MadSci Network: Genetics
Query:

Re: Why is the percentage rate of identical twins higher depending on the race?

Date: Wed Nov 8 20:43:25 2000
Posted By: Steve Mack, Post-doc/Fellow, Molecular and Cell Biology, Roche Molecular Systems
Area of science: Genetics
ID: 972872150.Ge
Message:

Well Wendi, I have to tell you that I think you might be laboring under a few misconceptions. Few if any of the factors involved in the development of monozygotic (identical) twins from a single fertilized zygote are known. In general, the worldwide frequency of twins is about 1 for every 250 births (0.4%), but this incidence of twin births is largely the same regardless of the mother’s race, age, the number of pregancies she has had, or any hereditary familial factors. On the other hand, the incidence of dizygotic (fraternal) twins depends heavily on all of these factors. For example, women who are fraternal twins give birth to fraternal twins about 0.8% of the time, and older women seem to be physiologically more likely to have dizygotic twins than younger women. But as far as I know, there are no biological determinants for racial differences in the frequency of monozygotic twin births.

In fact, I would like to take a quick moment to point out that there is really no biological basis for the concept of race. There is no gene or set of genetic characters which correlate with the racial distinctions that have so much meaning in our culture. From the perspective of a scientist, race is a purely cultural phenomenon. The few characters which we associate with race, (eye, hair and skin color, and facial characteristics) seem to be under the control of a very small number of genes, but the allelic variants of these genes are distributed all around the world.

I have found a study in which you might have an interest. This is a report from the Centers for Disease Control (CDC) on twin and triplet births between 1980 and 1997. In general, this study found that the number of twins being born increased dramatically over this 18 year period. Most of the growth in the number of twins was due to an increase in the age at which women were having children, so that more children were born to mothers aged 45 to 49 in 1997 than in all 10 years of the 1980s. Now, this study does not specify if the twins are monozygotic (identical) or dizygotic (fraternal), but it does say that the rate of twin births in non-spanish speaking European American women had risen to match the rate for African American women over the course of the study. 29/1000 for non-Spanish speaking European American women, and 30/1000 for African American women. Perhaps the earlier higher rate for African American women was the source of your idea that African American women were more prone to have identical twins.

References:

The Citation for the CDC study is:

Martin Ja, Park MM. Trends in twin and triplet births: 1980-97. National vital statistics reports; vol 47 no. 24. Hyattsville, Maryland: National Center ofr Health Statistics. 1999.

For information on the lack of any biological basis for race, take a look at these papers:

Owens K and King MC. (1999) Genomic Views of Human History. Science. 286 : 451-453.

Barbujani G, Magagni A, Minch E, Cavalli-Sforza LL. (1997) An apportionment of Human DNA diversity. Procedures of the National Academy of Science . 94 :4516-4519.

For information on the genes that control skin, hair, and eye color look at these papers:

Valverde P, Healy E, Jackson I, Rees JL, Thody AJ. (1995) Variants of the melanocyte-stimulating hormone receptor gene are associated with red hair and fair skin in humans. Nature Genetics. 11 : 328-30.

Rana BK, Hewett-Emmett D, Jin L, Chang BH, Sambuughin N, Lin M, Watkins S, Bamshad M, Jorde LB, Ramsay M, Jenkins T, Li WH. (1999) High polymorphism at the human melanocortin 1 receptor locus. Genetics. 151 :1547- 57.


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