Question

The coupling test cross- ratio is
(a) 1:7:7:1
(b) 7:1:1:7
(c) 1:1:1:1
(d) 9:3:3:1

Answer 1

This ratio is the result of testcross when independent assortment was lacking. Both the dominant and the recessive alleles are having the tendency of being together.

Complete step by step answer:
Coupling was first used by Bateson. In coupling dominant alleles are on the same homologous chromosome and both the recessive alleles are on the other homologous chromosome crossed together.
Bateson described coupling by using the cross in sweet pea, where a deviation from independent assortment was exhibited.
A plant of a sweet pea variety having blue flowers and long pollen (BBLL) were crossed with the plant of another variety having red flowers and round pollen (bbll) . In the { F }_{ 1 } generation all individuals had blue flowers and long pollen (BbLl) . Then these individuals with (BbLl) were crossed with plants having red flowers and round pollen (bbll) .
Normally, we should expect a 1 : 1: 1:1 ratio in a testcross if independent assortment takes place. But actually, we get a 7: 1: 1:7 ratio, this indicates that dominant alleles are having a tendency to remain together. Similarly was the case with recessive alleles also.
Therefore this deviation in the test cross ratio was, explained by Bateson as the gametic coupling.
So, the correct answer is,’(b) 7:1:1:7’.

Additional Information:
- As coupling was observed, similarly it was also observed that when two such dominant alleles or two recessive alleles come from different parents, they tend to remain separate. This was named as gametic repulsion.
- In Bateson's experiment during the repulsion phase, 1: 7:7: 1 ratio was obtained instead of the expected 1: 1: 1:1 from the test cross.
- Bateson explained the lack of independent assortment in the above experiments by means of a hypothesis known as the coupling and repulsion hypothesis.
- A testcross of a heterozygous individual resulted in a 1:1 ratio in the monohybrid cross. With the dihybrid cross, we get a 1:1:1:1 ratio.
- 9:3:3:1 is the Mendelian ratio for a dihybrid cross in which the alleles of two different genes assort independently into gametes.

Note:
- In Bateson's experiment during the repulsion phase, 1: 7:7: 1 ratio was obtained
- In Bateson's experiment during the coupling phase, a 7:1:1:7 ratio was obtained.
- The coupling phase indicates that dominant alleles are having a tendency to remain together, similarly the recessive alleles are also having that tendency of being together.
- The repulsion phase indicates that dominant alleles and recessive alleles from different parents are having the tendency of remaining separate from one another.