What are the Basics of Population Genetics? 

A gene is a unit of heredity and contains both physical and functional information that shapes an individual. Genes are made up of DNA (deoxyribonucleic acid), which carry genetic information from one generation to another, from one set of parents to their offspring, and so on. Every cell in a human body, or any living organism, has the same DNA, which implies that every cell in an individual’s body has all the information it needs to build and sustain the body! 

What is an Allele? 

An allele, also called allelomorph, is a variant of a gene- one part of the whole. A child gets one variant of genes, an allele, from his mother and one allele from his father. The allele can be imagined as one piece of a two-part puzzle which fits perfectly with another piece of the puzzle- one allele from each of the two parents- to make the individual. Alleles occur in pairs, and they are responsible for phenotypes of different traits. Now, if the allele pair is the same, the organism's genotype is called homozygous for a certain trait. On the other hand, if the paired allele is different, then the genotype is heterozygous. There are dominant and recessive alleles, while some may even be neutral, or codominant.  

Thus, the alleles account for the genetic similarity with parents, and members of a particular family that often resemble each other in a few or many characteristics. These features include having the same or similar eye color, hair color, skin color, height, as well as the shape of eyes, chin, nose, lips, face, etc. If a person resembles his mother or father or his sister looks like a great-grandmother, it is because of alleles and genetics, and the probability of them being passed on to the person. 

What is Allele Fixation? 

In the context of population genetics, fixation is when one allele dominates another out of existence. Over time, in a gene pool, there may be two or more alleles coexisting simultaneously. Eventually, one allele becomes the norm, and the others disappear from the gene pool. This allele becomes ‘fixed’; it becomes permanently established and has 100% frequency in the future population.

Note that allele fixation is an entirely different process compared to gene mutation or heterozygote advantage, wherein a heterozygous genotype might sometimes possess better fitness, and chances, relative to other genotypes like either the homozygous dominant or homozygous recessive. 

In simple words, when multiple alleles exist, they can either be permanently lost from a population or permanently exist in it. If the alleles appear, then they are said to be fixed. In turn, the gene made up by the fixed allele becomes the dominant one for future generations.  

The loss or fixation of alleles depends on several factors, mainly on selection coefficients and random fluctuations in the proportions of alleles.  

Does Gene Mutation Affect Fixation? 

Sometimes, a previously non-existent allele comes into being due to genetic mutations such as gene substitution. This newly mutated allele could very well become fixed if it spreads through the effective population size by positive selection. This kind of selection is also called Random Genetic Drift.  

Genetic drift happens when the number of alleles in a population starts to fluctuate at random. It causes alleles to increase, or decrease, over time. It is a key component of allele frequencies. 

Now, it is important to note that genetic drift, most typically, happens in small populations where certain alleles that aren’t as frequent as others stand a greater chance of being lost. Once the process of genetic drift starts, it will either cause the allele to be lost or become fixed. Now both these scenarios can affect genetic diversity; in most cases, they decrease how diverse the gene pool becomes, causing a population bottleneck.  

A population bottleneck is observed when the size of a population reduces considerably. It could be the result of various natural or man-made factors such as environmental disasters, overhunting, or reckless habitat destruction. Genetic drift is most common after such bottlenecks and can cause rare alleles to be lost and further decrease genetic diversity. In this way, a new population can become genetically distinct, with little or no genetic resemblance to their ancestors. Therefore, one could infer that genetic drift has a role to play in the evolution of new species, ardently leading to enhancement in the diversity of life.  

Is Genetic Drift Connected to Evolution? 

Evolution is the gradual change in the distinctive characteristics of species over time, several generations over several millions of years, that makes the organism better adapted to sustain life. Evolution relies on natural selection, and sometimes chance, the mechanisms that introduce changes in the characteristics of an organism. These phenomenadescribe how simple cellular organisms blossom into complex birds and animals, and even humans, over millennia! 

In terms of population genetics, however, evolution is viewed from a different lens. It is defined in terms of alleles. More specifically, in terms of the change in the frequency of alleles in a population over some time. So, evolution is any change in allele frequencies; it is not limited to natural selection, other related mechanisms, or even adaption. Genetic drift is random and changes allele frequencies without any selection bias.  

What is the Hardy-Weinberg Principle? 

This principle states that allele and genotype frequencies in any population will remain constant from one generation to another. But this principle only holds true when no other evolutionary influences like genetic drift, choice of mate, assortative mating, sex selection, natural selection, mutation, meiotic drive, bottleneck, founder effect, inbreeding are present.Thus, the principle essentially states that an equilibrium is reached when the evolutionary influences (as mentioned above) are absent.  

G. H. Hardy and Wilhelm Weinberg demonstrated the principle mathematically in papers that aimed to debunk the then-popular notion that a dominant allele almost always tends to increase in frequency. More recently, the Hardy-Weinberg principle is used for testing genotype frequency for population stratification and other non-random mating forms.  

Common Misconceptions 

1. Contrary to popular belief, genetic mutations and changes in allele frequencies are good for the diversity of life as they do not restrict genes or the passing on of characteristics in the long run. 
2. A chromosome, a gene, and an allele are all different! They may be closely related, but are distinct entities in population genetics. 
3. Over a long enough time, there are no dominant alleles. Every allele has a random chance of being fixed and therefore is expressed in the phenotype.  
4. Because the whole process of genetic drift is random, the risk of genetic malfunction or diseases cannot be predicted with accuracy. For instance, a couple with a one-in-four probability of passing on a disease to their children cannot determine which of their four children will have the disease. Even if the firstborn contracts the disease, it has no bearing on whether the others will be risk-free; they can all have the disease. It is a pure chance! 
5. Again, contrary to popular belief, genetically modified crops are not bad for people! All the crops that make up the food have been genetically modified at some point in their evolutionary history to make them cultivable or increase the yield per crop.   

Context and Applications  

This topic is significant in the professional exams for both Bachelors and Masters courses related to biology. Some of the courses are listed below:  

• Bachelors in Genetics  
• Bachelors in Biochemistry and Molecular Biology  
• Bachelors in Ecology and Evolutionary Biology  
• Masters in Biological Science  
• Masters in Biotechnology  

Related Topics 

1. Genetic drift 
2. Evolution 
3. The bottleneck effect 
4. The founder effect 
5. Allele frequency 
6. Evolutionary biology