{"id":4805,"date":"2021-10-13T10:02:06","date_gmt":"2021-10-13T10:02:06","guid":{"rendered":"https:\/\/www.guyhowto.com\/?p=4805"},"modified":"2021-10-13T10:03:16","modified_gmt":"2021-10-13T10:03:16","slug":"linkage-maps","status":"publish","type":"post","link":"https:\/\/www.guyhowto.com\/linkage-maps\/","title":{"rendered":"Linkage Maps and Construction of Linkage Maps"},"content":{"rendered":"
What are Linkage Maps?<\/strong><\/h5>\n

A chromosome map of species that shows the position of its known genes or markers relative to each other, rather than as specific physical points on each chromosome is called a linkage map<\/strong>. It is different from a gene map.<\/p>\n

History of Linkage Map<\/strong><\/h5>\n
Thomas Hunt<\/strong><\/h6>\n

Thomas Hunt noticed that the amount of crossing over between linked genes is different. This gave rise to the idea that the frequency indicates the distance separating genes on the chromosome.<\/p>\n

Alfred Sturtevant<\/strong><\/h6>\n

Alfred Sturtevant, a student of Morgan developed the first genetic map, he called it linkage map. He proposed that the greater the distance between linked genes, the greater the chance of crossing over between non-sister chromatids. If the number of recombinants is measured, then the distance between the genes can be measured.<\/p>\n

Genetic Map Unit<\/strong><\/h5>\n

The distance is known as the genetic map unit. It is defined as:<\/p>\n

\u201cThe distance between the genes for which one product of meiosis in 100 is recombinant. A recombinant frequency of 1% is equivalent to 1 m.u. This can be called as genetic map unit or a Centimorgan\u201d.<\/p>\n

Construction of Linkage Map<\/strong><\/h5>\n

The linkage map is constructed by finding the map distances between a number of traits that are present on the same chromosome. The linkage map is based on the frequencies of recombination between markers during the crossing over of homologous chromosomes<\/a>. The greater the frequency of recombination between two genetic markers, the farther apart they are will be.<\/p>\n

Conversely, the higher the frequency of association between the markers, the smaller the physical distance between them. Previously, the markers generally used were detectable phenotypes derived from coding DNA sequences such as eye color, enzyme production, etc.<\/p>\n

Procedure of Constructing Linkage Map<\/strong><\/h5>\n
1. Frequency of Recombination<\/strong><\/h6>\n

Firstly, there is the measurement of the recombination frequency of different genes from the given data. The proportion of recombinant types between two genes as compared to the sum of all combinations is called cross-over or recombination frequency.<\/p>\n

\"Frequency-of-Recombin\"<\/p>\n

Two linked genes are backcrossed for calculating the recombination frequencies. The cross between the heterozygote to a homozygous double recessive is called the back cross.<\/p>\n

2. Proportional Recombination Frequency<\/strong><\/h6>\n

The recombination frequency is directly proportional to the distance between the linked gene loci. Genes can be mapped on a chromosome on the basis of their recombination frequencies.<\/p>\n

3. Measurement of Linked Gene<\/strong><\/h6>\n

The recombination frequencies of the entire linked gene are measured. These genes are arranged on chromosomes according to those recombination frequencies.<\/p>\n

Let us take an example:<\/strong><\/p>\n

In Drosophila, the fruit fly, ebony body color (b), and vestigial wings (vg) are linked. The F1 progeny backcrossed or say test crossed with the recessive parent. Following results are obtained:<\/p>\n

No. of recombinants = 206 + 185 = 391<\/p>\n

Total numbers of progeny = 2300<\/p><\/blockquote>\n

\"example\"<\/p>\n

\"Linked-Gene\"<\/p>\n

So, the gene map unit between ebony color and vestigial wing genes is 17 m.u. similarly, the gene frequency of different genes is calculated. These genes are arranged on chromosomes according to these frequencies.<\/p>\n

Gene Mapping with Three-Point Cross<\/strong><\/h5>\n

A three-point testcross gives the relative distance between these genes. It also gives the linear order of genes present on chromosomes. It is necessary to find out the presence of genes in the middle of three. By this, the gene will arrange linearly.<\/p>\n

Let us take an example:<\/strong><\/p>\n

Let suppose, we have three genes A, B, and C. Their alternative alleles are present on the other chromosomes are a, b and c. The heterozygous of these alleles is ABC\/abc. A test cross is arranged with this heterozygous individual by crossing ABC\/abc with abc\/abc. Recombination frequency between these can be found out.<\/p>\n

It will give map distance between these three genes and we can arrange these genes linearly. It will also provide us with information about the presence of gene present in the middle.<\/p>\n

Suppose gene w, y and m<\/strong> are in line. Then one of them must be in the middle of two. There is a greater map distance between y and m (35.4 m.u). This map distance is almost equal to the sum of y-w (0.5) and w-m distances (35.4 m.u). Thus, the linear arrangement of these genes is y, w and m. Here the position of w is in the middle of y and m.<\/p>\n

Purpose of Linkage Map<\/strong><\/h5>\n

Critical for identification of the location of genes that cause genetic diseases.<\/p>\n

In a normal population, genetic traits and markers will occur in all possible combinations. The frequencies of combinations are determined by the frequencies of individual genes.<\/p>\n

Helps researchers to locate other markers, such as other genes by testing for genetic linkage of already known markers.<\/p>\n

 <\/p>\n","protected":false},"excerpt":{"rendered":"

A chromosome map of species that shows the position of its known genes or markers relative to each other, rather than as specific physical points on each chromosome is called a linkage map. 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