogeneous, that is to say, genetically identical and phenotypically similar. Thus there are homozygous and homogeneous pure-lines. Thus, options A and C are incorrect.
-The pure line is true-breeding genotypes. It is a line that in successive generations has been made homozygous with all genes under consideration. It is a line in which homozygous individuals only bear homozygous offspring like parents. Therefore, pure line breed only applies to homozygosity.
-Assortment notes that an assortment of each pair of traits is independent of the other during a dihybrid cross (crossing of 2 sets of traits). In other words, during gamete development, one set of traits is isolated separately from another set of traits. Thus, option D is incorrect.

Hence, the correct answer is option (B).

Note: For self-pollinated crops, a pure line breeding method is normally used, but it is important in the breeding of inbred lines which are used in self- or cross-pollinated crops to make hybrids. The variance within a pure-line is completely due to factors in the environment. Therefore the variation in the pure-lines is non-heritable. Due to the absence of genetic variation, a strain produced through pure-line selection is highly uniform inconsistency.