Biological Sciences
Linked Genes
Linked genes are genes that are located close together on the same chromosome. They tend to be inherited together because they are physically linked and do not assort independently during meiosis. This linkage can be broken by crossing over, which can result in the exchange of genetic material between the linked genes.
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3 Key excerpts on "Linked Genes"
eBook - PDF- Gregory Carey(Author)
- 2002(Publication Date)
- SAGE Publications, Inc(Publisher)
The purpose of linkage analysis is to find the approximate location of a gene for a trait . Usually the trait is a disease, but in other circumstances the trait could be a continuously distributed variable like height or IQ scores. The importance of linkage analysis in modern human genetics cannot be underestimated. Searching through the whole 3.2 billion base pairs of the human genome for a few nucleotides that may cause a Mendelian disorder is akin to searching through the whole state of Iowa (the largest corn producer in the United States) for a specific abnormal corn plant or two. Linkage analy-sis allows us to narrow the search to a specific county or township in Iowa. Even though linkage analysis may not detect the specific corn plant(s), it saves a tremendous amount of time and effort in the search for the mutant plant(s). To understand linkage analysis, we must first have a firm knowledge of several basic terms. They are homologous chromosomes , recombination , haplotypes , marker genes, and the recombination fraction . Let us examine each in turn. Homologous Chromosomes We humans are diploid organisms. That means that we have two copies of every gene, except for those genes on the X and the Y chromosomes. 1 We inherit one gene from our mother and the other from our father. Because the chromosome (and not the gene) is the physical unit of inheritance, it is more appropriate to say that we inherit one maternal chromosome (that happens to have that gene on it) and one paternal chromosome (that also happens to have a sequence of DNA that codes for the same kind of polypeptide as the gene on the maternal chromosome). Those two chromosomes that have the same ordering of genes on them are termed homologous chromosomes and are illustrated in Figure 10.1. Homologous chromosomes have the same ordering of genes, but they do not always have the same alleles (i.e., DNA spelling variations) at the gene.
eBook - PDF- Gurnah, Akinloye(Authors)
- 2018(Publication Date)
- Agri Horti Press(Publisher)
Genes that lie close together are closely coupled, those that are far apart must lie farther apart on the same chromosome. The mechanism which is thought to account for coupling is the phenomenon of “crossing-over “. Sometimes chromosomal strands seem to touch at one or more places, and when the strands break apart, the top half of one strand may be attached to the bottom part of the other. A new pair of chromosomal strands originates, but if on both sides of a break there are loci where the strands differ in chromosomal content, those loci are redistributed, so that, as far as the independent inheritance of separate genes is concerned, crossing-over may help to redistribute the genes, even if they are located in the same chromosome. an important of Gene Mapping An important, though by no means an essential, step in genetic analysis is to produce genetic maps of the marker loci. Such maps are often referred to as ‘framework maps’ because they provide a framework within which important genes can be located, as well as providing a means of comparing chromosome organization in other closely or distantly related species. There are two stages to mapping. Firstly, to arrange the markers in a linear sequence separated by an appropriate map distance, i.e. to construct a linkage map. The second is to relate the linkage maps to particular recognizable chromosomes. The latter is often the most difficult and generally not essential for breeding or conservation work. We will, therefore, concentrate on the former. Chromosomes contain a single linear molecule of DNA and hence the markers on that chromosome occur at particular positions along that molecule. Typically, each chromosome contains 107 to 108 base pairs (bp), This ebook is exclusively for this university only. Cannot be resold/distributed. 138 Biochemistry of Plant Genetics i.e. 104 to 105 kilo-base pairs (kbp) of DNA, while a typical structural gene, coding for a polypeptide chain, would be between 1 and 2 kbp long.
eBook - PDF- Tomar, Arun Kumar(Authors)
- 2021(Publication Date)
- Daya Publishing House(Publisher)
The probability of crossing over between very closely linked loci is increased with higher numbers of progeny produced. Therefore, linkage is not important in farm animals compared to that in fruit fly. This is one of the problems associated with linkage studies of genes closely located. 5.7 CHROMOSOME MAPPING The genes are located on chromosomes in a linear arrangement having their fixed relative position on the chromosome. The genetic map (or chromosome map), known as chromosome mapping, is the graphical representation of genes on the chromosome indicating distance of genes between loci and their relative position from each other in each linkage group. The distance of genes indicates the map distance that how far the two genes are located on the chromosome while the relative position of genes indicates the gene order (sequence of genes) on the chromosome. The map distance (distance between genes) is represented in percentage of cross over (recombination) between loci along the chromosome. This is because the amount of crossing over, (percentage of crossover progeny produced), is proportional to the distance between two loci on a chromosome. This is based on the hypothesis formulated by Sturtevant (1913) that each gene has a definite position (locus) on its chromosome and the place for occurrence of chiasma (synapsis or pairing of chromosome for occurrence of crossing over) is a matter of chance and hence the frequency of crossover is proportional to the linear distance between genes on the chromosome. He thus suggested that the percentage of crossover can be used as a quantitative index of the gene distance on the chromosome. This helped in chromosome This ebook is exclusively for this university only. Cannot be resold/distributed. mapping. Thus, map distance is measured from the frequency of crossovers obtained either from test cross or F 2 data. The map distance is measured in map units (anti Morgan).
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