Question

What results from crossing experiments would suggest 2 loci are on the same chromosome?

Answer 1

Results of the crossing experiments depends upon the relative location of loci of the genes of two traits considered on the same chromosome.

Explanation:

The two traits considered in a dihybrid cross may show independent assortment or linkage, depending upon the relative location of the genes controlling these traits.

The genes controlling these traits may be located far apart or closer to each other.

There is no possibility of crossing over to occur, If the genes are located very close to each other. These traits will show complete linkage . Only parental phenotypes will appear in the offsprings resulting from this cross. Taking example of a hypothetical plant, a pure tall plant bearing red flowers is crossed with a pure dwarf plant bearing white flowers will produce only tall plants with red flowers and dwarf plants with white flowers in the off springs of `F2` generation .

There is 100% possibility of crossing over to occur, if the genes of two traits are present very far apart from each other. These two traits will assort independently. The off springs will have parental phenotypes as well as recombined phenotypes, e.g. A pure tall plant bearing purple flowers is crossed to pure dwarf plant bearing white flowers, the off springs in `F2` generation will be tall bearing red flowers, tall bearing white flowers, dwarf bearing red flowers bearing and dwarf bearing white flowers.

The two traits may show incomplete linkage , if the loci of the genes of two traits are located away from each other, but not very far apart. This is because, crossing over does not occur in 100% of cells. The gametes produced by meiosis in the cells, in which no crossing over occurs, will only be of parental types. The percentage of recombined gametes will be lesser than those of the parental gametes in such a cross.