Humans usually give birth to a single baby. Occasionally, two embryos develop together, each with its own placenta and umbilical cord, resulting in twins.
There are two types of twins and they arise in different ways.
Fraternal and identical twins Ovary Ovary Foetus Foetus Foetus Foetus The ovum is fertilised by one sperm A single ovum is released from the ovary The embryo splits into two identical embryos The zygote divides to form an embryo. Each zygote forms an embryo. Each ovum is fertilised by a different sperm Two ova are released from the ovary at the same time
Comparing fraternal and identical twins Offspring are not genetically identical to parent or other off spring. Off spring are genetically identical to each other. Parents and offspring are genetically identical . Identical twins Fraternal twins Description
The empty unfertilised egg cell gains a new nucleus from the adult body cell.
The embryo of this cell was then grown for a few days.
This embryo was then implanted into a sheep's womb.
The new animal was a clone of the animal that donated the nucleus.
Artificial cloning Unfertilised egg cell (ovum) Body cell 1. Nucleus is taken out of unfertilised egg cell Unfertilised egg cell (ovum) 2. Nucleus is taken out of body cell 3. Body cell nucleus is placed in empty unfertilised egg cell.