Download LACK OF LACTASE ACTIVITY: FOLLOW UP READING

LACK OF LACTASE ACTIVITY: FOLLOW UP READING
Genetic and Evolutionary Background
Baby mammals feed exclusively on milk from their mothers. After a species-specific amount of
time, they are weaned off of mother’s milk and onto other foods. One species, however,
continues to feed on milk long after they have been weaned – Homo sapiens (humans). And
actually, only a small proportion of that species can actually digest milk.
Young mammals naturally produce the enzyme lactase so they can digest mother’s milk. The
lactase gene is located on chromosome number 2. The lactase protein (enzymes are proteins)
produced from this gene is almost 2000 amino acids long.
As young mammals are being weaned, production of lactase slows down and stops. The gene
that contains the DNA sequence that codes for the production of the lactase stops being
transcribed. In other words, that stretch of DNA no longer produces mRNA, which means that
no mRNA goes to the ribosomes for translation, and therefore, no proteins get produced. This
stoppage of transcription is controlled by a regulatory section of DNA near the lactase gene.
In humans, the gene is usually shut down by about age four. For most people, this means that if
they do eat or drink lactose containing food items, they suffer from the effects described earlier.
They are considered lactose intolerant and can’t digest milk products as adults.
In some people though, a mutation in the regulatory region means that transcription of the lactase
gene is not shut down. These people continue to produce lactase into adulthood and can
continue to digest milk products. Since this is not the usual pattern, nor is it the ancestral
condition, some people don’t like to use the term lactose intolerant. The normal condition is to
not be able to digest lactose, and those that possess this recent mutation are described as being
lactase persistent.
Overall, about 40% of the world is lactase persistent, meaning they can still digest lactose. The
numbers though, are highly variable with the region of the world. Almost no Native Americans
are lactase persistent, compared with 5% of Asians, 25% of African and Caribbean peoples, 50%
of Mediterranean people, and 90% of Northern Europeans. There is generally a strong
correlation with a cultural history of having dairy animals. Whereas the percentage is low
overall in Asia, in Mongolia where the people regularly drank horse milk, the lactase persistent
mutation is generally more common. Similarly in Africa, the overall percentage is low, but some
tribes had dairy cattle and these locations tend to have a higher prevalence of the mutation.
The first mutation is thought to have arisen amongst a group of dairy farming people in central
Europe about 7500 – 8000 years ago. They had been dairy farming in that region for
approximately 1000 years at the time. This mutation provides many advantages – it allows for
an additional nutrition source, it is a resource that is steadily available throughout the year, and it
is parasite free. As people with this mutation thrived, they spread throughout northern Europe,
bringing this mutated gene with them. The gene is spread as they move but also as they mate
with other peoples.
It is thought that lactase persistence evolved in Africa by a completely different mutation that
also allowed the lactase gene to continue to be expressed into adulthood.
References:
Itan Y, Powell A, Beaumont MA, Burger J, Thomas MG (2009) The Origins of Lactase Persistence in Europe. PLoS
Comput Biol 5(8): e1000491. doi:10.1371/journal.pcbi.1000491
http://www.usatoday.com/tech/science/2009-08-30-lactose-intolerance_N.htm?csp=34
http://www.eurekalert.org/pub_releases/2007-02/ucl-eeu022607.php