In a dihybrid cross between individuals that are heterozygous for each trait, the four possible offspring phenotypes will be observed in the ratio of

3 : 1

Heterozygous trait is when for a particular gene or genes there are two different alleles.

Dihybrid cross between individuals that are heterozygous for each trait

Cross        A  x B

Genotype Hh x Hh

         H          h

H      HH       Hh

h       Hh        hh

Four possible phenotype : HH Hh Hh hh

Since 'H' is dominant allele and 'h' is recessive allele so dominant allele will express itself in both heterozygous and homozygous form.

3 : 1

d. the weight of the book