Linkage and crossing over

 Linkage groups A linkage group refers to a group of genes that are present in one chromosome. In other words, all those genes which are located in one chromosome constitute one linkage group. The number of linkage groups is limited in each individual. The maximum number of linkage groups is equal to the haploid (n) chromosome number of an organism. However, in the case of species having dissimilar sex chromosomes, linkage groups are one more than the haploid chromosome number (n+1). For example, there are ten linkage groups in corn, 7 in garden pea, 7 in barley, 4 in Drosophila melanogaster and 23 (22 somatic chromosomes + 1 X chromosome) in women, and 24 in man (22 somatic chromosomes + 1 X chromosome + 1 Y chromosome). Genes are assigned to various chromosomes of a species with the help of deletion, monosomic, and nullisomic analyses. Linkage groups are assigned to different chromosomes in a linear fashion and the same sequence as they normally list. Generally, a relative length...

 Linkage is generally classified on the basis of three criteria, viz: • Presence or absence of crossing over, • Genes involved, and • The chromosome involved. Based on crossing over (i) Complete linkage- Linkage in which crossing over does not occur is known as complete linkage or absolute linkage. In other words, when only parental types are obtained from the testcross progeny, it refers to complete linkage. A good example of complete linkage is the Drosophila male and female silk moth. (ii) Incomplete Linkage- If some frequency of crossing over also occurs between linked genes, it is known as incomplete linkage. To put it another way, when recombinations are also observed in the testcross progeny, besides parental combinations, it refers to incomplete linkage. Incomplete linkage has been observed in maize, pea, Drosophila female, and several other organisms. Based on genes involved (i) Coupling linkage- It refers to linkage either between dominant genes or between recessive genes...

 There are two phases of linkage, viz., coupling phase and repulsion phase. These phases were given by Bateson and Punett (1905), but they could not give a proper interpretation of these terms. Later on, Morgan (1910) based on his studies with Drosophila explained that coupling and repulsion are the two aspects of the same phenomenon what we call linkage. i. Coupling phase The linkage between two or more either dominant (AB) or recessive (ab) alleles is referred to as coupling. A good example of coupling was reported by Hutchinson in maize for the genes governing the color of seed (colored and colorless) and shape of seed (full and shrunken). The colored seed is governed by a dominant gene (C) and the full seed is also governed by a dominant gene (S). He made a cross between plants having colored full seeds (CCSS) and colorless shrunken seeds (ccss). The F1 seeds were colored full. When the F1 was test crossed with the double recessive parent the following results were obtained ins...

 Following are the characteristic features of linkage • The genes for linked traits are located on the same chromosome • Linked genes are located closely on the same chromosome • Strength of linkage depends on the distance between the genes, the lesser the distance more the linkage • The linkage may involve dominant-dominant, recessive-recessive or dominant-recessive genes • More parental type progenies are observed than recombinant type • Test cross progenies deviate from 1:1:1:1 • The inked genes may be of all desirable, all undesirable, or both desirable and undesirable • In addition to pleiotropy, the linkage is another cause for genetic correlation between various plant characters • Maximum number of linkage groups equal to haploid chromosome (n) number in homogametic individual and n+1 in case of heterogametic individual • Repeated intermating of randomly selected plants for generations breaks the linkage

 The tendency of two or more genes or traits to inherit together is called linkage. During the process of meiosis, genes that are present on the same chromosome tend to be inherited together as a unit, called linkage, but in some cases, alleles of the linked genes recombine by crossing over and the linkage is broken. Linked genes thus rarely ever assort independently unlike genes present on separate chromosomes. Linked genes travel together in the same gamete as they are not independent of each other. The behavior of such kind of genes are said to deviate from Mendel’s law of independent assortment. In the Mendelian experiment, the common sweet peas with two or more “elements” (now called genes) are considered heterozygous. In a hybrid, the segregation of anyone is completely independent of the rest. That means, all the genes are distributed to the gametes at random. Mendel considered the inheritance of seven characters and all the characters showed random assortment. It was later ...