Linkage and its Types - Practice Questions & MCQ

• Linkage is the tendency of genes to stay together during inheritance through generations without any change or separation due to their being present on the same chromosome.
• The genes present on the same chromosomes are called linked genes. 
• Linked genes do not show independent assortment. They show a dihybrid ratio of 3:1 and test cross ratio of 1:1.
• A few years after the rediscovery of Mendel’s laws of inheritance, Bateson and Punnett (1905) observed in sweet pea that two pairs of alleles do not assort independently.
• Morgan (1910) found the same phenomenon in Drosophila and Hutchinson observed a clear cut case of linkage in maize.

Chromosome Theory of Linkage:

• In 1911, Morgan and Castle proposed this theory. It states that
  • Linked genes occur in the same chromosome.
  • They lie in a linear sequence in the chromosome.
  • There is a tendency to maintain the parental combination of genes except for occasional crossovers.
  • Strength of the linkage between the two genes is inversely proportional to the distance between the two.

Linkage Groups:

• Genes that are present on the same chromosome make one linkage group.
• Because the two homologous chromosomes possess either similar or allelic genes on the same loci, they constitute the same linkage group.
• Fruitfly Drosophila melanogaster has 4 linkage groups (4 pairs of chromosomes).
• Human females have 23 linkage groups (23 pairs of chromosomes).
• In human males, there are 22 autosomal linkage groups plus the X and Y linkage groups in males. This gives a total of 24 linkage groups.

Complete Linkage:

• Linkage in which crossing over does not occur is known as complete linkage or absolute linkage. In other words, when only parental types are obtained from the test cross progeny, it refers to complete linkage.

Incomplete Linkage:

• If some frequency of crossing over also occurs between linked genes, it is known as incomplete linkage. To put it another way, when recombination’s are also observed in the test cross progeny, besides parental combinations, it refers to incomplete linkage.

Coupling Linkage:

• It refers to linkage either between dominant genes or between recessive genes. Such linkage has been reported in pea, maize and several other crops.

Repulsion Linkage:

• It refers to the linkage of some dominant genes with some recessive genes. This type of linkage has also been observed in pea, maize and several other crops.