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| Linkage and Crossing Over
Easily Learn Crossing Over and Linkage 1. Why is not Mendel’s second law always valid for two or more phenotypical traits of an individual? Mendel’s second law, or the law of the independent assortment, is valid for genes located in different chromosomes. These genes during meiosis segregate independently. Mendel’s second law however is not valid for phenotypical features conditioned by genes located in the same chromosome (genes under linkage), since these genes, known as linked genes, do not separate in meiosis (except for the phenomenon of crossing over). Image Diversity: genetic linkage 2. Why is drosophila a convenient animal for the study of linked genes? The fruit fly drosophila is suitable for the study of Genetics because it presents many distinct traits but only four chromosomes (one sex chromosome and three autosomes). Image Diversity: drosophila karyotype 3. What is linkage? Two genes are said to be under linkage, or linked, when they reside in the same chromosome. For example, the research of the human genome discovered that the factor III of clotting gene and the factor V of clotting gene are located in the same chromosome (the human chromosome 1). The factor VII gene however is not linked to those genes since it is located in the chromosome 13. 4. What is crossing over? How is meiosis related to this phenomenon? Linked alleles, for example, A-b and a-B, form the gametes A-b and a-B that maintain the linkage of the alleles. This type of linkage is called complete linkage. In the first division of meiosis (meiosis I) however the crossing over phenomenon may occur. Chromosomes from a pair of homologous may exchange extremities and some once linked alleles, for example, A-b and a-B, recombine to form different gametes, in the case, A-B and a-b. Crossing over may happen when the arms of the chromatids of each homologous are paired during meiosis. Matching portions of the extremities of two nonsister chromatids (one from one homologous of the pair) break and the pieces are exchanged, each of them becoming part of the arm of the other chromatid. For example, if the allele A is situated in a side of the arm relating to the point of breaking and the allele b is located in the other side, they will be separated and gametes A-B and a-b will be formed, instead of A-b and a-B. (The percentage of recombinant gametes relating to normal gametes depends upon the crossing over rate that in its turn depends upon how far distant the given alleles are in the chromosome.) Image Diversity: crossing over 5. In genetic recombination by crossing over what is the difference between parental gametes and recombinant gametes? Parental gametes are those gametes that maintain the original linkage of genes (alleles) in the chromosome. Recombinant gametes are those in which the original linkage is undone due to exchange of chromosomal pieces by crossing over during meiosis. 6. What is recombination frequency? Recombination frequency, or crossing over rate, is the percentage of recombinant gametes made by crossing over (in relation to the number of parental gametes made). It always refers to two genes located in the same chromosome. Image Diversity: recombination frequency 7. Why does the recombination frequency of genes vary with the distance between them in the chromosome? The farther the distance between the loci of two genes in a chromosome the higher the recombination frequency between these genes. This is true because once alleles are nearer in the chromosome it is more probable that they are kept united when chromosomal extremities are exchanged by crossing over. On the other hand, if they are farther apart it will be easier for them to separate by crossing over. 8. What is a centimorgan? Centimorgan, or recombination unit, by convention is a distance between two linked genes that corresponds to 1% of recombination frequency of these genes. 9. How can the concept of recombination frequency be used in genetic mapping? Genetic mapping is the determination of the location of the genes in a chromosome. By determining the recombination frequency between several different linked genes it is possible to estimate the distance between them in the chromosome. For example, if a gene A has a recombination frequency of 20% with the gene B, this gene B has recombination frequency of 5% with the gene C and this gene C has recombination frequency of 15% with the gene A, it is possible to assert that the gene A is 20 centimorgans distant from the gene B and that between them lies the gene C at 15 centimorgans of distance from the gene A. Image Diversity: genetic mapping 10. Is crossing over important for the diversity of biological evolution? Sexual reproduction and recombination of linked genes (crossing over) are, along with mutations, the main instruments of biological variability. Sexual reproduction allows many combinations between genes situated in different chromosomes. Crossing over, however, is the only means to provide recombination of alleles located in a same chromosome. Crossing over probably emerged and has been maintained by the evolution because of its importance to biological diversity. |