Linkage and Crossing Over

Linkage and Crossing Over    

  Linkage: 

• It is a known fact that several genes are closely located on the chromosome. As chromosomes are carriers of heredity, these genes have a 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. 

Types of linkage:  

1) 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 

e.g. X chromosome of Drosophila males- show complete linkage. 

2) 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. 

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. 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. 

1)  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, etc.  

2) Incomplete 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 exchanging 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. 

Morgan’s Experiments showing linkage and crossing over 

• Morgan used Drosophila melanogaster (fruit fly) for his experiments because Drosophila can easily be cultured in the laboratory. Its life span is short, about two weeks.  

• Moreover, it has a high rate of reproduction. Morgan carried out several dihybrid cross experiments in fruit fly to study genes that are sex-linked. 

• 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 significantly from 9:3:3:1 ratio. 

• The genes for yellow body and white eye were strongly linked and showed only 13 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. 

Linkage and crossing over