Mother's (not father's) genes can cause fraternal twinning, How sundogs got their
name
 If a man has twins
running in his family, can he in any way cause his sex partner to become
pregnant with twins? I say 'no' because twins happen
either because two
eggs are available to be fertilized (fraternal twins), OR one egg splits
(identical twins). He can't influence either event. Julie, Nassau, Bahamas
Twin sisters. Photo courtesy of Dustin M. Ramsey and Wikipedia.
Genetics is complicated. You are mostly right, probably. Until
1977, the general consensus was: identical twins do not
run in families and, therefore, are not caused by either parent's genes. But
Harvey et al challenged this viewpoint in 1977. He reported 10 families
who had multiple pairs of identical twins, and suggested we reexamine the
question. Both mothers and fathers were represented in the identical-twin
parents of identical twins.
In 2004, Hamamy et al reported following one extended
multi-generational family and its thirteen sets of identical twins. Hamamy
hypothesized a gene accounting for the twins and traced it back five generations
to a common grandfather. If this theory proves correct, identical twins
may, indeed, be attributable to a father's genes.
On the other hand, there is no question but heredity can influence the probability
of fraternal twins, and due to the mother's genes.
"The probability of a subsequent twin pregnancy is increased 4-fold in
mothers of twins, and the risk of having dizygotic [fraternal] twins is roughly
doubled for women whose mother or sister has dizygotic twins," writes
medical geneticist
Victor A. McKusick, professor at John Hopkins University.
How is the trait for fraternal twinning passed along through a family?
"Most experts on the subject believe fraternal twinning is transmitted only
through the female line either as a recessive trait or as a dominant
female-specific trait," emails
Mark Curran of
the San Gabriel Valley Perinatal Medical Group.
By the way, about twelve in every 1000 deliveries results in fraternal twins, but this
probability varies with ethnic group and geographic area, writes Curran. In Nigeria
fraternal twinning occurs at a rate of 49 twins per 1000 births. In Japan
the rate of fraternal twinning is 1.3 per 1000 births. Furthermore,
a comparison of twinning rates of 14 ethnic groups in California found twinning
rates per 1000 maternities of 13.20 for blacks, 10.05 for whites, and 7.18 for
Asians.
Approximately three in every 1000 deliveries are identical twins. This
average seems to be the same worldwide and the reasons are uncertain.
Further Reading:
What
causes identical twins?, WonderQuest
Twinning by Mark Curran, M.D., OB Focus
Twinning, monozygotic by
Harvey, M. A. S.; Huntley, R. M. C.; Smith, D. W, Journal of Pediatrics, 1977.
Familial monozygotic twinning: a report of an extended multi-generational
family by HA Hamamy, HK Ajlouni, KM Ajlouni, Australian Academic Press, 2004
Why are Sundogs called sundogs?
Lucy, Asheville, North Carolina
 Sundog.
Photo courtesy of M. Metz and Wikipedia.
I was playing tennis on a cold winter's day when I saw my first sundog.
"Look!" I said to my partner, "There's a rainbow in that cloud!" "A
sundog," she replied. What a strange name, I mused.
Maybe the ancient Norse first started calling sundogs 'sundogs.' Norse
epic poems (the Eddas and the Edda, which dates from before 1000 AD) spoke of
two wolves hunting the Sun and the Moon. Perhaps the Norse poems pictured
one 'dog' running in front of the Sun and one after — as sundogs.
Physicist
Jonas Persson of the University of Agder in Norway offers this interpretation in his essay, "Norse
Constellations."
In fact, the present Danish name for sundog is 'solhunde' (sol = sun, hunde
= hound, wolf or dog), emails Persson. The old Norwegian and Swedish names
are lost. 'Sundog' (meaning a small halo or rainbow near the sun) first
occurred in English literature in about the 1580's.
Further Reading:
What is
a sundog, and what causes it? WeatherQuesting
Norse constellations by Jonas Persson, Digitalis
(Answered April 7, 2008)
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