Newspaper Page Text
Part fifteen in a series of articles
In search of
X Chromosome
Linked
Homosexuality.
by Dr. Donald W. Smith, PhD.
Although the study of pairs of twins and
family trees had established that homosexuality
appeared to run in families, proving that homo
sexuality was genetically based, at least in some
cases, would require isolating a gene that was
unique to a group of homosexuals. The problem
in doing this is daunting. Humans, with very rare
exceptions, have 46 chromosomes in 23 matched
pairs. Each parent contributes one set of the 23
chromosomes, one for each matched pair. Each
chromosome contains thousands of genes. In
order to isolate a trait confined to one chromo
some would require analyzing the genes on all 46
chromosomes of multiple individuals in several
families. Therefore, the search for a “gay” or
“lesbian” gene (if it exists) could take decades
unless there were some way to narrow the search.
If a characteristic is determined by more than one
gene on several chromosomes, the search can
become nearly impossible because number of
different sets of 46 chromosomes that can poten
tially be produced by one set of parents is in
excess of 70 trillion. However, effects of genes on
one type of chromosome, the X chromosome, is
easily studied in males.
The genes that determine maleness and
femaleness are largely confined to the pair of so-
called sex chromosomes X and Y. If one has two
X chromosome (one X from the mother and one
X from the father), that person is anatomically
female almost all the time. If an individual has
one X and one Y chromosome (the X from the
mother and the Y from the father), that individual
is anatomically male most of the time. (For excep
tions, see article 14 in this series.)
The X chromosome contains genetic infor
mation influencing a lot more that anatomic sex.
For example, it is one this chromosome that carries
the gene for one of the most common forms for
hemophilia as well as a form of retardation. Both
hemophilia and fragile-X retardation are diseases
primarily of males. Genetic traits carried on the X
chromosome are more likely to be seen in males
because, unlike females, they have only one X
chromosome. A female who carries the gen,e for
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hemophilia or fragile-X chromosome retardation is
likely to have a second X chromosome normal that
offsets the influence of the genes for hemophilia
and fragile-X retardation.
A result of the fact that men can only receive
an X chromosome from their mothers and pass that
Figure 1
NR = not reported, % = per cent gay/lesbian.
Father's
brother
1.7%
Father's
sister
sister brother
NR 7.3%
Mother Father
Mother's Mother's
sister's brother's
Father's Father's
brother's sister's
Brother
13.5%
X chromosome on only to their daughters, there is
a particular pattern in the appearance of a xlinked
trait among relatives that makes it fairly easy to
spot. ■
Because X-linked traits in a man comes from
his mother, the trait should be seen in her male
relatives. Therefore, she is expected to have broth
ers as well as uncles, nephews and male cousins
who have the same trait. Conversely, there should
be few if any male relatives from his father’s side of
the family with the same trait. A man with an X-
linked trait is likely to share that trait his brothers
and his sisters are likely to carry the trait because
they share the same mother. In turn, his sisters are
likely to pass on
the X-linked trait to their sons. If he has children,
none of his sons will have the trait because they can
only receive a Y chromosome from him, never an
X. Therefore, none of his sons’ male children will
have the trait. All of his daughters will be carriers of
the trait, because he has only one X chromosome
to pass on to them. Therefore, he is likely to have
grandsons with the trait bom to his daughters.
In that this pattern is fairly easy to establish by
studying family frees, it was one of the patterns
looked for by a research team at the Laboratory of
Biochemistry in the National Cancer Institute, a
branch of the National Institutes of Health (NIH).
The team headed by Dr, Dean Hamer recruited
76 self acknowledged homosexual men from an
HIV clinic at NIH, the Whitman-Walker Clinic
and homophile organizations in The Washing
ton, D.C. area. One hundred-twenty-two rela
tives of the sample of 76 homosexual men also
took part in the study. An additional 38 pairs of
homosexual brothers and their relatives were
recruited through national and local gay publica
tions.
Sexual orientations of the recruits and their
relatives were confirmed by asking them to rate
themselves on measure of sexual self-identifica
tion, sexual attraction, sexual fantasy and sexual
behavior. These rating were based on recent
behavior, thoughts and feelings. One hundred
and fourteen gay men were asked to rate their
male relatives as an admitted homosexual (Kinsey
5 or 6) or not definitely known to be homosexual
(straight, bi, or unknown). The reliability of the
gay men’s assessment of their male relatives was
tested by contacting a sample of ninety-nine of
their male relatives. The gay men were extremely
accurate in their classifying their male relatives. Of
the 69 identified as being homosexual all of them
were. Of the 30 relatives identified as not definitely
known to be homosexual, 27 confirmed this sta
tus, two refused toanswerall thequestionsand one
said he was asexual.
In order to identify families with exception
ally high rates of homosexuality, Hamer team
estimated the rate of homosexual males in the
general population as two percent. Their estimate
was based on a study of the rate of homosexuality
in the uncles and male cousins of lesbians which
has not been published yet. Although the rate of
Fall 1993