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The failure of pollen formation results in male sterile plants. In some crop plants male sterile mutants occur in which inheritance of male sterility follows one of the following two patterns: those in which the trait is inherited through a single recessive chromosomal gene segregating in Mendelian ratios; secondly, those that show maternal transmission. Rhoades in 1933 described maternal inheritance of male sterility in Zea mays.
In the cross shown above, the process of repeated backcrossing results in substitution of all the chromosomes of the fertile line for those of the male sterile line. The experiment therefore, not only shows cytoplasmic maternal transmission, but also confirms that chromosomal genes are not responsible for male sterility.
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Iojap Strain of Maize:
The iojap strain of maize is characterised by green and white stripes on the leaf. The name iojap originates from two parental strains Iowa which is green and Japonica, a striped variety.
In crosses between iojap and green varieties when iojap is used as the male parent, the trait is inherited according to Mendelian pattern with F1 progeny all green, and F2 segregating into 3/4 green and 1/4 iojap. But in the reciprocal cross where iojap is used as the female parent, the F1 plants showed all three phenotypes viz. green, white and striped.
It was found that the iojap gene in the homozygous recessive condition (ii) causes some of the plastids to mutate giving rise to colourless plastids. The mixture of green and colourless plastids accounts for the origin of striped plants. Once created, further transmission of the striped character takes place maternally through the egg cytoplasm as evident from the phenotypes of the F2 progeny of the cross above.
