Question

A true-breeding plant producing red flowers is crossed with a pure plant producing white flowers. The allele for the red color of the flower is dominant. After selfing the plants of the first filial generation, the proportion of plants producing white flowers in the progeny would be
(a)$\dfrac { 3 }{ 4 }$
(b)$ \dfrac { 1 }{ 4 }$
(c)$\dfrac { 1 }{ 3 }$
(d)$\dfrac { 1 }{ 2 }$

Answer 1

Hint: Genotype refers to the genetic makeup of an organism or in simple words it describes an organism's complete set of genes. Dominant genes are represented by capital letters and recessive genes by small letters.

Complete answer:
Let therefore the cross genotype of a true-breeding plant producing red flowers(dominant trait) be RR
So the genotype of a pure plant producing white flowers(recessive trait) will be rr
Parental generation : RR x rr
So, ${ F }_{ 1 }$ generation : Rr (red flowers)
Selfing ${ F }_{ 1 }$ generation plants : Rr x Rr
Gametes : R r R r
${ F }_{ 2 }$ generation after selfng : RR Rr Rr rr
Where,
RR and Rr represents red flowers
Rr represents white flowers
So, white flower producing progeny is = $ \dfrac { 1 }{ 4 }$.
So the correct answer is option (B).

Additional information:
The monohybrid cross is generally used to determine the dominance relationship between two alleles. The cross begins with the parental generation where one parent is homozygous for one allele, and the other parent is homozygous for the other allele. The offspring that are produced after the cross is called the first filial (${ F }_{ 1 }$) generation. According to the result, every member of the ${ F }_{ 1 }$ generation is heterozygous and the phenotype of the ${ F }_{ 1 }$ generation expresses the dominant trait. Crossing the two members of the ${ F }_{ 1 }$ generation will produce the second filial (${ F }_{ 2 }$) generation. According to probability theory, three-quarters of the ${ F }_{ 2 }$ generation will have the dominant allele's phenotype. So, the remaining quarter of the ${ F }_{ 2 }$ will have the recessive allele's phenotype.

Note: The cross between the two alleles is generally carried out in the Punnett square. It is generally a square shaped diagram that is used to know the genotypes of a particular cross or breeding experiment. It was named after Reginald C. Punnett, who devised this approach. Nowadays this system is used by biologists to determine the probability of an offspring having a particular genotype.