Question

In the summer squash, Cucurbita pepo, the fruit is normally disk-shaped. Two recessive mutations, g (globe) and r, (round), are known in different geographical races of the species, both of which cause fruit to be spherical. In crosses between homozygous g/g and r/r stocks, the F1 is always disk-shaped. However, when these F1 hybrids were interbred, it was discovered that squash plant homozygous recessive at both g and r make fruit that is elongated. What are the genotypes and phenotypes of the F2 from the cross of the F1 hybrids in Question 1?

Answer 1

Answer:

Let the interbred alleles be represente dy the letters A and B for two genes. Gene A will exist in two alleles, that will icnlude A and g while Gene B exists in two alleles, B and r.

The uppercase alleles are dominant to the lowercase alleles. The interbred long-shaped squash is AABB (aabb) and the true-breeding disk-shaped is ggrr  AABB.The F1 offspring are AaBb.

The outcome of interbred of the F1 plants and genotypes and phenotype sof F2 for:  

1) the disk-shaped phenotype, an offspring must inherit at least one dominant allele from both genes.

1 ggrr+ 2 Agrr+ 2 ggrB+ 4 AgBr = 9 disk-shaped offspring

2) To get the round phenotype, an offspring must inherit at least one dominant allele for one of the two genes but must be homozygous recessive for only one of the two genes.

1 ggrr+ 1 ggBB+ 2 AArB+ 2 gABB = 6 round-shaped offspring

3) To get the long phenotype, an offspring must inherit all recessive alleles:      

1 AABB= 1 long-shaped offspring

Hence, the phenotype and genotype of F2 is is 9 disk-shaped offspring and 1 ggrr+ 2 Agrr+ 2 ggrB+ 4 AgBr; 6 round-shaped offspring and 1 ggrr+ 1 ggBB+ 2 AArB+ 2 gABB; and 1 long-shaped offspring and 1 ggrr+ 1 ggBB+ 2 AArB+ 2 gABB respectively.