Question

A dihybrid test cross yielding a result of 1 : 1 : 1 : 1 ratio is indicative of

• A. Homozygous condition of the F${}_1$ dihybrid
• B. 4 different types of gametes produced by the F${}_1$ dihybrid
• C. 4 different types of gametes produced by the F${}_1$ parent
• D. 4 different types of F${}_1$ generation dihybrids

Answer 1

Answer: (B)

4 different types of gametes produced by the F${}_1$ dihybrid

A test cross is a cross of any genotype with a recessive homozygote. The gametes of the recessive homozygote contain only recessive alleles and therefore, alleles that the gametes of the other parent carry will determine the phenotypes of the offspring. If a gamete from the tested genotype contains a recessive allele, the resulting F${}_1$ organism will have a recessive phenotype. But if it contains a dominant allele, the F${}_1$ organism will have a dominant phenotype. Here, a dihybrid test cross yields 1 : 1 : 1 : 1 which means that the dihybrid forms four different types of gametes in equal which fuse with the single type of gamete produced by a recessive parent to give four different genotypes of progeny. Option B is correct.

If the F${}_1$ parent was homozygous, only one type of gametes was formed and progeny of a single type was obtained but this is not the case which means that the F${}_1$ parent in test cross is a hybrid, not a homozygous parent. Options A and C are wrong. Test cross determines the gametes produced by dominant parents to identify if it is homozygous or heterozygous; it is not concerned with F${}_1$ generation hybrids; option D is wrong.