[Jacob-list] Sex ratios

[Anderson, Gary B.]

I should be at home doing yard work, or at least sitting in our pasture
watching our Jacob sheep lollygag in belly-deep grasses of springtime.
Instead, I'll summarize what I know about artificial control of sex ratio.
My original message posted on the List was one that I sent to an individual
in response to questions that she directed to me.  I subsequently gave
permission to post my reply, which wasn't intended as general coverage on
the subject of sex ratios.  This one is.

Throughout history, humans have been intrigued with the prospect of exerting
control over the sex of their offspring and their animals' offspring, and
they developed clever ways to enhance the chances of producing male or
female progeny.  The ancient Greeks advocated having women lie on their
right side during intercourse to produce boy babies and on their left side
to produce girl babies.  A later Slavic custom involved squeezing the right
testis at ejaculation to produce a boy; if this proved to be awkward, a man
could achieve the same result by biting his wife's right ear.  (And the
Slavs were right: biting the wife's ear is just as effective at influencing
the sex ratio as squeezing the right testis.)  Regarding control of sex
ratio in animals, techniques have included affecting the side from which a
male dismounts after mating, whether animals mate into the wind or with the
wind at their backs, and when mating occurs relative to a full moon.

While I was a graduate student in the early 1970s, a book was published
titled 'Your Baby's Sex'.  It topped the best-sellers list for months; the
author made a bundle.  The author explained how couples could affect the sex
of their soon-to-be-conceived child via timing and frequency of intercourse
relative to ovulation.  The technique was based on changes in vaginal pH,
sperm survival and a number of other principles either for which no
scientific documentation of their validity was available or that had not
withstood scientific scrutiny.  Twenty-something years later, after having
presented my annual lecture on efforts to affect the sex ratio, senior
undergraduates enrolled in my Reproductive Physiology course came forward to
tell me that their parents had used the technique; about half the time their
parents had gotten the sex they are after.

A large number of patents have been awarded for sex-control gadgets and
techniques.  They involve centrifuging sperm cells, exposing them to an
electrical field or to various antibodies, making sperm swim through a
viscous matrix, and just about anything else one can think of to do to a
male's body parts or his sperm cells.  Private companies will provide the
service; if you give them your money, they will sex your semen.  One company
(no longer operating) claimed a money-back guarantee; if a client didn't get
the desired sex, his/her money was returned.  On average, I suspect that the
company got to keep the client's $$ about half the time.

The idea that orthodox Jews have more boys circulates from time to time, as
does the one that British deep-sea divers and bulls taken from sea level to
a high elevation produce an excess of sons.  One should evaluate the data
and the circumstances under which they were collected.  (Data might exist,
but I haven't see them.  I would like to see these data if anyone knows
where they are available.)  One proponent of a highly touted procedure
espouses the swimming speed idea, but the only data that support it are from
human clinical trials conducted in obscure places and provided by clinicians
who profit from the procedure.  Results of controlled and peer-reviewed
experiments showed no effect.  The results of enormous field trials with
cattle showed no effect.  A scientific review on timing of insemination and
sex ratio published in Theriogenology (vol 52, beginning on page 1273)
presents the overall conclusion that if there is an effect, it is small.

Researchers at USDA Beltsville developed a procedure using expensive and
sophisticated equipment (a cell sorter) using lasers and other stuff that I
can not begin to understand.  The researchers demonstrated conclusively that
they could separate X- from Y-bearing spermatozoa based on the different
amounts of DNA in the two types of sperm.  (Generally, but not always, the Y
chromosome is smaller than the X chromosome; thus, an X-bearing spermatozoan
would be expected to have slightly more DNA than a Y-bearing spermatozoan.
Since the X and Y chromosomes generally are large compared with other
chromosomes, the difference in total DNA can be a few percentages.  This
difference is sufficient for the precision of a cell sorter.)  A company
associated with Colorado State University (XY Inc.) has licensed the
technology and is attempting to improve it for practical use in animals.
There is little doubt that X- and Y-bearing sperm can be separated, but
originally the machine had to run all day to produce a sufficient number of
'sexed' sperm cells to inseminate one female.  Most research documenting the
validity of the procedure was done with surgical insemination into the
oviduct (impractical) or in vitro fertilization, both of which require small
numbers of spermatozoa.  XY Inc. has conducted field trials in cattle using
sexed semen and standard AI techniques and achieved reasonable fertilization
rates and excellent skewing of the sex ratio (e.g., 90% in the desired
direction).  The results of these trials have withstood scientific review
for publication in professional journals.

Separation of X- and Y-bearing spermatozoa based on documented differences
in size of the X and Y chromosomes sounds reasonable, and it took the
development of sensitive and precise equipment to achieve the separation.
Procedures based on other characteristics of the two types of sperm are
difficult to justify from a biological standpoint.  With only a few
documented exceptions, genes in a sperm cell are considered to be inactive
(because the DNA is bound by proteins called protamines, which are removed
from the sperm DNA only after the sperm has penetrated the egg).  If the
genes of an X- versus a Y-bearing spermatozoan are not being expressed, how
would differences in the sperm cells be expected or explained?

I'm going home to watch my sheep play in the grass.