In the summer squash Cucurbita pepo the fruit is normally di

In the summer squash, Cucurbita pepo, the fruit is normally disk-shaped. Two recessive mutations, g and r, are known in different geographical races of the species, both of which cause fruit to be spherical. In crosses between homozygous gg and rr stocks, the F1 is always disk-shaped. However, when such F1 hybrids were interbred, it was discovered that ggrr plants make fruit that is elongated. Are the mutations g and r allelic? How can you justify your answer to A? If the F1 hybrids described above are interbred, write the genotypes and phenotypes expected in the F2 and their expected ratios.

Solution

a) Yes, the two mutations g and r are allelic.

b) The complementation test provides a useful means to detect whether the two mutations affect the same gene or not. Here the wild type phenotype results when mutations occur in both the cis and trans position.

Here in F1 generation the mutations (g and r) occur in the trans position and a mutant phenotype is expressed, because the mutations appear in the same gene on both the chromosomes. i.e., allelic mutations do not complement when two mutations are in trans position. In this case the mutations are trans as two homozygous individuals are crossed.

And in F2 generations the mutations (g and r) may result in new combinations and results in wild type phenotype.

c) If the F1 hybrids with genotype (gr) were inherited, the expected ratios will be

genotypic ratio gg:gr:gr:rr = 1:2;2:1.

phenotypic ratio will be same as both the genes are recessive.