In this article we will discuss about the relationship between genetic and cytological crossing over.
The exchange of homologous segments of chromatids between homologous chromosomes during meiosis was cytologically demonstrated by Stern in 1931 in D. melanogaster and by Creighton and McClintock in the same year in maize.
Stern used stocks of D. melanogaster carrying chromosomal structural changes. These stocks were crossed to produce a female that had a segment of Y chromosome trans-located to one of its X chromosomes, while its remaining X chromosome was broken into two equal segments and the acentric segment was trans-located on to small IV chromosome.
The centromeric segment of the deleted X chromosome carried recessive allele for eye colour carnation (car) and the dominant eye shape Bar (B). The normal X chromosome, having a segment of Y chromosome earned the wild type alleles (+ +) for the two genes. Thus the two X chromosomes of this female could be recognized under the microscope as well as by the alleles of the two genes.
This heterozygous female fly was crossed to a male carrying car + genes (test cross). The female offspring’s from this cross can be grouped into four, two non-crossover and two crossover type classes. These groupings are based on theoretical expectations of genetic recombination being dependent on cytological crossing over (Fig. 11.11).
1. Carnation and bar-eyed (car B/car +): these had one parental broken X chromosome (from the female) and one normal X chromosome (from the male).
2. Wild type eye colour and normal eye shape (+ + /car +) : such flies had one normal parental X chromosome with the Y segment (from the female) and one normal X chromosome (from the male).
3. Carnation eye colour and normal eye shape (car + /car +): flies of this type had two normal looking X chromosomes.
4. Wild type eye colour and bar shaped eye (+ B /car +): such flies had one broken X chromosome with the trans-located piece of Y (from the female) and one normal X chromosome (from the male).
Stern scored the non-crossover and crossover females both genetically and cytologically; he found that the genotypes and X chromosome morphologies matched perfectly as per the above predictions. This experiment demonstrated that genetic recombination is accompanied by a reciprocal exchange of chromatid segments between homologous chromosomes (Fig. 11.11).
In 1931, Creighton and McClintock used stocks of maize in which the short arm of chromosome 9 was marked by a terminal knob, which could be easily recognized at pachytene. This short arm carried the dominant alleles yg+ (normal plant colour) and wx+ (non-waxy-endosperm).
There was another stock in which the short arm of the chromosome 9 was knobless and contained the recessive alleles yg (yellow-green colour) and wx (waxy endosperm); this chromosome also had a translocation in the long arm which was distinguishable at pachytene.
Both stocks were crossed and the F1 was testcrossed to the double recessive strain. Theoretically the following four types are expected in the test cross progeny; these types can be grouped into either crossover or non-crossover classes (Fig. 11.12).
1. Plants having normal plant colour and non-waxy endosperm (yg+wx+/ygwx)\ their chromosome 9 would carry the knob in its short arm (same as that of the parent).
2. Plants with the recessive phenotype, yellow-green plant colour and waxy endosperm (ygwx/ygwx)\ their chromosome 9 would be without the knob but would have the translocation (same as that of the parent).
3. Plants having normal colour and waxy endosperm (yg+wx/ygwx): their chromosome.
9 would have both the knob and the translocation (produced due to crossing over) (Fig. 11.12).
4. Plants with yellow-green, non-waxy traits (ygwx+/ygwx)\ the chromosome 9 of these plants would be without the knob as well as translocation.
Cytological and genetical analyses of the test cross progeny revealed that the recombination between the two genes was always accompanied by an exchange between the chromatids of the two homologous chromosomes.