Coupling and Repulsion Hypothesis of Bateson and Punnett

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Coupling and Repulsion Hypothesis of Bateson and Punnett

Coupling and Repulsion Hypothesis of Bateson and Punnett
Bateson and Punnett (1905-1908) formulated the hypothesis of coupling and repulsion to explain the unexpected F2 results of a dihybrid cross between a homozygous sweet pea (Lathyrus odoratus) having a dominant alleles for blue or purple flowers (RR) and long pollen grains (Ro Ro) with another homozygous double recessive plant (rr, roro) with red flowers and round pollen grains. When they test crossed a heterozygous blue or purple long (Rr, Roro) plant with recessive parent (rr, roro), besides getting the 1: 1:1: 1 test cross ratio, they received phenotypic ratio of 7:1:1: 7 as has been illustrated here:

Parent:	Blue of purple long	x	Red round
	(RR Ro Ro)		(rr ro ro)
F₁:	All blue or purple long (Rr Ro ro)
Test cross:	F₁ blue or purple long	x	Red round
	(Rr Ro ro)		(rr ro ro)
Test cross progeny:	Blue or purple long	=	192
	Red round	=	182
	Blue or purple round	=	23
	Red long	=	30
			-----
			427

Test cross ratio: 7 Blue or purple long: 1 Blue or purple round: 1 Red long: 7 Red round or 7: 1: 1: 7.
The 7: 1:1 : 7 test cross ratio clearly indicated that there was a tendency in the dominant alleles (RRo) to pass together to the same gamete. Similar was the case with recessive alleles (rro). This tendency of dominant or recessive alleles to inherit together was explained as "gametic coupling" by Bateson and Punnett.
Further, when they crossed blue or purple round (RR roro) with red long (rr RoRo), the F₁ hybrids were found to be heterozygous blue or purple long (Rr Roro). The F₁ hybrid when test crossed with recessive (rr roro) parent, the test cross ratio was 1 blue or purple long: 7 red long: 7 blue or purple round: 1 red round. as has been illustrated in following figure:

Parent:	Blue or purple round	x	Red long
	(RR roro)		(rr RoRo)
F₁:	All blue or purple long
	(Rr Ro ro)
Test cross:	F1 blue or purple long	x	P₁ Red round
	(Rr Ro ro)		(rr ro ro)

Test cross progeny: 1/16 Blue or purple long (Rr Ro ro) : 7/16 Blue or purple round (Rr ro ro) : 7/16 Red long (rr Ro ro) : 1/16 Red round (rr ro ro) or 1 : 7 : 7 : 1.
Hence, the two dominant pairs of alleles repelled each other. The tendency of both dominant or both recessive alleles to repel each other, so that the gametes of genotypes of Rro and rRo are formed more frequently, was termed repulsion.

Bateson and Punnett could not explain the exact reasons of coupling and repulsion, and it was T.H. Morgan who while performing experiments with Drosophila, in 1910, found that coupling or repulsion was not complete. He further suggested that the two genes are found in coupling phase or in repulsion phase, because they are present on the same chromosome (coupling) or on two different homologous chromosomes (repulsion). Such genes are then called linked genes and the phenomenon of inheritance of linked genes is called linkage by Morgan.

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