Linkage
A body possesses several genes & all genes gift within the body are heritable along.
Linkage: Study of inheritance of all genes present in an exceedingly body together. All genes in an exceedingly body are together referred as cistron & they form a cistron. The total range of cistron in an organism is adequate its haploid range of chromosomes.
Bateson & Punnet:
While functioning on sweet pea (Lathyrus odoratus) ascertained that genes for flower color form stay together and don’t assort independently in step with Mendel’s law of freelance assortment.
They gave the Coupling & Repulsion theory:
Coupling: once genes come back from a similar parent they enter a similar reproductive cell heritable along.
Repulsion: genes are inherited alone when they come from different parents & they enter various different gametes
T H Morgan’s Linkage:
He worked on Drosophila. Linked genes are genes present on a similar body and these genes are inherited together. Genes are linearly arranged in a chromosome.
Strength of linkage: Genes that are closely placed show robust linkage & genes that are placed so much show weak linkage. He explicit that Coupling & Repulsion are 2 aspects of Linkage. Linked genes show 2 types of arrangement:
Cis arrangement: dominant alleles of two or additional genes are present in one body & its recessive alleles in its homologue. AB/ab. This is called Coupling.
Trans arrangement: the dominant allele of one pair & recessive of the other pair together lie in a chromosome.Ab/aB. This is called Repulsion.
Linked genes are of 2 types:
Complete linkage: genes for two or additional characters seem along for 2 or additional generations in their parental combination. They are closely located in the chromosome.
Incomplete linkage: the parental combination of two or additional characters aren’t maintained within the next generations. They are not closely located in the chromosome.
Incomplete linkage occurs due to crossing over.
Morgan’s experiment:
Crossing over:
When non-sister chromatids and homologous chromosomes exchange segment between them it is known as crossing over. It occurs during pachytene stage of prophase I in meiosis. It also, always occurs between linked genes.It produces recombination of cistron that play vital role in evolution.Recombination frequency helps find out the gap between genes. Given by Sturtevant. It also helps within the construction of genetic maps of the chromosomes.
