MadSci Network: Genetics

Re: Is it possible to clone a human when it has just been born?

Date: Fri Feb 2 08:45:19 2001
Posted By: Paul Szauter, Staff, Mouse Genome Informatics, The Jackson Laboratory
Area of science: Genetics
ID: 981048309.Ge

As you may know from the news, cloning of mammals was first achieved in 1997 
when Dolly the sheep was cloned by Dr. Ian Wilmut and his collaborators in 
Edinburgh, Scotland. Since then, a number of different species of mammals 
have been cloned, including mice and cattle.

When we say that an animal has been cloned, we mean that another animal 
having exactly the same collection of genes has been produced. All animals 
of a particular species have essentially the same genes, but there are 
variants of each individual gene in a particular species. These variants are 
called alleles. For example, mammals with dark hair have a gene that 
produces an enzyme (a kind of protein that catalyzes chemical reactions) 
called tyrosinase. The enzyme tyrosinase produces a dark pigment called 
melanin that is present in hair and skin. In mice, cats, humans, and other 
mammals, there are alleles of this gene that produce an enzyme that has very 
little activity, so very melanin is produced. Mammals have two copies of 
most of their genes. If both of the copies of the tyrosinase gene are the 
allele with little or no enzyme activity, the animal is an albino, with 
white hair or fur, pink skin, and pink eyes, and looks very different from 
other members of its species.

In a similar way, there are different alleles (often many different alleles) 
of each of the tens of thousands of genes that all mammals have. In 
addition, an individual can have two different alleles of most genes, 
because mammals have two copies of most genes. What this means is that most 
individuals in a population of a species are genetically unique. No two 
individuals in the population have exactly the same collection of all of the 
alleles of all of their genes.

One exception to this is identical twins. You might know some brothers or 
sisters who are identical twins. Identical twins are born when for some 
reason the early embryo (made up of two or four cells) splits into two 
embryos. Because the embryo started out as a single cell, all of the cells 
derived from that embryo are genetically identical, so the two embryos 
develop into a pair of genetically identical individuals. You could say that 
one member of a set of identical twins is the clone of the other.

When sheep, mice, cattle or other mammals are cloned, a nucleus (the part of 
the cell that contains the genes) is transferred from some kind of cell from 
a completely developed individual into a egg cell from which the pronucleus 
(the unfertilized nucleus) has been removed. The egg is then activated 
(caused to start development) by some sort of experimental treatment. The 
embryo is allowed to develop for a few cell divisions in a culture dish 
before being implanted into a 'foster mother' who will give birth to the 

The success rate on cloning mammals is not very high. One of the problems is 
that cells from a fully developed individual are differentiated into 
specialized cell types, such as skin cells, muscle cells, liver cells, and 
so on. These cell types are different because a different collection of 
genes is active in each cell type. Once a cell has differentiated, it has a 
very limited ability (or even no ability at all) to turn into a cell that is 
able to form other cell types. The cells of an early embryo are special 
because they are able to form all of the different cell types that make an 

In cloning experiments, it is not uncommon for only a few embryos out of a 
hundred to make it to live birth. In addition, some of the animals are born 
with birth defects that appear to result from an incomplete reprogramming of 
the differentiated nucleus back into a nucleus that can form all cell types.

Biologists don't think that people are very different from other mammals in 
terms of their basic genetics and development, so if you ask a biologist 
whether it would be possible to clone a human, they would answer that, yes, 
it is. So far it has not been done. There are two sorts of reasons fro this.

The first sort of reason is technical. In order to clone a human, scientists 
would have to obtain a supply of hundreds of unfertilized human eggs. This 
is technically possible. Unfertilized human eggs are collected at fertility 
clinics from women who very much want to have children but who, for some 
reason, are not able to have them normally. This involves treating a woman 
with hormones to cause an unusually large number of eggs to be released at 
the same time (instead of the one or two a month that are normally 
released). The eggs are then collected surgically, which involves some risk 
to the woman's health. It is possible to collect up to about twenty 
unfertilized eggs from a woman undergoing this procedure. Normally, these 
are fertilized with her husband's sperm, some are implanted, and others are 
usually frozen for later attempts.

Suppose that we could get twenty or thirty women to agree to undergo this 
procedure not to have children of their own, but to provide eggs for a human 
cloning experiment. We would then use hundreds of these eggs to accept 
nuclei from cells from a person that we wanted to clone. Most of these 
embryos would fail to develop normally, and there is a very good chance that 
even if we got to a live birth, the clone would have birth defects resulting 
from a failure to completely reprogram the nucleus.

This brings us to the second reason why this has not been done, which is 
that it would be unethical. We would have to ask dozens of perfectly healthy 
women to undergo a procedure involving some risk to their health to provide 
the raw material for an experiment that might produce a few children who 
have an excellent chance (much higher than for normal births) of being born 
with birth defects. The purpose of this experiment would be to produce a 
cloned human. There isn't any good reason to do this. We can learn all we 
need about genetics, biology and development in humans from doing ethical 
experiments, including the cloning of experimental animals.

As for using a newborn baby as the donor for the nuclei, there isn't any 
substantial difference between a newborn baby and an adult with regard to 
the significant processes of development. A newborn is far less different 
from a adult than it is from a fertilized egg.

This is an excellent web site on cloning:

The site above includes this Glossary:

You might also find this Glossary useful:

This is a Scientific American article on cloning:

This is a good cloning site also:

This is a site on the ethical issues of cloning:

Thank you for your interesting question. I hope that you find this answer 
useful, and that it helps you learn more about all the exciting work in 
biology and genetics that is going on right now.


Paul Szauter
Mouse Genome Informatics

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