Linkage of Genes in Plants: Kinds and Theories

Linkage of Genes in Plants: Kinds and Theories!

Mendels law of independent assortment is applicable both to genes and chromosomes. During meiosis, the maternal and paternal members of each pair of chromosomes are distributed independently to the gametes. It is for this reason that genes carried in different chromosomes undergo independent assortment and produce the ratios of differentiating characters which Mendel discovered and explained so successfully.

It has been found that in most individuals the number of genes exceed the number of pairs of chromosomes. For example, in Drosophila hundred of genes have been studied yet there are only four pairs of chromosomes. About 400 pairs of genes are known in maize, yet there are only 10 pairs of chromosomes.

Stern has estimated that the total number of pairs of genes in man is not less than 5000 or more than 1,20,000 but man has only 23 pairs of chromosomes. Thus, it is clear that numerous pair of genes must be located on each pair of chromosomes and genes located in the same chromosome will not be assorted independently. So, Mendel’s second law is not universal but is limited to genes in different chromosomes.

Coupling and Repulsion Theory:

Bateson and Punnett in 1906 discovered that independent assortment of factors does not take place always as assumed by Mendel in some cases. This difference from the law of independent assortment was first studied by them in sweet peas. In a cross between purple long and red round the F₂ progeny did not give the 9:3:3:1 ratio as expected.

On the other hand, purple long and red round were more numerous than expected where as the purple round and red long were less in frequency. Likewise, when a purple round variety was crossed with a red long variety, the parental combinations appeared more frequently in the F₂ than the new combinations.

On the basis of the above results Bateson and Punnett formulated a coupling & repulsion theory. They back crossed the hybrid purple long (PpL1) with recessive parent (ppll) and got the phenotypic ratio of about 7 purple long to 1 red long to 1 purple round to 7 red round. It is clear from this that the hybrids produced gametes of types PL and pl about 7 times as frequently as those of types PL and pl.

Hence Bateson and Punnett suggested that these dominant determiners of two pairs must in some way be coupled, so that they tended to pass in the same gametes at gametogenesis. This tendency of same pair of characters (PP or LL) to unite together and to reappear hand to hand in next generation was termed Coupling by them.

Secondly, they crossed the purple round peas (PPll) with red long (ppLL). The F₁ hybrid was again heterozygous (PpLl) purple long. When this hybrid was back crossed with recessive parent (ppll), the F₂ ratio was 1 purple long to 7 purple round to 7 red long to 1 red round.

Now it is clear that gametes PI and pL appeared seven times more than PL and pl gametes (This is just opposite to the previous experiment). This tendency of unlike pairs (Pl and pL) to remain together and to avoid union with their dominant and recessive partners is termed as Repulsion.

In fact, the nature of coupling and repulsion was not completely understood by Bateson and Punnett. No satisfactory explanation of coupling and repulsion was given until Morgan and his associates Muller, Bridges, Sturtevant and others discovered that coupling and repulsion are essentially two aspects or facts of the same phenomenon, linkage.

T.H. Morgan (1910) postulated by his extensive experiments on Drosophila that those genes which are located on the same chromosome are linked and pass together from generation to generation while those chromosomes on other hand become freely assorted or segregated during gametic formation.

Furthermore, Morgan advanced the basic idea that the degree of strength of linkage depends upon the distance between the linked genes in the chromosome. This proved very fruitful idea and led to the construction of genetic or linkage maps of chromosomes. This tendency of genes to live together in the same chromosome during hereditary transmission is called linkage.

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Linkage is the tendency of two or more genes to transmit together in the same gamete due to which the parental types are obtained in greater than expected in the progeny

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The tendency of genes inherited in groups is known as linkage.

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Linkage is the consequence of the concerned genes being located in the same chromosome. Linked genes do not show independent segregation, as a consequence, the ratios found in F₂ and test cross generations are significantly different from the expected ratios of 9:3:3:1 and 1:1:1:1, respectively in the case of two linked genes.

This effect of linkage is more clearly noticeable in a test cross generation. The frequency of parental characters combination are clearly more than expected while those of new or non-parental character combinations are lower.

Linkage in Maize:

A good example is the results obtained by Hutchinson who crossed a variety of maize having seeds that were coloured and full to one with colourless and shrunken seeds.

In other experiments it had been shown that colour gene “C” was a simple dominant over colourless ‘c’ where as full endosperm, gene ‘S’ is dominant over shrunken ‘s’. Accordingly the parents were CCSS and ccss and the F₁ as expected, had coloured full seeds that must have the genotype CcSs.

If C and S assort (segregate) independently in accordance with Mendel’s second principle, these plants should produce four types of gametes CS, Cs, cS, cs in equal numbers.

The easiest way to test this gametic ratio is to make a test cross of F₁ to the double recessive cross which according to the expectation stated above, would produce four classes of progeny in the ratio of 1:1:1:1. When the cross was made, however, this expectation was not realized, but the following result was obtained.

Coloured full CS/cs = 4,032

Coloured shrunken Cs/cs = 149

Colourless full cS/cs = 152

Colourless shrunken cs/cs = 4,035

Total = 8,368

In the experiment shown, the situation is as follows:

Parental combination CS = 4,032

cs = 4,035/8,067 or about 96.4% of the total

Recombination’s CS = 149

cs = 152/301 or about 3.6% of the total

It is clear that the two pair of genes C-c and S-s have not assorted independently. The parental combinations greatly exceed the expected 50%; they remain combined or linked in 96.4% and are recombined in only 3.6% of the gametes.

When another experiment was so arranged that the same genes entered the cross in different associations, that is, when parents with colourless full seeds (ccSS) were crossed with those coloured shrunken (CCss) seeds, it was found that again the parental combination were in excess, although now these parental combinations are just the opposite of what they were in the first experiment.

The results of the second experiment were as follows:

Here the parental combinations are 21,379+21,906 = 43,285 or 97.06 percent of all. Where as the recombination’s, cross-over type or non-parental combinations are 638+672 = 1,310 or 2.94 percent of the total.

The ratio between parental combinations and recombination’s is only slightly lower than it was in the first experiment. It is now clear that whatever the parental combinations of two different pairs of linked genes may be, linkage tends to keep them together in about the same proportion of the gametes of the double heterozygotes.

Linkage in Drosophila:

If an ordinary Drosophila fly with grey body and long wings (BBW) is crossed with another Drosophila having the two recessive characters of black body and vestigial wings (bbyy), the F₁ di-hybrids are like the wild parent.

If now a male fly from the F₁ generation is back crossed with a double recessive black vestigial female, there should be, as a result of independent assortment, four kinds of offsprings in equal numbers e.g., Grey long; Grey vestigial. Black long and Black vestigial.

The actual experiment, however, showed only two types of offsprings which resembled the two grand parents. This suggests that the genes for grey body and long wings and black body and vestigial wings are linked together.

This can be represented diagrammatically as below:

In the above case the two classes of F₂ generation individuals were like the parents, i.e., characters responsible for grey body and long wings in Drosophila female and these responsible for black body and vestigial wings in male fly remain unchanged during the process of un-heritance. Therefore, genes for parental characters remained linked in their offsprings showing phenomenon of linkage.

It points out that linkage brings similarity between parents and offspring. It is a strict restriction in the production of variability among organisms.

Linkage Groups:

The genes found in one chromosome are usually linked to particular associations. The group of genes which exist in the same chromosome or the string of genes on a chromosome is known as linkage group. The number of linkage groups is proportional to the haploid number of chromosomes.

Thus in Drosophila, there are four linkage groups corresponding to the four chromosomes. Each linkage group contains many genes responsible for particular characters. Two linkage groups in Drosophila are very long, one is too small and another one is intermediate in size.

In maize there are 10 linkage groups (n = 10) and in barley, there are 7 linkage groups (n = 7). In cereal plants and animals all the linkage groups have not been discovered e.g., in mouse 16 linkage groups for 20 pairs of chromosomes and in rabbit 11 linkage groups for 22 pairs of chromosomes are known so far.

Chromosome Theory of Linkage:

Castle, Morgan with his associates have formulated chromosome theory of linkage as follows:

1— The genes showing linkage are situated in the same pair of chromosomes. The substance of chromosomes bind these linked genes together during the process of inheritance.

2— The distance between the linked genes determines their effective tendency of linkage. Closely related genes show strong linkage while genes widely located show weak or poor linkage.

3— The genes are arranged in linear fashion inside the chromosome.

Kinds of Linkage:

It is of two types:

1. Complete linkage:

Complete linkage is the phenomenon in which two or more parental characters are inherited together and uniformly appear through two or more generations. In complete linkage genes are closely associated and tend to transmit together. This is due to the fact that there is no break in the male genes combinations in the chromosomes. It is found in male Drosophila.

It is clear from the above cross that there is no breakage of chromosomal segments. Because of this reason in male Drosophila only two types of gametes are formed BV and by which on mating with gamete; ‘bv’ produces only parental type progenies like grey long ‘BbVv’ and black vestigial ‘bbvv’ in F₂ generation.

2. Incomplete linkage:

It involves the accidental or occasional breakage of chromosomal segments. This arrangement is called crossing over, It was first of all noticed by Morgan in white eyed and miniature winged Drosophila flies.

In cross between grey long (BBW) X black vestigial (bbvv), the F₁ hybrid is grey long (BbVv). The F₁ hybrid is female and produces gametes of four kinds. Two gametes will show the linked gene and no chromosomal change (non-crossovers) and other two will show chromosomal interchange. Thus, non-cross over gametes formed without chromosomal interchange are in 82% ratio and cross-over gametes formed as a result of crossing over are 18%.

If we cross the F₁ hybrid to a black vestigial male (Bb Vv X bbvv ), the F₂ will show 82%’ offspring of parental ratio i.e., 41% grey long and 41% black vestigial and 18% offspring of new cross-over ratio, 9% grey vestigial and 9% black long.

Theories of Linkage:

(i) Theory of differential multiplication:

This theory was proposed by Bateson (1930) to explain the phenomenon of linkage. According to this theory, the set of gametes possessing parental combinations multiply more rapidly than the set having non- parental combinations after the segregation of characters during gamete formation. This results in the formation of a greater number of gametes with parental combinations.

This theory has no cytological basis hence has been condemned by other cytologists. We know from our knowledge of gametogenesis, that after segregation, only a single division comes before the gametes are formed. Moreover the theoretical results do not agree with the statistics obtained.

(ii) Chromosomal theory of linkage:

This theory was proposed by Castle and Morgan. They claimed that genes situated in the same chromosomes are inherited together i.e., they are linked while those located in different chromosomes are inherited freely or independently i.e., they are not linked. The extent of linkage is correlated with the distance between the genes in the chromosomes— closer the genes, stronger the linkage & vice- versa.

The linkage of genes, according to this theory, is linear. An important distinguishing feature is that genes are located in the chromosomes longitudinally in linear fashion. The chromosome theory of linkage is well supported by other cytologists and is widely accepted.