03 Inheritance and Variation - part 08 - Linkage and Crossing Over

03 Inheritance and Variation - part 08 - Linkage and Crossing Over


Linkage and Crossing Over :

 Linkage : 

  • It is a known fact that several genes are present on the chromosome. 
  • As chromosomes are carriers of heredity, these genes have tendency to be inherited together.Such genes are called linked genes. 
  • This tendency of two or more genes present on the same chromosomes that are inherited together is known as linkage. 
  • Linkage was discovered in plants by Bateson and Punnett and in animals by T. H. Morgan. 
Linkage is of two kinds - 

  1. Complete linkage and 
  2. Incomplete linkage
I. Complete linkage : 

  • The linked genes which are closely located on the chromosome do not separate (no crossing over) and inherit together. They are called completely linked (strongly linked) genes and the phenomenon of their inheritance is called complete linkage. 
  • Thus the parental traits are inherited in offsprings. 
  • e.g. X chromosome of Drosophila males- show complete linkage. 
II. Incomplete linkage : 

  • The linked genes which are distantly located on the same chromosome and have chances of separation by crossing over, are known as incompletly linked (weakly linked) genes. 
  • The phenomenon of their inheritance, is called incomplete linkage. Thus, new traits occur in offsprings. 
  • e.g. In Zea mays - colour and shape of grain show incomplete linkage. 
Linkage Groups : 

  • All the linked genes in a particular chromosome, constitute a linkage group.
  • The number of linkage groups of a particular species corresponds to its haploid number of chromosomes.
  •  e.g. Drosophila melanogaster has 4 linkage groups that corrrspond to the 4 pairs of chromosomes. 
  • Garden pea has 7 linkage groups and 7 pairs of chromosomes. 
Sex-linkage : 

  • The transmission (inheritance) of X - linked and Y-linked genes from parents to offspring, is called sex-linked inheritance. 
  • Sex-linked inheritance is of three types viz.-
  1.  X-linked
  2. Y-linked and 
  3. XY-linked. 
  • Sex linkage is of two kinds : 
  1. Complete sex linkage 
  2. Incomplate sex linkage
a. Complete sex linkage :

  •  It is exhibited by genes located on non-homologous regions of X and Y chromosomes. 
  • They inherit together because crossing over does not occur in this region. 
  • Examples of X-linked traits are haemophilia, red-green colour blindness, myopia (near sightedness) and for Y-linked are hypertrichosis, Ichthyosis, etc. 
b. Incomplate sex linkage : 

  • It is exhibited by genes located on homologous regions of X and Y chromosomes. 
  • They do not inherit together because crossing over occurs in this region. 
  • Examples of X-Y linked traits are total colour blindess, nephritis, retinitis pigmentosa, etc. 
Crossing Over : 

  • Crossing over is a process that produces new combinations (recombinations) of genes by interchanging and exhanging of corresponding segments between non-sister chromatids of homologous chromosomes. 
  • It occurs during pachytene of prophase I of meiosis. 
  • The term crossing over was coined by Morgan. 
  • The mechanism of crossing over consists four sequential steps such as -  
  1. Synapsis
  2. Tetrad formation
  3. Crossing over and 
  4. Terminalization. 
  • The phenomenon of crossing over is universaland it is necessary for the natural selection,because it increases the chances of variation.


Know the Scientist : 

  • Thomas Hunt Morgan was an American biologist. 
  • He used fruit fly (Drosophila melanogaster) in genetic reserch and established the chromosomal theory of T. H. Margan heredity. 
  • He also discovered the principle of linkage, sex (1866-1945) linkage and crossing over. 
  • Margan's work played key role in the field of genetics. He was awarded a Nobel Prize in 1933, in Physiology and Medicine.


Morgan's Experiments showing linkage and crossing over : 

  • Morgan used Drosophila melanogaster (fruit fly) for his experiments because, Drosophila can easily be cultured in laboratory. 
  • It's life span is short, about two weeks. 
  • More over, it has high rate of reproduction. 
  • Morgan carried out several dihybrid cross experiments in fruit fly to study genes that are sex-linked. 
  • The crosses were similar to dihybrid crosses, as carried out by Mendel in Pea. 
  • For example, Morgan and his group crossed yellow-bodied, white eyed female to the wild type with brown-bodied, red eyed males and intercrossed their F1 progeny.
  • He observed that the two genes did not segregate independently of each other and F2 ratio deviated very signiticantly from 9:3:3:1 ratio. 
  • Morgan and his group knew that the genes were located on X chromosome.
  • Stated that when two genes in a dihybrid cross are situated on the same chromosome, then the proportion of parental combination is much higher than non-parental type. 
  • This occurs due to physical association or linkage of the two genes. 
  • He also found that, when genes are grouped on the same chromosome, some genes are strongly linked. They show very few recombinations (1.3 %). 
  • When genes are loosely linked i.e. present far away from each other on chromosome, they show more (higher) recombinations (37.2 %).
  •  For example, the genes for yellow body and white eye were strongly linked and showed only 1.3 percent recombination (in cross-I). 
  • White bodied and miniature wings showed 37.2 percent recombination (in cross-II). 
  • Cross I shows crossing over between genes y and w. 
  • Cross II shows crossing over between genes white (w) and miniature wing (m). Here dominant wild type alleles are represented with (+) sign.

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