Question

Genotypic ratio of a dihybrid cross is
A. $12:3:1$
B. $15:1$
C. $1:2:1$
D. $1:2:2:4:1:2:1:2:1$

Answer 1

Hint: Genotype means the expression of a gene in DNA for a particular trait. Phenotype means the physical appearance of that gene. For example when we cross a red coloured flower (RR) with white coloured flower (rr). After the cross, the gametes produced is (Tt). Hence Tt is the genotype of that trait but the phenotypic is ‘tall’.

Complete answer:
Dihybrid cross means taking two traits for the cross such as shape and colour of the pea plant.
Mendel uses a dihybrid cross to explain the law of independent assortment.
Let us understand the dihybrid cross and the phenotypic and genotypic ratio with an example :
In this example, the two traits are the yellow and green colour of the pod and round and wrinkled shape of seed in the pea plant.
The dominant traits are a yellow colour and round shape.
Dominant traits are expressed by yellow – YY and round – RR
However recessive traits are green colour and wrinkled shaped. They can be expressed by green – yy and wrinkled – rr.
For better understanding the independent segregation of the two pairs of genes, we can use Punnett square.

	RY	rY	Ry	ry
RY	RRYY: Round yellow	RrYY: Round yellow	RRYy: Round yellow	RrYy: Round yellow
rY	RrYY: Round yellow	rrYY: wrinkled yellow	RrYy: Round yellow	rrYy: wrinkled yellow
Ry	RRYy: Round yellow	RrYy: Round yellow	RRyy: Round green	Rryy: Round green
ry	RrYy: Round yellow	rrYy: wrinkled yellow	Rryy: Round green	rryy: wrinkled green

Now let us try to find out the ratio:
Phenotypic ratio –

Round yellow	wrinkled yellow	Round green	wrinkled green
$9$	$3$	$3$	$1$

That is, $9:3:3:1$
Genotypic ratio –

RRYY	RrYY	RRYy	RrYy	rrYY	rrYy	Rryy	Rryy	rryy
$1$	$2$	$2$	$4$	$1$	$2$	$1$	$2$	$1$

That is, $1:2:2:4:1:2:1:2:1$

Therefore the correct option is D.

Note: G. Mendel is the father of genetics. In the 19th century, he did experiments on pea plants. First, he did monohybrid cross and based on that he proposed the principle law of inheritance- the first law is the law of dominance and the second law is the law of segregation. When he crosses with two traits he stated the law of independent assortment.