Question

In Mirabilis jalapa when homozygous red flowered and white flowered plants are crossed, all $F_1$ plants have pink coloured flowers. In $F_2$ produced by selfing of $F_1$ plants, red, pink, white flowered plants would appear respective in the ratio of
A. 1:1:2
B. 2:1:1
C. 1:0:1
D. 1:2:1

Answer 1

Hint: Mirabilis jalapa is known as four O’clock plant, this plant show the another character of inheritance which is incomplete dominance, and this plants also produces the two types of flower red and white coloured, which is an example of contrasting characters.

Complete answer:
As this plant flower bears a pair of contrasting characters which means a single character reflects two different characters, like the colour of the flower in this plant is either red of white. When a plant holds homozygous for red flower (RR) and homozygous (rr) for white flower and when they cross then all plants of $F_1$ generation produce pink colour flowers which is a heterozygous condition (Rr) hybrid plant. When $F_1$ generation plants self cross then they will produce $F_2$ generation plants, which bear red coloured, white coloured and pink coloured flowering plants. Pink colour flower in plants in $F_1$ generation and $F_2$ generation characterized as incomplete dominance, in $F_2$ generation all plants come in the ratio of 1:2:1 for red coloured flowered : white colour flowered : pink colour flowered plant, such phenomenon occur according to the law of segregation.

This plant mirabilis jalapa provide us example which describe the feature of inheritance such as, the incomplete dominance phenomenon, law of segregation as we saw in $F_1$ generation in which all plant are heterozygous and appear as pink colour flowered plant and in next $F_2$ generation, the red, pink and white colour flowered plant are appear. The recessive allele of flower colour do not produce any pigment while in case of heterozygous condition the dominant allele produces half amount of colour or pigment.

Hence, the correct answer is option (D).

Note: The genotypic appearance of such kind of plant character is (RR) for red colour flowered plant, (rr) for white colour flower plant and (Rr) for pink colour flowered plant, but in case of phenotypic in first generation a new character arise in form of pink colour plants(Rr).