Question

Which of the following conditions is a hybrid breakdown?
A. Failure of a hybrid adult to produce functional gametes
B. Failure of the fusion of ova and egg of two species
C. Failure of the hybrid zygote to develop into an offspring
D. None of the above

Answer 1

Hint: Type of reproductive failure condition when the offspring of the ${{F}_{2}}$ generation are physiologically inferior to ${{F}_{1}}$ generations.
The hybrid breakdown is mainly due to the disruption of interaction of genes present on different loci as the genes segregate after the ${{F}_{1}}$ generation. Thus the correct answer is the failure of hybrid zygote develops into an offspring.

Complete answer:
-The hybrid breakdown is largely due to the incompatibility of genes at the unlinked epistatic loci.
-It appears after the ${{F}_{2}}$ generation i.e. the grandchildren of crosses between species.
-The hybrid breakdown is the inviability or sterility observed only in the ${{F}_{2}}$ generation or later on generations.
-Another reason for the hybrid breakdown is incompatibilities between the nuclear genome and the organellar genomes present in the mitochondria and the chloroplast.
-It is a reduction in the survival of ${{F}_{2}}$ or back-cross population from fertile hybrids produced by intercrossing genetically disparate populations or species.
Examples- hybrid breakdown can be seen in the cottonseed.
So, the correct answer is (c) Failure of the hybrid zygote to develop into an offspring.

Note:
-When the two interspecies crosses give ‘ ${{F}_{1}}$ hybrid’. ${{F}_{1}}$ hybrid derived may encounter pollen sterility or hybrid lethality.
-Hybrid breakdown refers to the ${{F}_{2}}$ or later on generation.
-The hybrid breakdown is seen in the offspring of hybrids rather than hybrids themselves.
-It is a postzygotic reproductive isolating mechanism.