LINKAGE�COUPLING AND REPULSION HYPOTHESIS�CHROMOSOMAL THEORY OF LINKAGE

Mr. Ravi Kumar

Department of Zoology�Hans Raj Mahila Maha Vidyalaya�Jalandhar (Punjab)

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 Coupling

• The linkage between two or more either dominant (AB) or recessive (ab) alleles is referred to as coupling.
• A good example of coupling was reported by Hutchinson in maize for the genes governing colour of seed (coloured and colourless) and shape of seed (full and shrunken).
• The coloured seed is governed by dominant gene (C) and full seed is also governed by dominant gene (S).
• He made cross between plants having coloured full seeds (CCSS) and colourless shrunken seeds (ccss).
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Coupling

• The F₁ seeds were colored full.
• When the F₁ was test crossed with double recessive parent the following results were obtained instead of 1 : 1 : 1 : 1 ratio.
• This indicates that parental combinations are higher than recombination’s, indicating presence of linkage.
• The parental combinations occurred in 96.4% instead of 50% and re-combinations were 3.6% instead of 50% in this case. There are several other cases of coupling in other plant species.

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Repulsion

• The linkage of dominant allele with that of the recessive allele (Ab or aB) is known as repulsion.
• Hutchinson also observed repulsion phase of linkage in maize.
• He observed this type of linkage when he made cross between plants having colored shrunken seeds (Cs) with those having colorless full seeds (cS).

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Repulsion

• In F₁ the seeds were colored full.
• By crossing of F₁ with double recessive parent the following results were obtained instead of 1 : 1 : 1 : 1 ratio.
• Again parental combinations were higher (97.1%) than expected (50%) and recombination’s were lower (2.9%) than expected (50%).
• Thus in both cases linked genes tend to remain together during hereditary transmission.
• Haldane (1942) used the terms cis and trans for coupling and repulsion, respectively.

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Coupling and Repulsion hypothesis

• When two genes are close together on the same chromosome pair, they do not assort independently.
• In other words, coupling refers to the linkage of two dominant or two recessive alleles, whereas repulsion indicates that dominant alleles are linked with recessive alleles.

Coupling and Repulsion hypothesis

• Bateson and Punett in 1906, described a cross in sweat pea, where failure of gene pairs to assort independently was exhibited.
• Plants of a sweat pea variety having blue flower (BB) and long pollen (LL) were crossed with those of ano­ther variety having red flower (bb) and round pollen (II).
• F₁ individuals (BbLl) had blue flower and long pollen.
• These were test crossed with plants having red flower and round pollen (bbll).

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Coupling and Repulsion hypothesis

• In this case, the character for blue color of flower is dominant over red color, and long pollen character is dominant over round pollen.
• In case of independent assortment, one should expect 1:1:1:1 ratio in test cross.
• Instead 7:1:1:7 ratio was actually obtained, indicating that there was a tendency of dominant alleles to remain together.
• The case was similar for recessive alleles also.
• This deviation was explained in terms of gametic coupling by Bateson.

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Coupling and Repulsion hypothesis

• It was also observed that when two such dominant alleles or recessive alleles come from different parents, they tend to remain separate.
• This was termed as gametic repulsion.
• In Bateson’s experiment, in repulsion phase, one parent would have blue flower and round pollen (BBII) and the other would have red flower and long pollen (bbLL).
• The results of test cross in such a repulsion phase were similar to those obtained in coupling phase, giving 1:7:7:1 ratio instead of expected 1:1:1:1

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Chromosomal Theory of linkage

• The speculation that chromosomes might be the key to understanding heredity led several scientists to examine Mendel’s publications and re-evaluate his model in terms of the behavior of chromosomes during mitosis and meiosis.
• In 1902, Theodor Boveri observed that proper embryonic development of sea urchins does not occur unless chromosomes are present.

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INTRODUCTION

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• The number of genes in an organism far exceeds the number of chromosome.
• Gene tally for Daphnia pulex, around 31,000 genes, stack up against other organisms?
• Humans have about 23,000 genes close in number to mice,roundworms, and a small flowering plant called Arabidopsis thaliana.
• Fruit flies have about 14,000 genes, yeast has about 6,000, and E. coli bacteria about 3,200.Zebrafish has more than 5000 genes
• A single chromosome bears several genes.

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Features of Linkage of Genes

• Linkage involves two or more genes which are located in the same chromosome in a linear fashion.
• Linkage may involve either dominant genes or recessive genes or some dominant and some recessive genes.
• Linkage usually involves those genes which are located closely.
• Presence of linkage leads to higher frequency of parental types than recombinants in a test cross progeny. When two genes are linked the segregation ratio of a test cross progeny deviates significantly from the 1 : 1 : 1 : 1 ratio.
• Linkage may involve either two or more desirable traits or all undesirable traits or some desirable and some undesirable traits.

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Features of Linkage of Genes

• Linkage is observed for both oligogenic traits as well as polygenic traits. However, it is more common for the former than latter.
• Besides pleiotropy, linkage is an important cause of genetic correlation between various plant characters.
• If crossing over does not occur, all the genes located in one chromosome are expected to be inherited together. Thus the maximum number of linkage groups in an organism is equal to its haploid chromosome number.
• Linkage can be broken by repeated inter-mating of randomly selected plants in segregating populations for several generations.

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�LINKED GENES

• Genes present at the same locus have the tendency to be linked together from one generation to the other and are not disturbed by the meiotic recombination.
• Linkage is defined as “the tendency of genes to remain together during the process of inheritance”.
• Extent of linkage - closer the genes, stronger the linkage and vice versa.
• Linkage of genes is in the linear fashion in the chromosomes.

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LINKAGE �

• The genes that show linkage are located on the same chromosome.
• A linkage group is formed by all the linked genes in a chromosome.
• The strength of linkage between two genes is directly proportional to the distance between them.

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LINKAGE

Chromosomal Theory of linkage

• That same year, Walter Sutton observed the separation of chromosomes into daughter cells during meiosis .
• Together, these observations led to the development of the Chromosomal Theory of Inheritance, which identified chromosomes as the genetic material responsible for Mendelian inheritance.

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Chromosomal Theory of linkage

• Morgan formulated the chromosome theory of linkage according to which the distance between the linked genes in the chromosome determines the strength of linkage.
• The closer the genes are located, stronger is the linkage. 
• Linkage is of two types – Complete and Incomplete linkage

Complete Linkage�

• The complete linkage is the phenomenon in which parental combinations of characters appear together for two or more generations in a continuous and regular fashion.
• In this type of linkage genes are closely associated and tend to transmit together.

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Example

• The genes for bent wings (bt) and shaven bristles (svn) of the fourth chromosome mutant of Drosophila melanogaster exhibit complete linkage.
• In 1919, T.H. Morgan mated gray bodied and vestigial winged (b+vg/b+vg) fruit flies with flies having black bodies and normal wings (bvg+/bvg+).
• F1 progeny had gray bodies and normal long wings (b+vg/bvg+), indicating thereby that these characters are dominant.
• When F1 males (b+vg/bvg+), were backcrossed (i.e., test crossed) to double recessive females (bvg/bvg or black vestigial), only two types of progeny (one with gray bodies and vestigial wings, b+vg/bvg and the other with black bodies and normal wings, to bvg+/bvg instead of four types of phenotypes were obtained

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Incomplete Linkage

• The linked genes which are widely located in chromosomes and have chances of separation by crossing over are called incompletely linked genes and the phenomenon of their inheritance is called incomplete linkage.
• The linked genes do not always stay together because homologous non-sister chromatids may exchange segments of varying length with one another during meiotic prophase.

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Incomplete Linkage

• This sort of exchange of chromosomal segments in between homologous chromosomes is known as crossing over.
• Example. The incomplete linkage has been reported in female Drosophila and various other organisms such as tomato, maize, pea, mice, poultry and man, etc.

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Incomplete Linkage in maize

• In Zea mays (maize) a case of incomplete linkage between the
• alleles for colour and shape of the seed has been observed by Hutchison.
• When a maize plant with seeds having colored and full endosperm (CS/CS) is crossed with another plant having recessive alleles for colorless, shrunken seeds (cs/cs), the F1 heterozygotes are found with the phenotype of coloured full and genotype of CS/cs.
• When F1 hybrid is test crossed with double recessive parent (cs/cs) four classes of descendants are obtained instead of two

Incomplete Linkage in maize

• The test cross results are clearly showing that parental combination of alleles (e.g., CS/cs and cs/ cs) are those expected from complete linkage and appear in 96% cases, the other two are new combinations (e.g., Cs/cs and cS/cs) and appear in 4% cases.
• Thus, in 4% cases crossing over has occurred between linked genes.
• Significance of Linkage
• The phenomenon of linkage has one of the great significance for the living organisms in that it reduces the possibility of variability in gametes unless crossing over occurs.

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• The F1 were heterozygous for blue flowers and long pollen grains (BbLl).
• When the F1 hybrid was test crossed with a double recessive parent (bbll), instead of the normal 1:1:1:1, an unexpected phenotypic ratio of 7:1:1:7 was obtained 
• When independent assortment takes place, the frequency of the four F₂ phenotypes is 25% each, with the parental types equal to 50% and the recombinants equal to 50%.
• In Sweet pea, the parental types accounted to 87.4% and the recombinant to 12.6%.
• This shows that the genes do not assort independently and that they are linked.

Synapsis

• The pairing of homologous chromosomes during prophase of meiosis I is called synapsis.
• Synaptic forces attract the homologous chromosomes and bring them close together.
• The paired homologous chromosomes are called bivalents.

Tetrad Formation

• The chromatids of each bivalent slightly separate and become visible and this group of four chromatids is called a tetrad Crossing over occurs in the four stranded stage.

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