INtroduction to Linkage and Crossing Over
The trend of genes / characters to be inherited together due to their position on similar chromosome is termed as linkage. Several hybridization experiments were conducted both on plants and animals relies on work of Mendel. The results of specific dihybrid crosses did not verify the law of independent assortment. It says that the inheritance of genes of every pair in a dihybrid throughout gamete creation is independent of the other.
Blue flower Red flower
Parent long pollen round pollen
BBLL bbll
Gametes BL * bl
Observed percentage frequency
44 6 6 44
Observed ratio 7 : 1 : 1 : 7
Expected ratio 1 : 1 : 1 : 1
Mechanism of linkage - coupling in Lathyrus odoratus
In the year 1906, William Bateson and Reginald Punnett carried out experiments in sweet pea, Lathyrus odoratus to verify dihybrid testcross of Mendel. They detected an exception to the free assortment of two genes in this plant. Now, blue flower (B) is leading over the red flower (b) and long pollen (L) leading over round pollen (l). They crossed true breeding plants comprising blue flower along with long pollen (BBLL) and red flower along with round pollen (bbll). All the F1 hybrids comprise blue flowers along with long pollen (BbLl). A testcross among heterozygous blue long (BbLl) of F1 hybrid and double recessive parental stock red round (bbll) did not result in ratio 1:1:1:1 but provided not supposed phenotype frequency as displayed below.
In this, blue long and red round are parental creates and display greater frequency 88%. Blue round and red long are recombinant creates and display lesser frequency 12%. The dihybrid test cross ratio acquired is 7:1:1:7 and not 1:1:1:1. This points out that the genes do not freely assort. From the above test cross, it is apparent that if leading alleles or recessive alleles are exists in similar plant, they tend to stay together resulting in increased parental forms. So, the two genes that inherit together are termed as linked genes. This feature is known as coupling.
They completed another cross among plants comprising blue flower with round pollen (BBll) and red flower along with long pollen (bbLL). A testcross among
Parent round pollen long pollen
BBll bbLL
Gametes Bl * bL
6 44 44 6
Observed ratio 1 : 7 : 7 : 1
Mechanism of linkage - repulsion in Lathyrus odoratus
Heterozygous blue long (BbLl) of F1 hybrid and double recessive red round (bbll) did not product in ratio 1:1:1:1 but provided not supposed phenotype frequency as displayed below.
In this, blue round and red long are parental creates and depict greater frequency 88%. Blue long and red round are recombinant creates and depict lesser frequency 12%. The dihybrid test cross ratio acquired is 1:7:7:1 and not 1:1:1:1. This points out that the genes do not freely assort. From the preceding testcross, it is apparent that if leading alleles or recessive alleles are exists in the dissimilar plants, they are apt to stay separate resulting in increased parental forms. This feature is known as repulsion.
Coupling and repulsion presented description for higher frequency of parental forms. They are two features of a single phenomenon known as linkage. The genes which are carried on similar chromosome will not assort freely due to their tendency to stay linked together. This is termed as linkage. The genes positioned on similar chromosomes which are inherited together are termed as linked genes. They tried to again confirm the law of independent assortment. But they could not obtain supposed result because the genes are linked.
Crossing Over
The process that generates recombination of genes through interchanging the corresponding segments among the nonsister chromatids of homologous chromosomes, is known as crossing over. A crossing over among the linked genes permits their recombination during meiosis.
Diagram: crossing over
Crossing over occurs in pachytene stage of meiosis' prophase I. In pachytene stage, the bivalent chromosome turns into tetrad that is with 4 chromatids. The adjoining nonsister chromatids are connected together at specific points termed as chiasmata. Crossing over takes place among the nonsister chromatids of paired chromosomes in the area of chiasma. At every chiasma, the two nonsister chromatids break, exchange their segments and rejoin resultant the crossing over.
Diagram: Illustration to explain the probability of crossing over
Therefore, out of four chromatids the two adjoining chromatids are recombinants and another two are original chromatids. So, four types of gametes are acquired.
Importance of crossing over
1. Crossing over leads to the generation of new combination of genes and gives basis for acquiring new range of plants.
2. It plays a significant role in the evolution process.
3. The crossing over frequency assists in the creation of genetic maps of the chromosomes.
4. It provides us the proof for linear arrangement of linked genes in a chromosome.
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