Population Genetics - An Overview

Introduction

Population genetics is a basis for studying the genetic variations and various evolutionary changes that occur in a given population at a particular time. Mendelian inheritance and biostatistics together are involved in understanding population genetics. Population genetics also deals with mutation (the process by which structural changes occur in a gene and variants are produced, which gets transmitted to the next generation), which is responsible for all kinds of genetic variations as well as the evolutionary changes in the given population. Genetic mutations have three primary roles: beneficial changes and deleterious effects like cancer, and in many cases, mutations may produce no significant results. Various research was carried out in order to note the fate of mutations, and it was concluded that mutation and its effects depend on various factors.

The following are a few factors that affect mutation:

• Presence or absence of other mutations.

• Other environmental factors.

• Size and structure of the population.

Various theories work on understanding the concept of population genetics, including

• Muller's ratchet theory.

• Effective neutrality.

• Fixing beneficial mutations.

What Is The Molecular Basis of Inheritance?

The molecular basis of inheritance comprises heredity, genes, and their variations. It explains the role of genetic materials like DNA in living beings. For a better understanding of medical genetics, conceptual knowledge about the molecular basis of inheritance is fundamental. Friedrich Meischer, in the year 1869, discovered DNA and named it nuclein. Later, in 1953 double helix structure of DNA was proposed by James Watson and Francis Crick. The basis of inheritance revolves around the following:

• Chromosomes.

• DNA (deoxyribonucleic acid).

• Genes.

Inside the nucleus of every cell, chromosomes are present, which are made up of DNA, and DNA organizes itself to form genes. Each cell has 23 pairs of chromosomes which mean 46 chromosomes. In certain genetic disorders, the number of chromosomes varies. We inherit one set of chromosomes from each parent, representing 23 chromosomes from the mother and 23 from the father.

What is Meant by Inheritance?

A child inherits genes from both father and mother both. These genes code for traits (like hair color, height, etc.). Genes that code for traits from the parents are of two types, the first dominant and the other recessive. In the Indian subcontinent, black hair color is the dominant trait. So, for example, a gene for the father with black hair could be coded as BB (dominant) or BB (codes for black color), and genes for the mother with brown hair will be coded as BB (recessive).

Now, the following could be the probable hair color of the child depending on the combination of the gene the child inherits

• BB- (black) 25 percent of the children.

• BB- (black) 50 percent of the children.

• BB- (brown) 25 percent of the children.

So, the ratio between black and brown hair color is 3:1; hence 25 percent of the children born could have brown hair. The above-mentioned inheritance pattern is based on the Mendelian rules.

What Is the Role of DNA in Human Genetics?

DNA serves the following actions:

1. Replication.

2. Transcription.

3. Mutation.

Apart from DNA, the other genetic material is RNA (ribonucleic acid). RNA acts like the messenger of information, and unlike DNA, it cannot replicate itself.

• Replication represents the ability of DNA to make copies.

• Transcription describes the ability of DNA to transfer genetic information from DNA to RNA.

• Whereas mutation represents the ability of DNA to alter the genetic structure, which leads to genetic variation. The DNA programs our appearance, behavior, and other unique personal features, resulting in a resemblance between parents and children.

The following sequential steps can explain the central dogma of molecular genetics:

• The genetic information gets transferred from DNA to RNA.

• The process that uses the information given by RNA to form protein.

• The previously mentioned sequential steps produce all the unique characteristics that describe an individual. For example, if the DNA has to program for the height of an individual, then:

Firstly, DNA will collect all required sets of information about the size. Then it will be transferred to the RNA by the process of transcription. After that, the mRNA (messenger RNA), which carries the information given by DNA, will transfer it to the RNA. This information will then be converted into a protein. These proteins, in the end, execute the DNA program and activate subsequent processes required for the development of the height of an individual. So, the primary genetic information is located inside the nuclear DNA, and a few small pieces of information are found in the mitochondrial DNA.

What Are the Types and Rates of Mutations?

Mutations are caused due to alterations in the genetic material. Based on these alterations, mutations have been classified into six types:

1. A point mutation that causes single nucleotide polymorphisms. In simpler terms, it refers to an alteration in a single nucleotide base in a DNA sequence.

2. Insertion or deletion of DNA sequences can result in variations in DNA copy numbers. In terminal deletion, the segment of chromosomes or genes is missing from the terminal part with a single break. Intercalary deletion describes the type of genetic error in which the segment of a chromosome or the genes are missing from the between. In such a kind of deletion error, two breaks in the segment can be identified. Homozygous deletion describes the loss of chromosome segments or genes from the homologous chromosome pair, and heterozygous deletion represents the loss of chromosomal segments or genes from any homologous chromosome pair.

As stated above, deletion is the type of genetic error in which loss of genetic material and loss of genes are associated with deleterious effects. This error is expressed with a range of signs and symptoms specific to every genetic disorder.

1. Transposition type of genetic mutation occurs when the DNA sequences move from one location to another.

2. Inversion is the type of genetic mutation in which the orientation of the DNA sequence gets changed.

3. Genetic mutations can also occur at the ploidy level, where the entire genomic sequence is either gained or lost.

Researchers are still working on understanding the rate of mutations as the instances of mutation that cause significant changes are very few, and keeping track of it seems to be complicated.

Conclusion

Inheritance determines the individual's characteristics, and DNA's ability to mutate is a platform for human evolution. Various researchers are currently studying the inheritance pattern, and this research work also focuses on locating the genes that help identify the cause of a particular disease. Genetic mapping is the latest advancement in the field of genetic testing.