Hutchison has demonstrated linkage in two varieties of maize. One had colored and full seeds, while the other had colorless shrunken seeds. The gene for color is C and/is dominant over the colorless condition c.
Similarly the full seed F is dominant over shrunken condition. The genotypes of the parents which represent the pure line are CCFF and ccff. In a cross between the two, as expected the F is colored and full. The genotype is CcFf.
The genotype however is written as CF/cf to indicate C & F and c &f are linked. The gene combinations are written in the manner in which they enter the zygote. If the genotype is- written as CcFf, it is indicative of independent assortment.
If the F, follows the Mendelian pattern of segregation, four kinds of gametes should be -produced in equal proportions. These are CF, Cf cF, and cf. In order to find out, whether these gametes are formed in equal numbers a test cross in done and the F dihybrid is crossed with the double recessive parent (CF/cfx cf/cf).
If the F, dihybrid forms the 4 types of gametes in equal proportions it should yield the four classes of progenies. (Colored full, colored shrunken, colorless full and colorless shrunken) in the ratio of 1:1:1:1. But the actual numbers of progenies obtained are as given in the following table.
The parental combinations namely colored full and colorless shrunken are in large numbers among the progeny than the recombination viz., colored shrunken and colorless full.
If the assortment were to be independent all the four categories should have been produced in equal proportions. Instead, parental combinations abound. Out of a total of 8368 individuals about 96.4% (4032 + 4035) are parental combinations and only about 3.6% (149 + 152) are recombination.
This shows clearly genes C and F have not assorted independently, so also c and f. In other words, the linkage of 96.4%, while recombination i.e., break of linkage is seen in only 3.4% of the progeny.
In another cross in maize plant dealing with the same characters, different parental combinations were selected. The two parents selected for crossing were colored shrunken and colorless full.
Even in this cross, as the table below shows parental combinations were more than the recombination. But here the parental combinations are different from those of the first experiment.
This experiment shows clearly, that whatever parental characters are there, they tend to be inherited together and not assorted independently.
This example in maize, however is not an example of complete linkage, because at least in some instances recombination is seen indicating the break in linkage.
While it is true linkage is there in 96.4% and 97.06% cases in the above two experiments, it is also true that the genes have assorted independently in at least 3.6% and 2.94% cases.
Thus the type of linkage seen in maize is called incomplete linkage. In a complete linkage, parental combinations are retained in 100%, there is just no recombination.
2. Linkage in drosophila:
Genes for body color and wing shape are known to be linked in Drosophila. Morgan and his associate Bridges (1910) experimented with grey body and long winged fly.
They were able to obtain by mutation another stock of fly which had black body and vestigial wings. These mutations were located on the same chromosome.
The normal allele can be indicated by + and the recessive allele for body shape (b) and vestigial wing (v). When a pure line grey bodied long winged fly was crossed with another pure line black and vestige (++/++ x bv/bv), the offspring has grey body and long wing (+ +bv).
If Mendelian independent assortment takes place, the F, individual should produce gametes of all types in equal proportions.
The four gametes are + +, +v, +b. and bv. When a F female was crossed with a recessive male four kinds of offspring are produced not in equal proportion but in the ratio given below. In the genotype of the offspring, the male contributes to each only the recessive genes bv, rest coming from the female parent.
In the above, 1 and 2 accounting for about 82% of the progeny are parental combinations, while 3 and 4 accounting for about 18% are recombination. The strength of linkage is 82% and of cross over is 18%.
In another experiment the two characters body color and wing shape were taken in different combinations. A black long fly (b+/bv) was crossed with another grey vestigial (+v/bv).
Among the Four classes of progeny obtained, once again the parental combinations were more. In this experiment, the parental combinations are – black long and grey vestigial.
1. Grey vestigial +v/bv 41%
2. Black long b+/bv 41%
3. Grey long ++/bv 9%
4. Black vestigial bvlbv 9%
Again here parental combinations are 82% and recombination is 18%. Morgan pointed out that the two genes B and + and b and v are linked to each other (B to + and b to v).
This is because they are situated on the same chromosome. In the case of Drosophila males, complete linkage between genes is observed as there is no cross over.
Consequently the characters always appear together. When a grey long male is mated with black long double recessive female, the progeny has grey long and black vestigial.
3. Linkage in mammals:
In several mammals linkage groups have been identified and they have been identified with their phenotypic traits. In rabbits out of 22 pairs of chromosomes, about six linkage groups have been identified.
In guinea pig one linkage group has been found in human beings, but generally it is difficult to study linkage or any genetic phenomenon in man as one cannot have the desired type of crosses or test crosses.
Pedigree studies or family history studies will reveal some amount of information, where linked characters passed down together in several generations can be studied. Genes on sex chromosomes can be easily studied than those on the automosomes.
Some amount of linkage has been seen between deutan color blindness and hemophilia as reported by Haldane. In a study of 34 individuals Haldane reported that the two genes have a recombination value of 11.4%.
Similarly the gene for the absence of the enzyme G6PD (Glucose 6 Phosphate dehydrogenate) in RBC is known to have a cross over value of 6.5. The data about linked genes in human autosomes is very difficult to obtain.
One study reports linkage between the genes for At BO blood group is linked with the gene for nail patella syndrome with 10% cross over value. The gene for Rh factor and the gene for ciliptocytosis also known to linked with a recombination value of 30%.
Similarly the gene for blood antigen and the gene for dominant cataract of the eye are linked; so also the genes for hemoglobin B and hemoglobin S, but the cross over value for these genes is not known.
4. Linkage in bacteria:
In haploid organisms like bacteria, where there is-no homologue, it is not possible to study linkage strength by means of cross over value.
Geneticists however worked out a method by which the sequence of genes is established on the bacterial chromosome.
This is assessed and calculated during bacterial reproduction, when certain genes are transferred from one cell to the other. This transfer is calculated against time, and reproduction or conjugation is stopped mid way.
The genes that are transferred are then found out. The procedure is as follows. A male strain (Hfr) of Escherichia coli which has some marker genes is allowed to conjugate with the female strain (F.) which does not have these marker genes (genes whose phenotypic trait has not been established).
The circular chromosome of male enters into the recipient cell. The conjugation is stopped at different time intervals -5,10,15, minutes etc., The recipient cell is analyzed as to its acquisition of new genes, by culturing it on various media.
The gene acquired by the recipient strain after 5 minutes is located first, the second one after 10 minutes etc., In this way the distance and location of the genes can be calculated.
Some of the genes transferred from to female against particular time interval are as follows. Conjugation in the first five minutes will result m transfer D gene, after 10 minutes -pho gene, after 16 minutes gal A gene and after 24 minutes try C gene are transferred from male to female.