Question

Identical twins will be produced, when
A) One spermatozoan fertilizes two ova.
B) One ovum is fertilized by two spermatozoa.
C) Two eggs are fertilized.
D) One fertilized egg divides into two blastomeres and they become separate.

Answer 1

Identical twins are also termed as monozygotic twins and they share all of their genes and are always of the same sex.

Complete answer:
Fertilization occurs when male gamete (sperm) unites with the female gamete (ovum) forming a diploid zygote which develops into an embryo in the later stages of life, then into a fetus. There can be various events that can occur when sperm fertilizes the ovum. One such event is the formation of monozygotic twins who are identical to one another. Let us analyze the options to see how identical twins are produced.

Option A: One sperm cannot fertilize two eggs at a same time. This event is thus unlikely to happen and does not generate identical twins. Therefore, this is the incorrect option.
Option B: When one sperm fertilizes two eggs, it is termed as double fertilization. This happens in about 1% of the cases and the fertilized egg usually does not survive.Therefore, this is the incorrect option.
Option C: When two eggs are fertilized by two sperm the twins are called dizygotic twins. They have different genetic material and they are not identical to one another. Fraternal twins are formed in this case. Therefore, this is the incorrect option.
Option D: when one sperm fuses with an egg and when it is divided into two blastomeres, the genetic material gets divided equally and two identical eggs are produced. They are thus called monozygotic or identical twins. Therefore, this is the correct option.

Thus, the correct option is D) One fertilized egg divides into two blastomeres and they become separate.

Note: Fraternal twins share half of their genes and can be of different sex unlike identical twins.Environmental factors play a major role in modification of genetic material. Thus, even identical twins share 99.9% of genetic similarity.