Twins, test-tube babies, cloning and gamete banks
Usually one baby is born at a time, but multiple births happen, up to six in the book. Twins are the most common: the book gives about one birth in 86. There are two kinds. Fraternal twins form when two mature ova are released, from one or both ovaries, and each is fertilized by its own sperm. Two different embryos grow, genetically different from each other, each with its own sac and placenta, and they are no more alike than other brothers and sisters. Identical twins form from one ovum fertilized by one sperm. The developing cell mass separates into two masses, and each grows into an embryo. They have the same genes and mostly share one placenta. Rarely they are born partly joined (conjoined twins), and many can be separated by an operation. In test-tube babies a mature ovum from the wife’s ovary is fertilized outside the body with the husband’s sperm, in a dish with a nutrient medium. It is grown to the blastocyst stage and then replanted in the wife’s uterus, where development goes on until birth. Families have personal reasons for choosing this, and doctors carry it out. Cloning began with experiments on frogs and toads: the nuclei of embryo cells were put into egg cells whose own nuclei had been removed or destroyed, and normal animals grew, identical to the animal that gave the nuclei. This showed that the nucleus of an early embryo cell can direct the development of the egg just as the zygote’s nucleus does. Later work used an ordinary body cell, as with Dolly the sheep, made from a cell taken from the udder of an adult sheep. Gamete banks keep gametes of selected farm animals, such as cattle and horses, stored frozen at a very low temperature for many years, so they can be used even after the donors have died or for rare species. Some people also store their own gametes.
| Fraternal twins | Identical twins | |
|---|---|---|
| Ova and sperm | two ova, two sperm | one ovum, one sperm |
| How they start | two zygotes | one zygote whose cell mass splits in two |
| Genes | different | the same |
| Placenta | each has its own | mostly one placenta, shared |
| Technique | What is done | What it shows or gives |
|---|---|---|
| Test-tube baby | ovum and sperm meet in a dish; embryo grown to blastocyst; replanted in the uterus | development can continue in the uterus after fertilization outside |
| Cloning | a nucleus is put into an egg cell whose own nucleus was removed or destroyed | a body-cell or embryo-cell nucleus can direct development |
| Gamete bank | gametes are stored frozen at a very low temperature | gametes can be kept for many years |
Draw to compare, with a grown-up. On one side of a page draw two ova and two sperm joining to make two zygotes, and from each one a separate embryo with its own sac. On the other side draw one ovum and one sperm making one zygote that divides, splits into two masses of cells, and makes two embryos that mostly share one placenta. Write under each drawing whether the twins have the same genes or different genes.