Natural cloning also occurs in animals. Clones are produced by mitosis, so they are genetically identical to each other.Identical (monozygotic) twins are the main example of natural cloning in mammals, including humans. A single egg is fertilised to form a zygote. In the early embryo, the cells are totipotent. Very early in development, the embryo splits into two, and each part develops into a separate individual. The twins are genetically identical to each other, but not to either parent, because the zygote was formed by sexual reproduction.Many invertebrates reproduce asexually and form clones of the parent:• Budding: in Hydra, a small outgrowth develops on the body wall, grows into a new individual and breaks off.
• Fragmentation and regeneration: a starfish or flatworm can break into pieces, and each piece regrows into a complete animal.
• Parthenogenesis: in aphids and water fleas (Daphnia), females produce offspring from eggs that develop without fertilisation. When conditions are good, this lets the population increase very quickly.Asexual reproduction like this is fast and needs only one parent, but the offspring have no genetic variation, so the population may not survive environmental change.
Many of these animals also reproduce sexually at certain times, for example aphids when conditions worsen in autumn.Worked example: monozygotic twins occur in about 1 in 250 births. In a town with 2000 births a year, about 2000 ÷ 250 = 8 pairs of identical twins would be expected.
• Fragmentation and regeneration: a starfish or flatworm can break into pieces, and each piece regrows into a complete animal.
• Parthenogenesis: in aphids and water fleas (Daphnia), females produce offspring from eggs that develop without fertilisation. When conditions are good, this lets the population increase very quickly.Asexual reproduction like this is fast and needs only one parent, but the offspring have no genetic variation, so the population may not survive environmental change.
Many of these animals also reproduce sexually at certain times, for example aphids when conditions worsen in autumn.Worked example: monozygotic twins occur in about 1 in 250 births. In a town with 2000 births a year, about 2000 ÷ 250 = 8 pairs of identical twins would be expected.