Answer:
1/64
Step-by-step explanation:
This is a typical trihybrid cross involving three genes coding for plumage color, shank type and comb shape in leghorn chicken. White plumage (W), feathered shank (F), and pea comb (P) are all dominant over colored plumage (w), clean shank (f) and single comb (p) respectively.
A cross between an all homozygous dominant chicken (WWFFPP) and an all homozygous recessive chicken (wwffpp) will result in an F1 that exhibits all the dominant traits but heterozygous (WwFfPp). If the three genes assort independently, 8 types of gametes will be produced by this F1 offsprings: WFP, WFp, WfP, Wfp, wFP, wFp, wfP, wfp.
If the F1 offsprings are self-crossed, we use this gametes in a punnet square to obtain 64 possible F2 offsprings in a phenotypic ratio 27:9:9:9:3:3:3:1. This ratio is the phenotypic ratio of a trihybrid cross.
According to the question which says that what proportion of the F2 offsprings will produce all the dominant traits (white, feathered, pea) if crossed with an homozygous recessive chicken (wwffpp). This means that to produce only offsprings with an all dominant trait, the parent that combines with the homozygous recessive (wwffpp) must be homozygous dominant (WWFFPP) for all three traits.
In the ratio 27:9:9:9:3:3:3:1, the "27" represents the offsprings that have all dominant traits, but only one (1) of these 27 possesses a homozygous dominant genotype for all three traits (WWFFPP), which also means only 1 out of the possible 64 offsprings possesses an homozygous dominant genotype for all three traits.
Hence, the proportion that will produce only white, feathered pea-combed chicken (heterozygous) if mated to colored, clean, single-combed birds is 1/64.