Question

In Mirabilis jalapa, RR, Rr, rr determine red, pink and white colours respectively. When F1 hybrid of RR and rr was crossed with dominant parent, the ratio produced is
A) All red
B) 2 Red : 2 Pink
C) All white
D) 2 Pink : 2 White

Answer 1

Hint: Incomplete dominance is found in the plant species Mirabilis jalapa. When a dominant allele does not affect the effect of recessive allele and dominance is found in both forms. It is called partial dominance or semi dominance.

Complete Answer:
In Mirabilis jalapa, it shows incomplete dominance. If the red coloured plant (RR) is crossed with white coloured Mirabilis plant (rr), in the F1 generation, all plants make a hybrid plant (Rr) which is Pink in colour.
- According to the experiment, when the plant (Rr) made in F1 generation is crossed with dominant plant i.e red coloured Mirabilis plant (RR), the offspring will be pink (Rr) and red (RR). Their genotypic ratio will be 2:2 i.e. 1:1.

Alleles	R	R
R	RR	RR
r	Rr	Rr

- If two dominant experimented plants are crossed together then the offspring will be red in colour and the genome in offspring will express the dominant allele. As a result, all plants will be red.

Alleles	R	R
R	RR	RR
R	RR	RR

- If two recessive alleles experimented on plants are crossed together then the offspring will be white in colour and the genome in offspring will express the recessive allele. As a result, all plants will be white.

Alleles	r	r
r	rr	rr
r	rr	rr

- If the white coloured recessive plant (rr) is crossed with one of the plants (Rr) made in F1 generation , the plants produced will be pink (Rr) and white (rr). Here the ration will be 2:2 i.e. 1:1.

Alleles	r	r
R	Rr	Rr
r	rr	rr

Note: After Mendel’s monohybrid cross and dihybrid cross, some exceptional events were found and incomplete dominance is one of them. It was first found in a 4 O’ clock plant.