Re: cloning a female from a male

We have to distinguish in this answer between what is technically difficult 
but possible and what is impossible because of a known scientific reason. 
The feat that Heinlein describes does not seem to be scientifically 
impossible, just technically difficult. There is also an interesting 
genetics problem in your question.

So far, the mammals that have been cloned include sheep, mice, cattle, pigs, 
and goats. By cloning in this case I mean that live animals have been born 
as a result of replacing the pronucleus of an egg with a nucleus from a 
somatic cell. Please see the references in my earlier answer:
 /posts/archives/mar2001/
984508482.Ge.r.html

The recent announcement of human cloning was simply the creation of multiple 
genetically identical embryos with the same genotype; these embryos were not 
brought to term. It is unlikely that there will be any human cloning soon 
because of ethical considerations. The success rate in cloning other mammals 
is rather low, and some of the animals have been born with birth defects. 
There is no benefit to a given individual to be born as a result of cloning, 
and the procedure places their future health at risk.

No one has changed the sex of a future embryo by replacing an X chromosome 
with a Y chromosome or vice versa, but the sex of some animals has been 
changed experimentally by altering the genes that determine sex. This was 
done to confirm the hypothesis about which genes were responsible. Please 
see:

1. Swain A, Lovell-Badge R.
Mammalian sex determination: a molecular drama.
Genes Dev. 1999 Apr 1;13(7):755-67.

2. Werner MH, Huth JR, Gronenborn AM, Clore GM.
Molecular determinants of mammmalian sex.
Trends Biochem Sci. 1996 Aug;21(8):302-8.

It is difficult to handle whole chromosomes of mammals in the way the 
Heinlein suggests, but it is not impossible.

Now we come to the genetics question. A human male has an X and a Y 
chromosome. Because the X chromosome has many genes that are not found on 
the Y chromosome, a human male always has an X chromosome that is free of 
genes incompatible with viability. In addition, it is immediately apparent 
what the genotype of any human male is with respect to any genetic trait 
governed by an X-linked gene. Human females, with two X chromosomes, can be 
carriers of X-linked genetic diseases that show up in half of their sons. 
This is why diagnosis of X-linked genetic disorders like hemophilia, color 
blindness, and so on is much more frequent among males.

If a man were to have himself cloned, replacing the Y chromosome with 
another copy of his X chromosome, and everything worked, that person would 
be essentially genetically identical to the man, but would be female.

I haven't read the novel, but this is one way of exploring the idea of how 
men and women are different. It is a very interesting idea, because we would 
be able to see the differences between a man and a woman that were 
essentially the same genetically (there is no way to do this now). Of 
course, scientifically, this would be a better study if there were two 
clones, one male and one female, so that they could grow up in similar 
circumstances and have similar life experiences.

There are some differences between men and women that are easy to measure. 
For example, in terms of physical strength, the bottom 10% of men are about 
as strong as the top 10% of women. There are other differences that are 
commonly assumed as stereotypes, but which are harder to measure, or are 
just not verifiable. These would be things like musical ability, social 
skills, language skills, spatial reasoning, and aspects of personality.

There are many studies on identical twins (of the same sex, of course) being 
raised together vs. sets of identical twins raised apart. These studies can 
tell us the extent to which some characteristics are the result of 
environmental influences (which can be different for twins raised apart) or 
genetics (which will be the same for twins raised apart). It should be 
straightforward to find these studies. They do not, however, address the 
question that could be raised by studying male and female clones that were 
essentially genetically identical.

Thank you for an interesting question.

Yours,

Paul Szauter
Mouse Genome Informatics