Linkage Between Genes,Chromosomes,Morgan,Complete Linkage,Linkage,Genes are Arranged,Chromosome,Genes Located,Generation or Inheritance

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Linkage and Crossing Over Between Genes
Linkage Between Genes
Repulsion Phase of Genes

	Linkage Between Genes - After Sutton suggested the chromosomal theory of inheritance in 1903. evidence accumulated for the idea that genes were located on the chromosome. Morgan showed that there could be genes more than chromosomes. Sturtevant first showed that genes are arranged in a linear fashion on the chromosome. The concept of presence of genes on chromosomes is universally accepted. But genes present on single chromosome move together to the same pole in every meiosis step. As a consequence such genes would fail to show independent segregation.
But in real situation when a test cross is carried in pea plant (flower colour and pollen shape) the result showed 7: 1 : 1 : 7 ratio instead of 1 : 1 : 1 : 1 ratio as per Mendelian law of independent segregation. Bateson and Punnett observed these variations and were puzzled. Hence they named this peculiar behaviour as coupling and repulsion phase. But Morgan in 1911 proposed the new concept of linkage.
1.Genes are arranged on the chromosome in a linear fashion. 2.Genes located in the same chromosome tend to stay together during every generation or inheritance. 3.The distance between two genes decides whether they will be inherited together and if so with what frequency. Morgan proposed finally, the ability or tendency of two genes to stay together during inheritance is known as linkage. Coupling and repulsion phases are two aspects of linkage". If this always happened the Fl (UGg) would produce only two types of gametes (Le., LLGG long wings, grey bodies and llgg short wings, black bodies) in test progeny. When only combinations of parental characteristics are recovered in test cross progeny, it is called complete linkage.
However, linked genes do not always move to the same pole. Sometimes their alleles recombine to produce recombinant gamete Lg and 19. This yields the recombinant phenotype long winged black bodies (LLgg) and short winged grey bodies (llGG). Here the recombinant types are also recovered in the test cross progeny and are called incomplete linkage. For example, let us consider two characteristed coloured flowers and long leaves of a plant. The seeds from such plant are grown. If the plant with long leaf produces red coloured flower, the seed of Fl generations are grown again. The results are compared, Le., whether the plant with long leaves is producing red coloured flowers or not.
	If these two characteristics are present simultaneously together in every generation, then these genes are said to be linked, and this phenomenon or transmission pattern of the linked genes is called linkage. Linkage is also classified as coupling and repulsion phase linkage. If dominant alleles of the linked genes are present in the same chromosome, then it is referred to as coupling phase. In contrast, in repulsion phase linkage, the dominant allele of one gene is present with the recessive allele of the other gene in the same chromosome.

Two or more genes may stay together (during inheritance) because they are located in the same chromosome. Linked genes do not show independent segregation.

Hence the frequencies of parental character combinations are markedly more than expected, while those of recombinants or new character combinations are considerably low.