[Jacob-list] Sex ratios

[fay, gary]

Dr. Anderson,

One observation in humans is that if the woman percieves that the
man is inadequte, ro the resouces in the area are scarce, that
the ratios become squewed toward the female for "ease of reproduction".

The theory in nature being that it is easier for a female to  spread her
parents genes than it is for a male. ( Females just have to show up, males
must compete.) There was a 'slight' bias seen in ratios seen on income in
survey studies of humans where, richer families tended to have
more sons than poorer families.

Gary Fay

"Anderson, Gary B." wrote:

> 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.
>
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