GEORGE F.W. HAENLEIN
Extension Dairy Specialist
University of Delaware
NEWARK, Del.—Recently I
wrote about DNA as the latest and
most powerful tool for progress in
modem animal science and animal
production.
I reported on the reliable new
method for finding out whether a
calf is a red-color barrier. Now,
there is a second DNA-based pro
cedure in the very important area
of sex determination.
For years I have wondered
about the question of sex determi
nation. In our University of Dela
ware herd and in others I have
observed a preponderance of male
calves when older semen ampules
from famous bulls were used.
Often I received bull semen
donations from Holstein or Guern
sey breeders at their dispersal sale.
The semen typically has been
stored for a time and is often from
older bulls that have reached fame
as superior performers by the
majority of daughters they’ve pro-
duced, and a performance reputa
tion takes time to establish.
Although I had discussed this
older semen issue with bull stud
managers, I never succeeded in a
research project to collect appro
priate data for reliable analysis. I
am still convinced there is some
thing to this conjecture.
We know that there arc two
kinds of spermatozoa: those that
determine female sex and those
that determine male sex in
offspring.
And the physical difference
between the two is that the male
sex-determining spermatozoa
have only the small Y chromo
some, while the female
determining spermatozoa have the
big X chromosome.
If there is a difference in size,
that must also mean a difference in
weight or gravity or migration
during storage.
When semen is stored, some
spermatozoa could settle out
because of greater weight If more
of the big-chromosome spermato-
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More On DNA and Dairy
zoa settle out than the small
enromosome ones, this could
explain why more of the small
male chromosome spermatozoa
are left in “solution,” which, when
used on insemination, would pro
duce mostly male calves.
This is a plausible answer to my
puzzle of too many bull calves
from older semen—at least more
than a SO percent chance.
A reinforcement of this theory
came from botany.
I knew that when pollen grains
are added to a blooming flower’s
stamen, they migrate down inside
toward the egg cells of the flower
for fertilization.
But while they move down
ward, some pollen grains travel
faster than others. It’s the male
pollen with the smaller Y chromo
somes that migrates faster.
This has been proven by cutting
the channel in half at a certain
time interval and physically pre
senting the slower female pollen
from reaching the eggs, thus pro
ducing a different sex ratio in the
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seed.
Does that not also sound like
the case of semen from older bulls
that has been in storage for some
time?
Well, over the years, many sci
entists have tried to find methods
to separate Y from X spermatozo
Electric currents and vinegar
douches have often been success
ful, but the semen died in the pro
cess, much like the proverbial
surgery that was successful except
that the patient died.
Now along comes DNA, the
new magic.
Of course, for some time, we
have been able to see under the
microscope whether a fertilized
egg is male or female. The chal
lenge has been to predetermine, in
a practical way, which egg is
going to be male or female by
sorting out the respective semen.
Remember, it is only the sper
matozoa that make the difference;
eggs are always only female,
because females (cows) are
State Zip _
homozygous for the X chromo
some, while males (bulls) are
always heterozygous for X and Y
chromosomes, producing them
half and half.
Mating with the homozygous
female X chromosomes follows
the third lav/ of the discoverer of
genetics, Gregor Mendel, which
states that the offspring numbers
between a homozygous and a
heterozygous parent are always in
a 50:50 ratio; i.e., half of one
parent’s type, homozygous, and
half of the other parent’s type,
heterozygous.
Unlike the situation with black
or red color in Holsteins (where
black is dominant over red and red
is not visible in the animal’s
appearance unless it is homozyg
ous for red), in sex there is no
dominance.
A heterozygous animal for sex
is visibly male and there is no
homozygous YY animal, only
heterozygous XY and homozyg
ous XX.
The story of the new DNA test
for finding and separating the
male determining Y spermatozoa
comes from research at the USDA
Experiment Station in Beltsville,
Md., by Dr. Larry Johnson (Dairy
Profit Report, University of
Wisconsin-Madison, vol.
7(2): 1-2).
Johnson determined that X
chromosomes have about 4 per
cent more DNA than the smaller Y
chromosome; he used this fact to
separate spermatozoa.
Spermatozoa are treated with a
bluish fluorescent dye, and
because X spermatozoa contain
more DNA than Y sperm, the X
sperm absorb more dye.
This then is measured by an
optical detector in a so-called flow
cytometer, which can analyze and
sort microscopic cells.
Also, there, the more luminous
X sperm are earmarked with a
positive charge, the less luminous
Y sperm with a negative charge.
Invisible to the naked eye, the
dye-charged sperm flow single
file past deflecting plates, separat
ing the positive and negative
sperm into two containers.
' When cow ova are fertilized in
vitro by the separated sperm, the
success rate of obtaining male or
female calves has been 90 percent
With a new procedure of
- "jesting ova from cows’ ovaries
directly and repeatedly, called
ultrasound-guided follicular aspi
ration, the use of the separated
sperm is ready for practical wide
use, because much less sperm are
needed.
The cytometer separation of
sperm is actually not fast requir
ing about 40 hours to separate 20
million sperm needed for one arti
ficial insemination.
For the in vitro insemination
only a few thousand sperm are'
needed, thus 200 to more ova can
be fertilized from separated sperm
sorted in one hour. In a study with ’
a British company in 1592, pre
determined 40 bull chives were
produced from 50 births.
So AI (artificial insemination),
hie great dairy success story, and
ET (embryo transfer), the more
iccent extension of that success
story, now have the culmination of
progress potential for breeding
wily one sex from the top cows in
the herd with the top bulls in the
world.
What a difference from grand
father s cow farm! And what a dif
ference in profit potential! *
What next? In terms of worid
ecology and environment, maybe