Question

1 : 1 phenotypic ratio is obtained in the offspring when__________for the character considered in the cross.

• A. both the parents carry dissimilar alleles
• B. one parent carries both the dominant alleles and the other parent carries both the recessive alleles
• C. one parent carries both the dominant alleles and the other parent carries dissimilar alleles
• D. one parent carries both the recessive alleles and the other parent carries dissimilar alleles

Answer 1

The correct option is D one parent carries both the recessive alleles and the other parent carries dissimilar alleles

1 : 1 phenotypic ratio is obtained in the offspring when one parent carries both the recessive alleles and the other parent carries dissimilar alleles for the character considered in the cross.

This can be understood by considering the monohybrid cross in which the parents differ in height. Tall and dwarf are two contrasting traits for this character in pea plant.

The tall trait is dominant over the dwarf trait.
‘T’ represents the allele responsible for tallness while ‘t’ represents the allele responsible for dwarfness.

When the pea plant with dissimilar alleles (Tt) is crossed with the pea plant carrying recessive alleles (tt), tall and dwarf offspring are obtained in the ratio of 1 : 1.



Hence option d is correct.

Option a: When both the parents carry dissimilar alleles (Tt), then the phenotypic ratio of 3 : 1 is obtained.

Option b: When one parent carries dominant alleles (TT) and the other parent carries recessive alleles (tt), then all the offspring produced from this cross show dominant phenotype (tall).

Option c: When one parent carries dominant alleles (TT) and the other parent carries dissimilar alleles (Tt), then all the offspring produced from this cross show dominant phenotype (tall).