Concept of Linkage

Linkage

Linkage (Exception to Independent Assortment)

Linkage is the phenomenon of certain genes staying together during inheritance though generations without any change or separation due to their beign present on the same chromosomes. Linkage was first sugested by Sutton and Boveri (1902-1903) when they propoundd the famous “chromosomal theory of inheritance. “Bateson and Punnet (1906), while working on Sweet Pea found that the factors for certain characters do not show independent assortment. They then argued that since Sweet Pea has only a few pairs of homologous chromosomes, a chromosome some must possess several factors which should be transmitted together. It was Morgan (1910) who clearly proved and defined linkage on the basis of his breeding experiments in fruitfly Drosophila melanogaster Morgan’s famous chromosomal theory of linkage states that:-

  • Linked genes occur in the same chromosome.
  • They lie in a linear sequence in the chromosomes.
  • There is a tendency to maintain the parental combination of genes except for occasional crossovers.
  • Strength of the linkage between two genes is inversely proportional to the distance between the two i.e. two linked genes show higher frequency of crossing over if the distance between them is higher and lower frequency if the distance is small.

Linked genes are those genes which occur on the same chromosome while unlinked genes are the ones found on different chromosomes. Linked and unlinked genes can be easly known from breeding experiments. Unlinked genes show independent assortment, a dihybrid ratio of 9:3:3:1 and the two point test cross ratio of 1:1:1:1 with two parental and two recombinant types. The linked genes do not show independent assortment. The dihybrid ratio is only 3:1 and a test cross ratio of 1:1. Linkage is of two types, complete and incomplete.

  1. Complete Linkage (Morgan, 1919)-

    The genes located in the same chromosome do not separate and are inherited together over the generations due to the absence of crossing over. Complete linkage allows the combination of parental traits to be inherited as such. It is rare but has been reported in male Drosophila and some other heterogametic individuals.

Example- A red eyed normal winged (wild type) pure breeding female Drosophila is crossed to homozygous recessive purple eyed and vestigial winged male. The progeny of F1 males are test crossed to homozygous recessive female (purple eyed and vestigial winged), only two types of individuals are produced-red eyed normal winged and purple eye vestigial winged in the ratio of 1:1 Recombinant types are absent.

  1. Incomplete Linkage-

    Genes present in the same chromosome have a tendency to separate due to crossing over and hence produce recombinant progeny besides the parental type. The number of recombinant individuals is usually less than the number expected in independent assort most. In independent assortment all the four types (two parental types and two rcombinant types) are each 25%. In case of linkage, each of the two parental types is more than 25% while each of the recombinant types is less than 25%.

Example 1. A red eyed normal winged or wild type dominant homozygous female Drosophila is crossed to homozygous recessive purple eyed and vestigial winged male. The progeny of F1 individuals are hetozygous red eyed and normal winged. F1 female flies are test crossed with homozygous recessive males. It does not yield the ratio of 1:1:1:1 instead the ratio comes out to be 9:1:1:8.

Only 9.3% recombinant types were observed which is quite different from 50% recombinants incase of independent assortment. This shows that in the oocytes of the F1 generation only some of the chromatids undergo cross-over while the majority is preserved intact. This produced 90.7% parental types in the progeny.

Example 2- In Sweet Pea (Lathyrus odoratus) blue flower colour (B) is dominant over red flower colour (b) while the trait of long pollen (L) is dominant over round pollen (1). A sweet pea plant heterozygous for both blue flower colour and long pollen (BbLl). was crossed with double recessive red flowered plant with round pollen pollen (bbll), It is similar to test cross. In case the genes of the two traits are unlinked, the progeny should have four phenotypes (Blue Long, Blue Round Red Long. Red Round) in the ratio of 1:1:1:1 (25% each). In case the two genes are completely linked the progeny should in the ratio of 1:1:1:1 (25% each). In case the two genes are completely linked the progeny should have both the parental types (Blue Long and Red Round) in the ratio of 1:1 (50% each). Recombinants should not appear. However, in the above cross Bateson and Punnet (1906) found both parental and recombinant types but with different frequencies.

Only 12.6% recombinant types were observed against the exact percentage of 50% in case of independent assortment. Therefore, the genes are linked but undergo recombination due to cross-over in some of the cases.

Linkage Groups

A linkage group is a linearly arranged group of linked genes which are normally inherited together except for crossing over. It corresponds to a chromosome which hears a linear sequence of genes linked and inherited together. Because the two homologous chromosomes possess either similar or linkage group. Therefore the number of linkage groups present in an individual corresponds to the number of chromosomes in its one genome (all the chromosomes of haploid or homologous pairs of diploid).. It is known as principle of limitation of linkage groups. Fruitfly Drosophila melanogaster has four linkage groups (4 pairs of chromosomes), human beings 23 linkage groups (23 pairs of chromosomes Pea seven linkage groups (7 pairs of chromosomes), Neurospora 7 linkage groups (7 chromosome), Mucor 2 linkage groups (2 chromosomes while Maize has 10 linkage groups (10 pair of chromosomes). The size of the linkage group depends upon the size of chromosome. The smaller chromosome will naturally have smaller linkage group while a longer one has longer linkage group. This is subject to the amount of heterochromatin present in the chromosome. The Y-chromosome of man possesses over ten genes while X-chromosome has over 102 genes. Above 150 genes have been recorded in the X-chromosome of the Drosophila.

Importance of Linkage

  • The number of linkage groups is equivalent to number of chromosomes present in a genome. It proves that genes are present on the chromosomes.
  • Linkage prevents or reduces the incidence of recombination so that specific varietal or racial characters are retained over the generations.
  • It is highly useful for maintaining the good characters of the newly developed variety.
  • Linkage is a biggest headache for breeders because it does not allow the to freely bring all the desirable traits in one variety.
  • It dilutes the use of desirable out if undesirable ones are also present on the same linkage group; e.g., low gaining and naked seeds or fuzzy seeds and high gaining in American Cotton.
  • marker genes or genes which express their effect in early growth can indicate the effect of a linked gene which is to express lately wavy lamina and larger panicle in Milet.

Sex Linkage

Sex linkage or sex-linked inheritance is the transmission of characters and their determining genes alongwith sex determining genes while are borne on the sex chromosomes and, therefore, are inherited together from one generation to the next. Y-chromosome of the male carries very few genes along with TDF. The X-chromosome which is common to male and female carries a number of genes. It is estimated that in Drosophila melanogaster the X-chromosome bears about 150 genes while in human beings it is over 102 genes. All sex linked characters show criss-cross inheritance. It was discovered by Morgan (1910) when he studied the inheritance of red-white eye colour trait (locating genes on chromosomes. Two important sex-linked human diseases are haemophilia and colour blindness.

  • Sex Linked Traits- They are those traits the determining genes of which are found on the sex chromosomes. All the sex-linked traits present on a sex chromosome are inherited together.
  • Sex Limited Traits- They are traits which are expressed in a particular sex though their genes. It also occur in the other sex, e.g., milk secretion in mammalian females.
  • Sex Influenced Traits- The traits are not due to particular genes but are by-products of sex hormones, e.g., low pitched voice, beard, moustaches and baldness in human. The gene for baldness behaves as an autosomal dominant in males and autosomal recessive in females.

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