Question

In a plant, two genes have cumulative influence on the weight of fruits. Each dominant allele adds 10g to a minimum basic weight of 30g. In a cross between dihybrid plants having fruits weighing 50g each, what percentage of the offspring would have fruits weighing 40g?
A. 12.5 percent
B. 25 percent
C. 37.5 percent
D. 50 percent

Answer 1

Hint: The systematic breeding of two separate individuals, resulting in offspring bearing part of each parent's genetic material. The parent organisms must be genetically identical and can come from different varieties or closely related species.

Complete answer:
Let two genes regulating weight be A and B genes with recessive alleles as a and b. The minimum specific weight is 30g, so the genotypes of all such fruit-bearing plants will be recessive (aabb) genes. According to the question, each dominant allele contributes 10g to the base weight of 30g. So, it is important to have 2 dominant alleles for a plant to have a fruit of 50g. Since we are told in question that the plant is dihybrid, one dominant allele of both genes will be found in the fruit of 50g weight of the parent plant. Therefore the genotype of the parent plant appears to be ‘AaBb’. A dihybrid cross between AaBb X AaBb gives the following results as seen in the table:

Gametes	AB	Ab	aB	ab
AB	AABB=70g	AABb=60g	AaBB=60g	AaBb=50g
Ab	AABb=60g	AAbb=50g	AaBb=60g	Aabb=40g
aB	AaBB=60g	AaBb=50g	aaBB=50g	aaBb=40g
ab	AaBb=50g	Aabb=40g	aaBb=40g	aabb=30g

So, after analysing all the fruit weights in progeny plants, we find that 4 out of 16 plants would give fruit of 40g weight. So, the figure that we get after dividing 4 by 16 and then converting it in percentage is 25 percent.

So, the correct answer to this question is '25 percent'.

Note:
There are generally two types of traits dominant and recessive. When the connected allele is dominant, dominant traits still appear, even though only one copy of the dominant trait (exists with one recessive trait). Recessive Traits are only displayed if both the corresponding alleles are recessive.