Tag: linkage and crossing over

A.cis – arrangement - two dominant alleles linked on one chromosome is called cis arrangement of genes.

B.Trans arrangement – two dominant alleles present on different chromosomes is called trans arrangement of genes

C.Partly cis partly trans – dominant alleles partly present on same chromosomes and partly present on different chromosomes.

So, the correct option is ‘cis arrangement’.

Continuous phenotypic variation, where traits show a smooth gradient of phenotypes like height or skin color, is typically controlled by multiple genes working together, known as polygenic inheritance.

The garden pea, Pisum sativum was the model system used by Gregor Mendel to formulate his famous Mendel's laws. But the characters used by Mendel have been since shown to be controlled by genes belonging to different linkage groups or to distant parts of the same linkage group. Thus, Mendel failed to observe the phenomenon of linkage. Instead, linkage was first described and interpreted by Thomas Morgan in his works with Drosophila. Linkage in pea was first described in 1912 by Vilmorin and Bateson. Thus if the history of pea genetics begins with the Gregor Mendel, the history of the pea linkage map begins much later. There are seven linkage groups in pea; present on chromosomes 1, 6, 5, 7, 3, 2 and 4 respectively. So, the correct answer is '7'.

Linkage: Tendency of the genes located on the same chromosome, to stay together in hereditary transmission is known as linkage. The genes located on the same chromosome are called as linked genes. The principle of linkage was discovered by Bateson and Punnet in 1906 in the sweat pea plant, Lathyrus odoratus. However, linkage as a concept was put forth by Thomas Hunt Morgan in 1910 based on his experiment on Drosophila melanogaster. It has 4 linkage groups.

Bateson explained the lack of independent assortment by means of a hypothesis known as coupling and repulsion hypothesis. Although coupling and repulsion were discovered later to be the aspects of same phenomenon. It is called as linkage, the terms coupling phase and repulsion phase are still considered to be useful terms in scientific literature. Bateson and Punnet proposed that the F$ _{1}$ had actually produced more dominant and recessive gametes in sweet pea cross than would be produced by Mendelian independent assortment. Because these genotypes were the gametic types in the original pure lines, the researchers thought that physical coupling between the dominant alleles P and L and between the recessive alleles p and l might have prevented their independent assortment in the F$ _{1}$. However, they did not know what the nature of this coupling could be.

Mendel studied 7 characters in pea plant. The genes for these characters were located on 4 chromosomes. Mendel was lucky enough as none of the genes showed linkage and assorted independently. This is because the genes studied were located far apart on the same chromosome and thus, expressed independently. 

Genetic linkage is the tendency of alleles that are located close together on a chromosome to be inherited together during meiosis. Genes whose loci are nearer to each other are less likely to be separated onto different chromatids during chromosomal crossover, and are therefore said to be genetically linked. In other words, the nearer two genes are on a chromosome, the lower is the chance of a swap occurring between them, and the more likely they are to be inherited together. Linkage of genes in animals and plants is seen as deviations from independent segregation; recombination seen as deviation from complete linkage.
More linkage between genes, lesser will be the recombination frequency.

Genetic linkage is the tendency of alleles that are located close together on a chromosome to be inherited together during meiosis. Genes whose loci are nearer to each other are less likely to be separated onto different chromatids during chromosomal crossover, and are therefore said to be genetically linked. In other words, the nearer two genes are on a chromosome, the lower is the chance of a swap occurring between them, and the more likely they are to be inherited together.

Linkage is referred to the phenomenon wherein alleles located close together on a chromosome show tendency to be inherited together during meiosis. Genes present near to each other have less probability of being separated onto different chromatids during chromosomal crossover. Therefore, when a cluster of genes show linkage behaviour they do not show independent assortment. Genes showing linkage behaviour do not assort independently and hence, do not show recombination during meiosis. Linkage map shows relative position of genes on the chromosomes based on recombination frequency between them; the map show both linked and independently assorted genes. 

When the genes are present on the same chromosomes they are said to be linked. When gene remains together for two or more generation they are said to have complete or perfect linkage. Thus the bond between them becomes strong. Thus, the correct answer is option A.