Question

In a typical Mendelian cross which is a dihybrid cross, one parent is homozygous for both dominant traits and another parent is homozygous for both recessive traits in F2 generation both parental and recombinant appear. The phenotypic ratio of parental combination to recombination is?
A. 10:6
B. 12:4
C. 9:7
D. 15:1

Answer 1

Hint: Genotype of an individual refers to the sum total of genes inherited from both the parents irrespective of whether they are expressed or not.

Complete answer: If the homozygous trait is “YYRR” and the heterozygous trait is “yyrr”
Parents: “YYRR” x “yyrr”
Gametes: ‘YR’, ‘YR’, ‘yr’, ‘yr’
Punnett square for the F1 generation:

	YR	YR
yr	YyRr	YyRr
yr	YyRr	YyRr

Gametes : ‘YR’, ‘Yr’, ‘yR’, ‘yr’
Punnett square for F2 generation :

	YR	Yr	yR	yr
YR	YYRR(Yellow round)	YYRr(yellow round)	YyRR(yellow round)	YyRr(yellow round)
Yr	YYRr(yellow round)	YyRr(yellow round)	YyRr(yellow round)	Yyrr(yellow wrinkled)
yR	YyRR(yellow round)	YyRr(yellow round)	yyRR(green round)	yyRr(green round)

Therefore, the phenotypic ratio obtained is 10:6.
Hence, the correct answer is option A.

Additional information:
i. Homozygous organisms: an organism in which both the alleles of a character at the corresponding loci in homologous chromosomes are identical is said to be homozygous or genetically pure for that character. Homozygous individuals breed true for the specific trait i.e. produce offspring with similar genotype and phenotype on inbreeding.
ii. Heterozygous organism: an organism in which two alleles of a character at the corresponding loci in homologous chromosomes is unlike is said to be heterozygous for that character. Heterozygous individuals do not breed true and are said to be hybrid for that character.

Note: The law of independent assortment states that alleles of different characters located in different pairs of homologous chromosomes are independent of one another in their segregation during gamete formation and in coming together into the offspring by fertilization, both process occurring randomly this can be explained by dihybrid cross.