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    why is lactose tolerance also called lactase persistence?


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    Lactose Intolerance (Lactase Non

    VIVO Pathophysiology

    Digestive System > Small Intestine

    Lactose Intolerance (Lactase Non-Persistence)

    Lactose intolerance is a deficit in the ability to digest lactose, and is due to a relative lack of the lactase enzyme in the small intestine.

    Milk is rich in lactose (roughly 40 grams/liter for cow's milk), which serves as the major carbohydrate energy source for infants and young animals. However, dietary lactose cannot be absorbed intact. Rather, it must be hydrolyzed into its constituent monosaccharides glucose and galactose to allow transport across the epithelium. This hydrolysis is dependent upon lactase, a brush border enzyme in epithelial cells in the small intestine.

    Developmental Changes in Lactase Expression

    In mammals, the normal course of events is for the newborn to subsist on milk over the first few months of life, then be weaned and rarely if ever consume milk again. It thus makes perfect sense that mammals have evolved a developmental pattern of small intestinal gene expression that promotes high level production of lactase early in life, followed by a turnoff of lactase expression around the time of weaning. This is indeed what happens in almost all mammals, including most humans.

    A large majority of humans show the typical loss of lactase expression early in life. This downward progression begins at 2 to 3 years of age and is generally complete by 5 to 10 years. Thus, most teenagers and adults have minimal quantities of lactase in their small intestine, and are unable to digest all by a small amount of lactose - they are lactose intolerant and are said to have the trait of .

    Some humans do not turn off lactase expression during childhood, and continue to produce considerably quanities of the enzyme into adulthood. This trait is called and allows those individuals to tolerate consumption of lactose sources, including milk.

    Different populations of humans have different frequencies of the two lactase expression phenotypes. In general:

    (lactose tolerance) is seen predominantly in individuals with northern European ancestry, especially Scandinavian, and in certain other populations, including some of the nomadic peoples of the middle east and Africa.

    (lactose intolerance) is observed in a majority of the world's populations, including most of those with Asian or African forebearers.

    Genetics of Lactase Persistence

    Lactose persistence and non-persistence reflect inheritence of different alleles of the lactase gene. Lactase persistence, and therefore lactose tolerance, is inherited as a dominant trait. Lactose intolerance is the result of being homozygous for the recessive lactase allele that is poorly expressed after early childhood. Being homozygous or heterozygous for the persistence allele allows lactase expression after the time when lactase expression is down-regulated. In some circumstances, heterozygotes can manifest partial intolerance, indicating that this is an incompletely dominant gene.

    There are a number of polymorphisms within the human lactase gene, and "persistence genes" appear to have arisen multiple times independently in human populations. Data from several studies indicate that in European populations, the difference between persistence and non-persistence results from the difference in a single nucleotide located 13,910 bases upstream of the transcriptional start site of the lactase gene. In virtually all cases examined, a T at this position is associated with lactose persistence, while a C is observed in the non-persistent allele. Interestingly, this polymorphism that appears to potently affect lactase gene expression is embedded in an intron of an adjacent gene. Different single nucleotide polymorphisms at roughly -14,000 bp relative to the transcription start site of the lactase have been implicated in lactase persistence within African populations. The mechanism by which these minor differences in DNA sequence affect lactase gene expression is not known.

    The apparent convergent evolution of lactase persistence among human populations is best explained as an adaptive response to the shared selective pressure resulting from domestication of dairy animals and consumption of milk during adulthood. Indeed, sequencing of DNA from skeletal remains of eight neolithic individuals who lived across Europe between 5800 and 5000 years BC revealed that all were homozygous for the lactase non-persistence allele, lending support to the hypothesis that the lactose-persistence mutation was selected for following the domestication of dairy animals.

    Clinical Signs and Diagnosis of Lactose Intolerance

    Three forms of lactose intolerance are recognized:

    : the is the common form of lactose intolerance in which lactase gene expression turns off in childhood, and the individual is relatively lactase deficient as a teenager and adult

    : a number of diseases affecting the small intestine (inflammatory conditions, viral infections) can result in temporary lactase deficiency in individuals that are normally lactose tolerant. In most cases, this deficiency resolves in a few weeks.

    : this is a very rare disorder in which lactase is deficient from birth

    Clinical signs of lactose intolerance are triggered by consumption of more lactose than can be readily digested. There is substantial variability among intolerant individuals in the extent and severity of clinical response to ingesting even the same quantity of lactose.

    Source : www.vivo.colostate.edu

    Lactase Persistence

    Lactase Persistence

    Lactase persistence is a likely result of the advent of dairying and the result of natural selection.

    From: Encyclopedia of Human Nutrition (Third Edition), 2013

    Related terms:

    LactaseProteinPhenotypeGenotypeDNAAlleleLactose IntoleranceLactase DeficiencyLactose

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    How Genetic Transmission Works

    Martin Kohlmeier, in Nutrigenetics, 2013

    2.2.2 Inheritance patterns

    Many genetic traits are inherited in classical Mendelian fashion. This means that both parents contribute equally because one chromosome of each pair comes from the mother and the other from the father. The transmission to the next generation of a variation on one of the numbered chromosomes (autosomes) is called autosomal inheritance. If an inherited allele always prevails over alternative ones, the inheritance pattern is called autosomal dominant. If the allele is eclipsed by the alternative, it is called autosomal recessive. Let us take the example of adults who cannot drink large amounts of milk because their lactase gene in the small intestine is turned off. This is an autosomal recessive trait for all practical purposes. This means that inheritance from only one parent is not enough to make it apparent. If someone is lactose intolerant, the trait has probably come from both parents.

    Inheritance of X-linked traits is slightly different, of course. In this case all males inherit one gene dose of the trait from the mother, but none from the father (because he has contributed the Y chromosome instead). A classic example of an X-linked nutrigenetic inheritance pattern can be seen with glucose 6-phosphate (G6PD; OMIM 305900) deficiency. If males with G6PD deficiency eat broad beans or take certain medications, some of their red blood cells may break up (hemolyze). Females have the same problem only if both of their X chromosomes are affected, which happens less often. This means that the condition is usually inherited from mothers who are symptom-free carriers; rarely from fathers. Affected fathers can transmit the trait only to daughters, who will not be symptomatic unless they get a second affected X chromosome from their mother.

    The inheritance of other traits follows more complex patterns. Genomic imprinting, variable numbers of nucleotide repeats, copy number variants, multilocus traits, and mosaicism are mechanisms that explain deviations from the orthodox Mendelian pattern of inheritance.

    Let’s look at an example of two independently inherited traits in adults: the ability to drink significant amounts of milk (a cup or more at a time) and the ability to drink a can of sugar-containing soda without adverse effects (Figure 2.7). Milk contains lactose, large quantities of which (for instance the 12 g in one 240 mL glass) cause unpleasant digestive problems unless lactase is active in the small intestine of the drinker. Half of the sugar in soda consists of fructose (for instance 17 g in one 368 mL can), which triggers life-threatening hypoglycemia in people without a functional copy of the fructoaldolase gene (ALDOB; OMIM 612724) encoding fructose-1,6-bisphosphate aldolase (EC Adverse effects of either lactose or fructose consumption are recessive traits, which means that they occur only when both of the gene copies inherited from the mother and father provide inadequate amounts of functional enzyme. The law of independent assortment holds true in this example. The figure below shows the outcome (phenotype) when children inherit a particular set of alleles from each parent. If, for instance, the father provides the allele for milk intolerance (I), and the mother contributes the allele for milk tolerance (L), the child will have both an “l” and an “L” allele. Because the “l” allele is recessive, the “L” allele wins out and the child will be able to drink milk. Review some parental combinations and ask yourself (without looking) what the outcome (phenotype) will be in the child.

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    FIGURE 2.7. Inheritance of two independent traits: The ability to safely drink sugar-containing soda (indicated by the soda can) and the ability to drink milk (indicated by the milk carton) without abdominal discomfort are linked to different chromosomes and inherited independently of each other. The table shows the genotypes resulting from specific combinations of parental chromosomes. Since both traits are recessive, only children with two intolerance alleles have a problem.

    Along those lines, consider the following example. The father is homozygous for the lactose intolerance allele (due to lack of lactase persistence) and has a problem with drinking milk (Figure 2.8). The mother is homozygous for the fructose-intolerance allele (due to defective aldolase B) and gets low blood sugar (hypoglycemia) after drinking sugar-sweetened soda. Now, because both alleles are recessive, any children resulting from this union will be able to drink both milk and sugar-sweetened soda.

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    FIGURE 2.8. This is an example of two traits that are transmitted independently. Each parent is homozygous for a different recessive trait, one for the inability to drink a cup of milk and the other one for the inability to drink sugar-sweetened soda without causing discomfort.

    Mendelian principles of heredity

    Law of segregation

    An individual inherits one allele from the mother and another from the father.

    Source : www.sciencedirect.com

    lactase gene regulation activity

    Start studying lactase gene regulation activity- unit 6: genetics: proteins and traits. Learn vocabulary, terms, and more with flashcards, games, and other study tools.

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    What is the difference between lactose tolerance and lactose intolerance?

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    If you're lactose tolerant, you have the enzyme lactase which breaks down lactose into glucose and galactose and so you can digest milk because it contains lactose and you have the enzyme that can break it down.

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    Why is lactose tolerance also called lactasle persistence?

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    Because it's the trait that allows adults to continue digesting milk after infancy.

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    Terms in this set (21)

    What is the difference between lactose tolerance and lactose intolerance?

    If you're lactose tolerant, you have the enzyme lactase which breaks down lactose into glucose and galactose and so you can digest milk because it contains lactose and you have the enzyme that can break it down.

    Why is lactose tolerance also called lactasle persistence?

    Because it's the trait that allows adults to continue digesting milk after infancy.

    In which cells or tissues is lactase produced?

    the cells of the small intestine

    What is the role of lactase?

    break lactose into glucose and galactose

    In general, why is it important for a cell to regulate protein production?

    The cell needs enough protein for its needs, but at the same time, if there are too many, the system may be overwhelmed or energy could be wasted.

    What are the steps in gene regulation that affect protein levels in a eukaryotic cell?

    transcribing, translating, and processing

    What is the role of general transcription factors and where do they bind?

    They facilitate the binding of the RNA polymerase enzyme that catalyzes DNA transcription. GTF's bind to the promoter region of the gene.

    How do activators and repressors affect transcription?

    They regulate transcription. Activators increase transcription and repressors decrease it.

    Where do activators bind?

    To a region of DNA called enhancer region to enhance transcription. They also bind to the regulatory region.

    What are two ways in which repressors can interfere with transcription?

    Some can bind to the binding side of activators, thus preventing them from binding to DNA and so transcription cannot be activated.

    Some can order the chromatin structure to coil up tightly and that makes them unavailable for transcription.

    Why can different genes be expressed only in certain types of cells?

    Different combinations of multiple transcription factors such as activators and repressors regulate the same gene in different types of cells. This is why the lactase gene is on in a small intestinal cell but off in a skin cell.

    Is RNA processing a common way for regulating gene expression?


    What is alternative splicing and why is it important?

    Splicing different exons together. This is important for enriching the pattern of gene expression in eukaryotes.

    RNA interference is a major mechanism of gene regulation in eukaryotes. Explain how RNA interference works.

    Small pieces of RNA bind to mRNA to trigger degradation or block translation. This shuts down gene expression.

    Which proteins are marked for destruction?

    The ones that are no longer needed or damaged.

    How does a cell know which proteins should be destroyed?

    Large protein complexes called proteasomes recognize the ones that have been tagged with the molecule ubiquitin.

    How are these proteins destroyed?

    They are degraded by proteasomes

    At what level is the lactase gene regulated?

    at any step

    In what regions of the world is lactase persistence most prevalent?

    northern europe and africa

    How is lactase persistence an example of human evolution?

    The ability to digest milk is a recently derived trait. This means until very recently, we weren't able to digest milk. Therefore, we had to evolve to be able to digest milk.

    ... ...

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