The chromosome maps. The statements of chromosome theory of inheritance

Above we stated that distance between genes corresponds with crossing over percentage. So we can prove that conclusions by assessment gene distance between genes A and В and at the same time between genes В and C (pic 7.7).

Ріс. 7.7. The scheme of genetic chromosome mapping

So if we have distance between A and В 10% and distance between A and C 3% of crossing over, we may conclude that the gene is either between genes A and В or on a side of them (gene A is between В and C). So if the distance between В and C is 7% of crossing over, that means genes are in a range A,B,C (pic 7.7, upper scheme) So if the distance between В and C is 13% of crossing over, that means genes are in the range C,A,B (pic 7.7, lower scheme).

Thus, chromosome gene localization is linear and distance between them is related with crossing over frequency. But this thesis is good enough only for closely placed genes. For genes which are far from each other that rule appear to have some imperfections. The genes, placed in one chromosome, are linked and they make one linkage group. Number of linkage groups equals to haploid chromosome set.

Thus, frequencies with which crossing over occurs in crosses can be used to construct a genetic map, in which distance is measured in terms of frequency of recombination. The genetic map - is a conditional line with pointed genes according their relative distance in centiMorgans. Some organisms, which were studied more actively than others, have genetic maps of all chromosomes (drosophila, corn, and so on) (pic 7.8).

Ріс. 7.8. The genetic and cytological chromosome maps (by N.N.Ortov 1968): A - genetic map of Drosophila X-chromosome and corresponding part of X-chromosome from Drosophila salivary gland. В -gene localization with help of deletions method: 1 - part of two conjugated chromosomes of Drosophila salivary gland. 2 - deletion 1,3 deletion 2. 4 - localization if gene d resulting from those deletions; C - locus BAR in X-chromosome from Drosophila salivary gland: 1 norm, 2 - double region І6А of BAR mutant 3 triple region 16A of ULTRABAR mutant

Having determined linear chromosome discontinuity, scientists faced the necessity to make a cytological map and to compare them with genetic maps. The cytological map - is a chromosome map, where gene localization and gene distance is defined in chromosome. The construction of cytological maps is based on chromosome aberration analysis (translocatons) and differentiated staining of polutenic chromosomes (pic 7.8).

For today, we have made and compared cytological maps of Drosophila chromosomes. It proves linear gene arrangement in chromosomes. All 23 human chromosomes have been mapped for today, but these maps don’t include all human genes. So that work still needs to be done. The most detailed map was made for first chromosome (20 genes were placed).

The statements of chromosome theory of inheritance. Assuming all above, we may formulate the base points of chromosome theory of inheritance.

1. Genes are on chromosomes. Each chromosome is a gene linkage group. The number of linkage groups is equal to the haploid chromosome set.
2. Each gene has a definite locus in chromosome. There is a linear gene arrangement in chromosomes.
3. There is allelic gene exchange between homologues chromosomes.
4. The distance between genes is closely related with crossing over percentage between them.