Question

ln certain species of plant, one purebred
variety has hairy leaves and another pure-
bred variety has smooth leaves. A cross of
the two varieties produces offspring that all
have smooth leaves. Predict the ratio of
phenotypes in the F2, generation.

Answer 1

Answer:

If crossing plants with hairy leaves and smooth leaves produces plants with smooth leaves in F1, the phenotypic ratio of plants with smooth leaves:hairy leaves in F2 will be 3:1.

Explanation:

It is assumed that, if the first descendants of this cross presented smooth leaves, this represents the dominant characteristic.

If in the crossing of two plants, the parental generation consists of two pure plants for the characteristic aspect of the leaf, one with hairy leaves (s) and another with smooth leaves (S), the first filial generation (F1) will be of hybrid plants with a phenotype of smooth leaves (Ss), which represents the dominant characteristic.

Parental Generation:

• Smooth leaves SS
• Hairy leaves ss

Punnett Square  F1

Alleles      S        S

s              Ss       Ss

s              Ss       Ss

F1 : 100% plants with smooth leaves Ss

The second filial generation (F2) results from the crossing of F1 individuals, so:

Punnett Square  F2

Alleles      S        s

S              SS      Ss

s              Ss       ss

The result of the offspring in F2 is:

Genotype

• Pure smooth leaves SS 25%
• Smooth hybrid leaves Ss (heterozygous) : 50%
• Pure hairy leaves ss:  25%

Genotipic ratio: pure smooth leaves:hybrid:pure hairy leaves  1:3:1

Phenotype

• Smooth leaves SS + Ss:  75%
• Hairy leaves  ss: 25%

So the phenotypic ratio of smooth leaves: hairy leaves will be 3:1.