LINKAGE AND CHROMOSOMAL MAPPING

BY: RAHUL ANDHARIA (MSIWM001)

Introduction (Linkage):

  • Genes may be present on the same chromosome or different chromosome. Many genes are present one chromosome.
  • Characters controlled by genes express in next generation, when the genes are present on different chromosome. Here, we can say they assort independently as per the law of independent assortment given by Mendel.
  • Genes tend to inherit, if they are present on the same chromosome and are close to each other.
  • Thus, linkage can be defined as coexistence of 2 or more genes on the same chromosome.
  • Extensive work carried out in drosophila led to the discovery of term Linkage, which was given by Thomas Hunt Morgan.

On a given chromosome, if all the genes are located Physically, than it is called as Linkage group.

Characteristic of Linked Genes:

  • 1:1:1:1 test cross ratio is observed when genes assort independently on different chromosomes.
  • Linked genes remain in the same combination as they are in parents, and do not assort independently.

Chromosome Theory of Linkage:

  • It was given by Castle and Morgan.
  • During the inheritance process, linked genes remain attached to chromosomal material and are present on the same chromosome.
  • Strength of linkage is determined by the distance between linked genes. More stronger linkage is exhibited by closely related genes when compared to widely located genes, which has weaker Linkage.
  • In a chromosome, genes are arranged in linear fashion.

Types of linkage:

Basically two types of linkages are found based on works in drosophila by Morgan and co-workers.

  1. Complete Linkage:
  2. In this linkage, parental characters appears together for more than two generations in a continuous fashion.
  3. Genes transmit together and remain close to each other in this type of linkage.

Example:

  • Fourth chromosome mutant of Drosophila Melanogaster, exhibits complete linkage carrying genes for bent wings (bt) and shaven bristles (svn).
    • Between 2gene pair, there is absence of independent assortment indicating very strong complete linkage.
  • Incomplete Linkage:
  • Widely located linked genes on chromosomes which are capable of crossing over are called as incomplete genes and the pattern of their inheritance is termed as incomplete linkage.
  • As homologous non-sister chromatids(one part of chromosome) exchange varied length fragments during meiotic phase, the linked genes do not always stay together.

+ExampleThe phenomenon is observed in maize, female drosophila, tomatoes, pea, mice, man and poultry. Incomplete linkage in Maize, was studied by Hutchinson, where he observed alleles for incomplete linkage between colour and shape of the seeds.

 Chromosomal Mapping:

  • Genetic maps: It refers to representing the relative distance between linked genes in a diagrammatic manner.
  • It is also called crossing-over map, as it is the outcome of crossover.
  • Chromosome mapping is construction of genetic maps for different chromosomes.

Steps involved in construction of chromosome maps:

  • Crossing over frequency between 2 genes is directly proportional to the distance between them on the chromosome.
  • One unit map distance between genes(1%frequency of Crossing over between 2genes) is called as Centimorgan.
  • Step 1: Conduct hybridization experiments(among wild and mutants) to know number of genes and chromosomes of a particular species.
  • Step2: Determine the relative distance between linked genes  after knowing the linkage groups.
  • Step3: calculate 2gene distance based on the percentage of crossing over.(crossing over directly proportional to gene distance).
  • Example: Map distance between 2 linked genes is one centimorgan, if percentage of cross over between 2 linked gene is 1 percent.

Hypothetical Construction of chromosomal map:

  • Assume that there are 3types of genes: A, B and gene C.
  • Say 10% is the cross over percentage between A and B, they can be plotted on a linear scale.
  • 3% say, is the cross over percentage between B and C, than cross percentage between A and C will determine position of C. It can be away from A or 3units from B.(It can be interpreted by crossing A and C).

Now suppose say, 13% is the cross over percentage between A and C, than C will be plotted 3centimorgans right side to that of gene B.

  •  
  • Formula to determine Recombination frequency:

It can be given as: (Recombination frequency= Total no of recombinants/ Total no Progeny)

Significance of mapping Chromosome:

  • Exact location of genes in a Chromosome can be determined.
  • Approximate distance and correct order between the genes can be obtained and is useful for Genetic studies.
  • Chances of crossing over between genes and their linkage can be studied and obtained using mapping technique.
  • To identify in which loci of chromosomes exactly genes are present.
  • Used in genetic manipulation studies.
  • This chromosomal mapping proves beneficial in Autosomal Dominant testing, which helps to understand the ancestor history by knowing which DNA segments came from which ancestor.

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