Genetic Drift

Genetic drift is a change in in genetic variation due to chance changes in allele frequencies. For example, genetic drift may occur as a matter of luck; one phenotype may simply never meet a member of the opposite sex in order to mate and produce offspring. Genetic drift is not affected by the relative fitness of individuals to their respective environments. In most cases, genetic drift occurs in small populations to the smaller gene pool compared to large populations. After many generations, genetic drift may cause an allele to be lost or fixed- the allele will reach either 0% frequency or 100% frequency. Genetic drift may also be observed in large populations, but its effect (loss or fixation of an allele) happens much more slowly than it would in a small population.

Two examples of genetic drift are the founder effect and the bottleneck effect. The founder effect occurs when a subset of a population breaks off from the larger population and establishes its own colony. Because the subset is not likely a representative sample of the larger population, genetic drift will cause the smaller colony to have a much different gene pool than that observed in the larger colony. The bottleneck effect occurs when a random event decreases the size of the population dramatically. This can be caused by such events as natural disasters, such as hurricanes or earthquakes, or through the destruction of habitat by humans or other forces. Because the decrease in the gene pool is random, the allele frequencies in the surviving population may be different from that of the original population. Sometimes, alleles can be completely eliminated by these chance events.

Gene Flow / Migration

These bears lived in different populations with different allele frequencies. By meeting each other, mating, and producing offspring, they will have caused gene flow between their populations.

Gene flow refers to the movement of genes between two populations that may have different gene frequencies. For example, a deer population that lives on the east side of a river may have different allele frequencies than the deer population on the west side of the river. If one year there is a drought and some deer are able to move to the other side of the river, mate, and produce offspring, the allele frequencies of each population may change as a result of the new gene pool. Gene flow is also sometimes called migration because it is often caused by migration of one population into the territory of another population. Thus, the genes in each population also “migrate” to the other population.

Gene flow has two important consequences. First, allele frequencies in each population experiencing migration is reduced. Scientists can evaluate the relative similarities and differences in allele frequencies in order to determine how isolated  the populations are. The more similar the allele frequencies between two populations are, the less isolated the populations are in turn. Second, gene flow can introduce new alleles into a population. Since mutations are so rare, and beneficial mutations even rarer, populations that do have a rare beneficial mutation introduced into their population are most likely to share the allele via gene flow.

Non-Random Mating

Would you rather mate and produce offspring with the man above

or with this man? If you expressed a preference for either, you are practicing nonrandom mating based on phenotypic characteristics.

Non-random mating occurs when individuals choose mates based on phenotypic characteristics or genetic lineage. In non-random mating situations, the allele frequencies do not change, but the proportions of heterozygotes to homozygotes may change. Inbreeding is one example of non-random mating that increases the frequency of homozygotes in a population. Conservation biologists keep track of non-random mating in a population in order to make sure increased homozygosity does not lead to inbreeding depression. If evidence of inbreeding depression is seen, biologists may introduce animals from another population to increase the gene pool.

Natural Selection

Natural selection is a phenomenon in which the environment selects for individuals with beneficial traits and against individuals with unfavorable traits. Because natural selection is discussed in greater depth in other articles, it will only be mentioned as a factor here.

Sources of New Genetic Variation

Random Mutations

The different tail types for manx cats were brought about by random mutations.

Random mutations are changes in existing genes that can introduce new alleles into a population. However, mutations are very rare, and beneficial mutations are even rarer. In most cases, mutations are detrimental to an individual and those that do not result in death, disfigurement, or other forms of severe disadvantage are, at best, neutral. When a beneficial mutation does arise, other mechanisms such as genetic drift or natural selection must act upon them in order for the allele to rise in frequency in a population.

Gene Duplication

Abnormal events during crossover (meiosis) may increase the number of copies of a gene. Through several generations, the new gene family may have new presence in a gene pool and may have different gene products.

Exon Shuffling

Sometimes, instead of full gene duplications during crossover, exons may be inserted into another gene. The new gene may have new functions and will be acted upon by mechanisms such as genetic drift or natural selection before increasing in frequency in a population.

Horizontal Gene Transfer

Typically, horizontal gene transfer is seen in bacteria. Through events such as endocytosis, genes from one species may be introduced into the genes of another species.

Related posts:

1. Important Biology Terms
2. Population Genetics
3. Natural Selection

Population geneticists primarily study allele and genotype frequencies. They use quantitative methods to analyze the frequency of alleles. For example, a population geneticist may study the frequency of certain patterns on the fur of wild cats, then will revisit the same population several generations later to find how the frequency of patterns has changed from the initial measurement. This type of study would give scientists a good idea of what genetic changes are happening in a population.

Darwin’s theory of natural selection posits that only a certain percentage of offspring in any given generation will survive to reproduce. Whether an individual survives or not depends on the inheritance of alleles that will increase its ability to survive in its environment. Alleles that do not benefit an individual and increase its chances at survival will not be passed down to future generations, thus decreasing the frequency of that particular allele in a population. Population geneticists can study changes in allele frequencies from generation to generation in order to determine what mechanisms underlie the natural selection in a given population.

New genetic variations can arise through mutation, gene duplication, exon shuffling, and horizontal gene transfer. Alterations to existing genetic variation can occur by natural selection, random genetic drift, migration, and nonrandom mating.

Nonrandom mating does not change the allele frequencies in populations, unless other evolutionary forces are also present. The other ways of increasing genetic variation will affect the allele frequencies in populations because they tend to increase heterozygosity.

Inbreeding is one form of nonrandom mating. Inbreeding occurs when two genetically related individuals  mate and produce offspring. Homozygotes are more likely to be found in populations that have a high degree of inbreeding, due to the decreased genetic variation between breeding pairs. Sometimes, populations that have too much inbreeding will experience what is called an inbreeding depression. An inbreeding depression occurs when homozygotes are less fit to survive in their environment, resulting in decreased reproductive success in the population. Biologists will often intervene to introduce new genetic variation into a population by introducing new individuals, and thus new genes, into the population.

To calculate allele frequencies in a population, divide the number of copies of a particular allele in a population by the the total number of all alleles for that gene in a population.

To calculate genotype frequencies in a population, divide the number of individuals with a particular genotype in a population by the total number of individuals in a population.

For example, consider the following population:

• 49 dark green frogs with the genotype DD
• 42 brown frogs with the genotype Dd
• 9 yellow frogs with the genotype dd

Our population of frogs is diploid, meaning that each frog inherits one allele for a gene from each parent, so each individual has two total alleles for a particular gene. Homozygotes have two copies of the same allele; heterozygotes have one copy of two different alleles. This means that when we calculate our allele frequencies, we have to account for the fact that each individual frog has TWO copies of an allele for each gene.

To calculate the frequency of the r allele, we need to add up how many total d’s we have in our population. Each frog with the Dd genotype has one d, each frog with the rr genotype has two d’s, and each frog with the DD genotype has zero d’s. Once we get the figure for the total number of d’s in a population, we need to divide that by total number of ALL the alleles. We have:

(Dd) + 2(dd) / 2(DD) + 2(Dd) + 2(dd)

Keep in mind that we are multiplying the genotypes by two because we are counting alleles, and each individual has two alleles for each gene. Plugging in our population numbers to the above formula gives us:

42 + (2) (9) / 2 (49) + 2 (42) + 2 (9) = 60 / 200 = 0.3  = 30%

The allele frequency for d is 30%. Since we have only two alleles and each frequency must add up to 100%, we know that our other frequency, D, is 70%.

We can also calculate genotype frequencies. For this exercise, we are counting the genotypes, NOT individual alleles, so we do not need to multiply by two as we did in the above example. If we want to calculate the frequency of dd, we need to find the number of dd individuals in the population, and divide by the total number of individuals in the population. So we have:

dd / DD + Dd + dd

9 / 49 + 42 + 9 = 9 / 100 = 0.09 = 9%

So we know that 9% of the individuals in our population have the dd genotype. If we calculate the frequency of one of the other genotypes, we can add them together and subtract from 100% to find the frequency of the third genotype.

Understanding how to calculate allele and genotype frequencies is vital to understanding the Hardy-Weinberg Equilibrium and being able to use the Hardy-Weinberg equation. Be sure you understand how to do these calculations before you move onto Hardy-Weinberg problems.

Related posts:

1. Important Biology Terms
2. Increasing Genetic Variation
3. Mendelian Genetics Basic Definitions

• Personality changes
• Forgetfulness
• Involuntary movements
• Begins in middle adulthood
• Progresses over 15-20 years
• Leads to loss of motor control and intellectual function

Treatments:

• Nothing known to slow or stop the decline

Prevalence:

• 1 in 20,000 people

Famous People with the Disease:

• Woody Guthrie

Heritability & Environmental Factors:

• Caused by a dominant allele

Individuals with Huntington’s Disease have one dominant Huntington’s allele and one normal allele. It is unusual for a person to inherit two dominant Huntington’s alleles because this would mean both that person’s parents have Huntington’s. There is a 50% chance that a parent with Huntington’s will pass the gene on to his or her children.

Huntington’s Disease persists in the population because it does not take effect until after the reproductive years, during middle age. If it took effect prior to the reproductive years, Huntington’s Disease would be wiped out within a generation.

The Huntington gene was discovered in 1983 by the use of DNA markers. This was the first time DNA markers had been used to identify a linkage of a gene without any knowledge of the chemical mechanisms behind the disease. The gene is located near the tip of chromosome 4. It is possible to do a genetic test to determine whether an individual has the dominant allele for Huntington’s Disease. Amniocentesis can detect the presence of the Huntington allele in a fetus, as well.

The Huntington allele codes for a protein called huntingtin, which interacts with so many other proteins that it becomes difficult to find an appropriate drug therapy. The allele consists of a short sequence of DNA repeated many times (a triplet repeat), which is a common element in many types of genetic disorders. The particular repeat in the Huntington allele is CAG (cytosine, adenine, guanine) over and over again, hundreds of times. The gene product, huntingtin, contributes to neural death in the cerebral cortex and basal ganglia, which leads to the motor and cognitive defects of the disease.

For More Information:

Scientific American: Hunting Down Protein Interactions for Huntington’s Disease

Scientific American: Researchers Find Huntington’s Final Flaw

Science Daily: Protecting the Brain From Huntington’s Disease

Related posts:

1. Mendelian Genetics Basic Definitions
2. Important Biology Terms
3. Phenylketonuria (PKU)

• Mental retardation
• Becomes apparent in infancy/very early childhood, but is variable
• Excess of phenylpyruvic acid in urine

Treatments:

• Specialized low-phenylalanine diet

Prevalence:

• 1 in 10,000 people

Famous People with the Disease:

• Unknown

Heritability & Environmental Factors:

• Caused by a recessive allele

PKU is caused by a recessive allele, meaning an individual must inherit two copies of the allele, one from each parent, in order to be affected. If only one copy of the allele is inherited, the child will not be affected. If each parent is a carrier for the PKU allele, there is a 25% chance that their child will be affected with PKU.

The PKU allele can be detected with a genetic test. However, the genetic test is only somewhat reliable due to the many mutations of the PKU allele. A blood test will also screen for elevated phenylalanine levels in the blood of a fetus or infant, which aids in early diagnosis, but is not a definitive diagnostic tool. Different mutations can cause variability in the the blood phenylalanine levels of affected individuals, making an accurate diagnosis difficult. If caught early enough, the diet of the child can be switched to one low in phenylalanine in order to prevent the toxic build-up of the amino acid in the body, which leads to mental retardation.

Phenylalanine Structure

PKU is seen most often in children whose parents are genetically related. Because PKU is a recessive trait, the child must inherit the allele from both parents. If the allele is in a particular family, the chances of both parents having the allele are higher when the parents are genetically related. 1 in 50 individuals carry the PKU allele, making the chance of a non-genetically related spouse also carrying the allele about 2%. The risk is much greater than 2% if a carrier marries a relative. PKU has many known mutations, and can sometimes emerge in a family with no history of PKU.

The PKU gene was identified in 1984, and was found to be located on chromosome 12.

Related posts:

1. Huntington’s Disease
2. Increasing Genetic Variation
3. Deaf Parents With Hearing Children

During translation, tRNA may bind to three different regions of a ribosome. These are called the peptidyl site (P site), aminoacyl site (A site), and exit site (E site).

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2. Prokaryotic Cells vs. Eukaryotic Cells
3. How Cells Communicate

1. P generation – the “Parent” generation; the point of reference on which Mendelian predictions are based. Similar to an “index case” in a pedigree.
2. F1 generation – the first generation children of the P generation.
3. F2 generation – the second generation children of the P generation; the grandchildren of the P generation.
4. Dominant – the allele that is expressed even in the presence of other alleles for a characteristic.
5. Recessive – the allele that is present but not expressed in the presence of other alleles for a characteristic.
6. Gene – a sequence of DNA that codes for a particular functional product. Genes are responsible for particular characteristics of organisms, such as hair color or flower size.
7. Allele – different forms of a particular gene. A gene that codes for hair color may have alleles for brown hair, red hair, blond hair, etc.
8. Phenotype – the observable characteristics of an organism; the alleles that are expressed. If an organism has alleles for red hair and blond hair, but the observable characteristic is blond, then the phenotype is blond.
9. Genotype – all of the alleles present in an individual. In the example used in phenotype, both the red and blond alleles would constitute the genotype, even if only one is observably expressed.
10. Segregation – the process by which two alleles, one from each parent, separate from each other during heredity. Mendel’s first law of genetics refers to this process.
11. Trait – refers to the phenotype or possible phenotype within a genetic code. A characteristic refers to hair color, height, weight, etc. A trait refers to specific possibilities such as red hair, blond hair, tall, short, skinny, heavyset, etc.
12. Characteristic – see trait above.
13. True-breeding – a strain that has been self-fertilized or inbred for successive generations until the same characteristics are seen generation after generation. For example, Mendel’s initial pea plants were true-breeding in that every generation would give only tall plants from tall plants or dwarf plants from dwarf plants.

Related posts:

1. Important Biology Terms
2. Natural Selection
3. Population Genetics

In the earlier days of our knowledge regarding DNA, there were three possible models proposed for how DNA replication occurs. These three models were the conservative mechanism, the semi-conservative mechanism, and the dispersive mechanism. Currently, the only accepted model is the semi-conservative model.

In all these models, the two original strands of DNA are called ‘parent strands’, and the newly created strands of DNA are called ‘daughter strands’. In the semi-conservative model, the parent strands are separated and daughter strands are paired one each with the parent strands. This is called the semi-conservative model because in the conservative model, both parent strands of DNA are left together in the helix, thus conserving the original structure and pairing.

The initial evidence for the support of the semi-conservative model was researched by Matthew Meselson and Franklin Stahl in 1958. They used different isotopes of nitrogen, which is present in DNA, to identify the mechanism used in DNA replication. A heavy isotope of nitrogen was used in a medium to grow E. coli colonies for several generations. In this way, all of the DNA present in the E. coli after several generations was labeled with the heavy isotope of DNA. Then, the E. coli with heavy nitrogen was switched to a medium with a light isotope of nitrogen. By doing this, the scientists had a way to identify any new DNA created- any new DNA made after the switch would be created with the light form of nitrogen, but the original strands would keep the heavy form of nitrogen. Using centrifugation, the scientists were able to separate the DNA with different densities. A DNA helix with all heavy nitrogen will fall to the bottom of a solution, a DNA helix with all light nitrogen will rise to the top of solution, if a DNA helix contained both the light and heavy isotopes of nitrogen, it would float somewhere in the middle. The semi-conservative model was supported by the results; the E. coli tested had a mixture of light and medium-weight DNA after only two generations. These results are consistent with only the semi-conservative model of DNA replication.

For more information:
Wikipedia

emunichs.edu

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2. Semi-Autonomous Organelles
3. Mendelian Genetics Basic Definitions

Ha ha, NOW I’ve got your attention. The little buggy roaches don’t even PEE! Check out the article to find out how and why. And people dare say that humans are higher up on the evolutionary ladder? I heard somewhere- I forget where exactly- that one might like to postulate that God is a cockroach. The man may have a point- I have a hard time picturing God running to the bathroom after too much water during a long meeting with Michael and Gabriel and my great-great-grandmother. But I digress. Check out the article.

Cockroach Superpower No. 42: They Don’t Need to Pee | Wired Science

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2. Observations: Hair dye will soon debut in U.S. that has the power of ammonia without the smell
3. Bigger not necessarily better, when it comes to brains

We all learned in biology and genetics classes that the age of a mother has more effect on genetic disorders than does the age of the father. For example, the occurrence of Down’s Syndrome is closely tied to mothers over the age of 30 and increases over time, whereas the age of the father does not appear to be correlated with the occurrence of Down’s Syndrome.

Recent research is showing, however, that the father’s age does make a difference in some cases. Is there evidence for the “Biological Clock” for men? Check out the link above for more information on this new research.

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1. Grief: Miscarriage, Stillbirth, and Neonatal Death
2. Good News For People Who Have a Heart and Plan to Get Old
3. Disinfectants may promote growth of superbugs

Here is an interesting bit of new research regarding the genetics of age-related heart failure. For those who don’t want to read the whole scientific article, I have condensed the main points into a bullet point list under the link.

via Suppressing A Gene In Mice Prevents Heart From Aging, Preserves Its Function.

• Aging causes changes in our bodies that reduces the functionality of various systems, including the cardiovascular system.
• Suppression of a gene that is part of insulin production and tissue aging led to mice essentially seeming younger in measurable, biological ways compared to a “regular” old mouse.
• In short, suppression of the gene slightly changed the function of insulin and allowed heart cells to live longer. However, we aren’t sure exactly how this entire process worked in the mice; we just know that it did, and we need to find out if humans would be similarly affected.
• Additional research in this area will hopefully lead to mechanisms by which loss of heart cells and age-related cellular abnormalities can be reduced.

American Heart Association (2009, October 14). Suppressing A Gene In Mice Prevents Heart From Aging, Preserves Its Function

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1. Tissues vs. Organs
2. Why Cells Communicate
3. Important Biology Terms