Question

Explain,All back cross cannot be a test cross but any test cross has to be a back cross only.​

Answer 1

EXPLANATION:

• Back cross is defined as the cross between the parent and the offspring.

• Back cross is of two types

★ Test cross

★ Out cross

• To know about Back cross lets go in detailed discussion about the Test cross and the Out cross.

TEST CROSS:

• Test cross is a type of cross of offspring with a recessive parent.

• It is used to identify the genotype of an unknown offspring.

• Let us consider a tall pea plant, Its genotype can be either TT ot Tt. But for dwarf plant it is always tt.

• Cross a tall offspring with a dwarf plant. If the genotype is Tt we will get Tt : tt = 1 : 1 (Tall : Dwarf = 1 : 1)

• But if it has TT as the genotype, we will get all four plants as Tt( All tall)

OUT CROSS:

• Out cross is a type of cross of offspring with a dominant parent.

• It cannot be used to identify the genotype of an unknown offspring.

• If we cross either a plant with genotype Tt or TT with the dominant parent(TT), We will always get a tall plant(TT or Tt) and there is no use in it. As we already do not know the genotype of the plant.

CONCLUSION:

• All test cross are necessary to be a back cross as it is the cross between the parent and a recessive offspring.

• But all back cross is not necessary to be a test cross as the cross between offspring and a dominant parent which is a out cross(One type of back cross) is not a test cross.

NOTE 1: REFER ATTACHMENT FOR PUNNET SQUARE.

NOTE 2: IN PUNNET SQUARE I CONSIDERED tt AS FEMALE. IT CAN BE OF EITHER MALE OR FEMALE DEPENDING ON THE OFFSPRING.

Answer 2

Answer:

Testcross is a crossing of a F1 hybrid with one of its recessive parent or individual with similar genetic makeup. ... - But if back cross involves crossing of F1 generation with dominant parent, it's not a test cross. - Therefore, test cross is a backcross but back cross is not necessarily a test cross.