1. Linkage:
- Mendel’s principle of independent assortment is truly applicable for both genes and chromosomes. Genes control various characteristics of the organisms and gene is a basic unit of heredity.
- First-time German scientist Sutton studied the interrelationship between factors and chromosomes. He discovered linkage.
- According to him in each pair of chromosome one chromosome is from each parent, (paternal and maternal member).
- It is the reason by which the genes present on separate chromosomes are independently assorted, but all genes present on the same chromosome are said to be linked and should be inherited together.
- A single chromosome may carry a large number of genes.
- All the genes located on the same chromosome are said to be linked genes. The characters controlled by these genes are said to be Linked characters.
- These linked genes have a tendency to be inherited together but independently. Genes present on a single chromosome are not independently assorted.
- Bateson and Punnet (1906) explained about linked genes in his experiment on sweet pea (Lathyrus odoratus).
An experiment of Bateson and Punnet:
- Bateson and Punnet (1906) did a dihybrid cross on plants of blue flowers and long pollens with red flower and round pollen plant and found the following results:
[Abbreviations: Blue flower = B, Red flower = b, Long pollens = L and round pollens = L.]
- A cross between Blue flower and long pollens (BBLL), and red flower and round pollens yielded blue flowers and long pollens in the F1 generation.
Now they made a test cross between heterozygous F1 offsprings (BbLl) with homozygous recessive parent (bbll). By the concept of linked genes and independent assortment BbLl should give four kinds of gametes and hence four phenotypes in equal proportions 1:1:1:1 were expected.
But Bateson and Punnet obtained the following results –
- The above results in 7:1:1:7 ratio were obtained. This suggests that new combinations or recombinants were less and a number of parental plants were more.
- It means the selected genes for the study have not followed the law of independent assortment, but their nature showed linked inheritance or they show Linkage.
- The genes present on the same chromosome show their nature to inherit together are called Linked genes and the incidence is known as Linkage.
- Sometimes a group of genes found on a single chromosome are inherited together, is called Linkage group.
Various Types of Linkage:
Linkage is of two types –
(i) Complete linkage:
When certain genes located so nearly on the same chromosome and tend to remain linked together while passing from one generation to another without new combinations is termed complete linkage.
(ii) Incomplete linkage:
Linked genes are not inherited always in their combination. In the homologous chromosomes or at the time of meiosis exchange of such genes may be found due to crossing over. The genes located at distance on the chromosome are also incompletely linked because of more chances of isolation by crossing over in these.
2. Crossing Over:
- Bateson and Punnet obtained some recombinant type plants during experiments. It is because of a special process called crossing over.
- In crossing over an exchange of genetic material takes place between non-sister chromatids of homologous chromosomes. It results in the formation of new combinations.
- During prophase I of meiosis there is an exchange of parts of chromatids between homologous chromosomes. The homologous chromosomes become intertwined and the chromatids of homologous chromosomes cross over at certain point called Chiasmata.
- The Chromatids break at this point and then rejoin. Crossing over results in the exchange of corresponding segments at the same locus between the paternal and maternal chromosomes (means non-sister chromatids of homologous chromosomes). Linked genes separate from each other by crossing over and new combinations appear.
Types of Crossing over:
On the basis of the number of Chiasma formation in the chromosomes, the three types of crossing over are found –
- Single Crossing over When only one chromatid of homologous chromosomes participate in crossing over and only one chiasma is formed. It is called Single crossing over.
- Double-crossing over, When two, three or all four chromatids of homologous, chromosomes participate in crossing over and two chiasmata are formed. It is known as Double-crossing over.
- Multiple Crossing over When more than two chiasmata are formed between two non-sister chromatids. It is called multiple crossing over.
Significance of Crossing Over:
- This process occurs during the pachytene stage of prophase – I of meiosis.
- It occurs between non-sister chromatids of homologous chromosomes.
- Exchange of segments or genes between non-sister chromatids is known as Crossing over.
- Numbers of points of a chromosome where crossing over takes place at a single time depends on the length of the chromosome. Percentage of crossing over increases with the increase in the length of a chromosome.
- Distance between the genes in a chromosome affects the probability of crossing over. More distance between genes increase the probability of crossing over and if the distance is less than there is a decrease in the probability of crossing over.
