A chromosome possesses many genes & all genes present in the chromosome are inherited together.
Linkage: Study of inheritance of all genes present in a chromosome together.
- All genes in a chromosome are together referred as linked genes & they form a linkage group.
- The total number of linkage group in an organism is equal to its haploid number of chromosomes.
Bateson & Punnet:
- While working on sweet pea (Lathyrus odoratus) observed that genes for flower color & pollen shape remain together and do not assort independently according to Mendel’s law of Independent assortment.
- Test cross failed to produce 1:1:1:1 ratio & instead produced 7:1:1:7.
- They gave the Coupling & Repulsion theory:
- Coupling: when genes come from the same parent they enter the same gamete & are inherited together.
- Repulsion: genes are inherited separately when genes come from different parents & they enter different gametes
T H Morgan’s Linkage:
He worked on Drosophila.
- linked genes are genes present on the same chromosome.
- Linked genes are inherited together.
- Genes are linearly arranged in a chromosome.
- Strength of linkage: Genes which are closely located show strong linkage & genes which are located far show weak linkage.
- He stated that Coupling & Repulsion are two aspects of Linkage.
Linked genes show 2 types of arrangement:
- Cis arrangement: dominant alleles of 2 or more genes are present in one chromosome & its recessive alleles in its homologue. AB/ab. This is Coupling.
- Trans arrangement: the dominant allele of one pair & recessive of the other pair together lie in a chromosome.Ab/aB. This is Repulsion.
Linked genes are of 2 types:
- Complete linkage: genes for 2 or more characters appear together for two or more generations in their parental combination.They are closely located in the chromosome.
- Incomplete linkage: the parental combination of 2 or more characters are not retained in the next generations.They are not closely located in the chromosome.
- Incomplete linkage occurs due to crossing over.
- It is the exchange of segments between non-sister chromatids of homologous chromosomes.
- It occurs during pachytene stage of prophase I in meiosis.
- Crossing over always occurs between linked genes.
- It produces recombination of linked genes which play very important role in evolution.
- Recombination frequency helps in finding out the distance between genes. Given by Sturtevant.
- Recombination frequency helps in the construction of genetic maps of the chromosomes.
In a dihybrid cross:
- When the test cross result is 1:1, genes are linked and there is no crossing over.
- If the test cross result is 1:1:1:1 means the genes are independently assorting (present on separate chromosomes).
- If in the test cross result parental combination is more than 50% & recombination is less than 50%, the genes are linked and crossing over has occurred.