Question

We talked about the genes controlling our ability to digest lactose, the sugar found in milk. Collectively the enzymes produced by the set of genes involved are called lactase. "Lactose intolerance" describes the most common condition in human adults, which is an inability to digest lactose because lactase is not produced. These people could all produce lactase as infants, however, so they have the genes needed to produce functional enzymes. Somehow the regulation of expression of the genes changes as most people grow up.

People who can digest lactose as adults are said to be "lactase persistent", that is, they continue to produce lactase even in adulthood. Ponder the genetic mechanisms for regulating the expression of the genes producing lactase and describe three feasible hypotheses for mutations affecting the regulating mechanisms that would enable a person to produce lactase as an adult.

Answer 1

Lactose Intolerance:

Lactose intolerance may be caused due to various reasons like:

1. Inherited lactose intolerance- genes of lactase deficiency is inherited. It may be further subdivided as:

a) Primary lactase deficiency- In this case, lactase deficiency occurs gradually or declines after a certain age. This is the most common type of lactose intolerance and is also called lactose on persistence. This may occur at any age, but usually observed above the age of six years.

b) Congenital lactase deficiency- In this case lactose intolerance is observed from birth.

2) Lactase deficiency may also be formed due to external reasons, like:

a) Developmental lactase deficiency- This is a transient form and is observed in premature babies. The condition usually does not last when baby becomes older.

b) Secondary lactase deficiency- Due to injuries, infections, or other diseases, the intestinal may be damage, resulting in lactase deficiency.

Genetics:

• Lactose digestion is mediated by the enzyme lactase (produced by intestinal cells), which is encoded by LCT gene, long arm chromosome 2 (2q21).
• In infancy, LCT gene is active, but lactase production is supressed during adulthood.
• Individuals with cystine (C) residues (MCM6 gene) near LCT gene on both the alleles, are supressed to produce lactase in adulthood, and become lactose intolerant. If C is mutated to T, then the individual become lactose tolerant. The phenomenon is called lactase persistence.
• Lactose intolerance is a recessive trait, resulting in inherited or congenital disorder.

Hypotheses for mutation, resulting in lactase production in adulthood.

• Genetic mutations have led to evolution of lactase persistence (LP).
• LP is a genetically controlled trait, observed in populations of Northern part of Europe, Some parts of Africa, Middle east, South Asia.
• Mutations that were hypothesized are:

1. Darwinian theory: mutation resulted in a survival advantage to individuals, on milk diet. This theory applies to the regions that has lactose tolerant population.

2. Markov’s hypothesis: Evolution of lactose tolerance during course of evolution, associated with dairying group. Associated with culture that keeps animals that milks. Related to alternative hypothesis of latitude theory. Modified to Adaptation theory- co-existence of lactose tolerant/ non-tolerant group in dairying population.

3. Latitude hypothesis: Lactase gene mutation associated and characterised by latitude and climate. Associated with Northerly-dwelling culture group.