Genetical and Cytological Methods of Locating Inversion Break Points

In this article we will discuss about the genetical and cytological methods of locating inversion break points.

Cytological Location:

Cytological analysis is an important method of the physical location of the break points of inversion.

(a) In plants like maize, and other organisms where pachytene analysis is easy, the breakpoints can be located by measuring the position of the inversion loop on the chromosome; thus it is essential to study inversion heterozygotes.

(b) In Drosophila, inversion breakpoints can be determined by measuring the position of inversion loop in the salivary gland giant chromosomes. This is done in inversion heterozygotes.

(c) Inversion breakpoints in Drosophila can be located even in inversion homozygotes by studying the banding pattern of salivary gland chromosomes. This method has been used to locate the breakpoints in chromosome 3 of D. pseudoobscura.

Genetic Location:

(a) When recombination is either reduced in or eliminated from a chromosome region, it may be considered as inverted.

(b) Linkage test is an important method of locating the break positions of inversion. Recombination between the marker genes and sterility is studied. In general, the frequency of recombination between the markers in the inverted region is much lower than that in the normal chromosome.

In 1926, Sturtevant used an inversion stock for chromosome 3 of Drosophila for linkage analysis. This stock was the wild type, and it was crossed to a norm?’ 3rd chromosome back homozygous for the recessive mutations st-sr-e^s-r°-ca which are linked in the given order.

The F₁ females from this cross were backcrossed to the homozygous mutant parent. He found rather low frequency of crossovers within the inverted region. The recombination values were greatly reduced in the sr-e^s, e^s– r°and r°-ca regions indicating that the genes sr. e^s and r° were within the inverted region. Recombination between st and sr was reduced from 18.0% (normal) to 9.6%; this indicated that this area (st-sr) was outside but adjacent to the inverted region.

In yellow fever mosquito (Aedesaegypti), Bhalla in 1970 studied recombination between genes in normal and inversion heterozygotes. The inverted region included the genes re, m, bz and w in the sex chromosome. The total linkage distance from re to w was 23 units in the normal chromosome; it was reduced in the inversion heterozygote to only 2.8% in the female and to 3.5% in the male (Fig. 15.12). There was no reduction in recombination frequency in the region ra-re lying outside the inverted region.

In maize, the genes for normal (Lg2) vs. liguleless (lg2), and for coloured (A1) vs. colourless (a1) aleurone are located in the long arm of chromosome 3 giving a crossover value of 28%. A normal stock homozygous for both the recessive genes (lg2 lg2 a1 a1) was crossed with an inversion stock carrying the dominant alleles Lg2 and Al by Rhoades and Dempsey in 1953. The resulting F₁ was crossed reciprocally to the recessive normal stock.

They observed 14 cross over and 2625 non-crossover plants in the progeny when the F₁ was used as male, while 108 crossover and 5457 non-crossover plants when the F, was used as female. Thus the recombination between the two genes was greatly reduced as compared to their normal recombination frequencies. It was only 0.5% and 1.5% when F₁ (inversion heterozygote) was-used as male and as female, respectively.

(c) By using viable deficiency-duplications, obtained from crossing over in inversion heterozygotes, genetic location of breakpoints can be done. This method was used by Li in 1950 for the study of a pericentric inversion in chromosome 9 of maize.