The Bases of Genetics: Everything You Need to Know

Today, we'll talk about the basics of genetics, including DNA and RNA, genetic terms, and Mendel's three laws, among other things.

Written and verified by el biólogo Samuel Antonio Sánchez Amador.

Last update: 09 May, 2023

Genetics is the area of study of biology that seeks to understand how inheritance is transmitted from generation to generation in populations of living beings. In nature, the only vital engine of all animals – at least that we know of – is the continuation of their own lineage through reproduction, even above existence. Continue reading, because today we’ll present the bases of genetics.

Heredity codifies all animal behavior in one way or another. From sexual selection to genetic drift, evolutionary forces base their machinery on patterns of inheritance to encourage or discourage certain adaptations throughout the history of life.

Genes don’t only live by nature because genetics is also present in human society: From eye color to certain types of cancer, we too are conditioned by heredity patterns and their underlying mechanisms.

The bases of genetics: DNA and RNA

We can’t start without laying the foundations of the most important molecules as far as heredity is concerned. DNA is the structure of life, as in most living beings, this double helix contains all the information necessary for protein synthesis and the modulation of processes at the cellular level. Human DNA is a library of about 25,000 genes.

As summarized by scientific sources, DNA contains the code to create and maintain the entire body. This book is made up of various phrases that encode various processes, according to the order of the letters stored.

The term letters is quite literal, as each of the nitrogenous bases that characterize each nucleotide —the functional unit of DNA and RNA— is designated a letter: Adenine (A), cytosine (C), guanine (G), thymine (T ) and uracil (U). Thymine is unique to DNA, uracil to RNA, and the rest are common to both.

For example, serine is an amino acid that’s coded by the letters AGC, that is, adenine, guanine, and cytosine. A concatenation of various amino acids—each coded by three letters—will be encased in a larger segment of DNA and, as you guessed, these will form the proteins that give us life and metabolism.

On the other hand, RNA is in charge of transporting the information present in the nucleus of the cell to the ribosomes so that proteins are built in complex processes known as transcription and translation. This is how genes are transformed into tangible structures and physiological processes.

The bases of genetics: Terminology

Bioinformatics portals collect the basic terms related to the world of heredity and genetics. Once we’ve established the fundamentals of the molecules on which these processes are based, it’s time to explore them.

• Gene: This is a particle of genetic material that, along with others, is arranged in a fixed order along a chromosome. It encodes a gene product, either a protein or RNA.
• Allele: Each of the alternative forms that a gene can have. In diploid beings —that is, humans and many other living beings— we usually speak of two alleles per gene; one inherited from the mother and one from the father.
• Chromosome: A structure organized by DNA and proteins. Each cell in the human body has 23 pairs of chromosomes, 46 in all, half of which come from the mother and half from the father.
• Genotype: The genetic information possessed by a particular organism in the form of DNA.
• Phenotype: The expression of the genotype, that is, the physical qualities of an organism encoded in the genes, in a certain environment.

Mendel’s laws in genetics

Building the foundations of genetics without taking Mendel’s laws into account is like making soup without broth: It’s impossible. To understand these complex postulations, we must first define certain terms. Let’s take a look at an example.

Honoring Gregor Mendel himself, we’ll begin by saying that the characteristic of a pea seed color is encoded by two alleles: One for yellow (A) and one for green (a). As each gene has two alleles, one from the father and one from the mother, the possible combinations are AA, Aa, and aa. Let’s apply the necessary definitions.

• Dominance: Predominance of the action of a genetic factor over its alternative. In this case, the yellow allele (A) is dominant over the green (a), hence its representation with the capital letter.
• Recessiveness: A recessive allele only occurs when the dominant allele isn’t present, that is, when it’s accompanied by another allele equal to it. The allele that codes for green (a) is recessive.
• Homozygous: Both alleles of the gene code for the same information. This would be the case of the combination (AA) of yellow color and the combination (aa) of green.
• Heterozygous: The two alleles of the gene would be different from each other. This is the case of (Aa).

In these examples, the combination of alleles (AA) (Aa) and (aa) would be the genotypes and the green or yellow colors would be the phenotypes. The one that interests us the most is (Aa), as what color will the pea be? Because the allele that codes for yellow (A) is dominant over green (a), despite being heterozygous, it will still be yellow.

In conclusion, the green color will only be expressed with the genotype (aa). Once these terms are defined, it’s time to explore Mendel’s laws per se.

1. Mendel’s first law: The principle of the uniformity of hybrids in the first filial generation

The Virtual Science Museum (CSIC) explains what this first law consists of. When two homozygous individuals for a characteristic are crossed, in this case (AA) and (aa), following Mendelian principles, the first descendant generation will be equal to each other.

AA x aa= Aa, Aa, Aa, Aa

Because one allele comes from the mother and one from the father, the offspring have only one possible combination (Aa). As the yellow allele is dominant over the green allele, all descendant peas will be yellow and heterozygous for the trait. It’s that simple.

2. Mendel’s second law: The principles of segregation

The issue is complicated by crossing the individuals of this first offspring generation with the genotype (Aa). In this case, the offspring of these heterozygotes will again present —in a lower proportion— the recessive color green. Why?

Aa x Aa: AA, Aa, Aa, aa

Gametes only have 23 chromosomes (they’re haploid) —instead of the 46 of normal cells (diploid)— so, as we’ve said, a father will contribute one allele to the offspring and a mother the other, thus giving rise to a diploid cell. Therefore, there are four possible scenarios when combining (AA) and (aa) :

• 1/4 will be homozygous dominant (AA): Yellow color.
• 1/4 will be homozygous recessive (aa): Green color.
• 2/4 will be heterozygous (Aa): Yellow color, as (A) is dominant over (a).

Mendel’s second law explains the reappearance of recessive characters in the second generation through the crossing of the heterozygotes of the first.

3. Mendel’s Third Law: The Law of Independent Association

We can summarize this law in the following concept: Different traits are inherited independently of each other and there’s no relationship between them. Therefore, the inheritance pattern of one trait won’t affect that of another.

This is only true when the traits are encoded by unlinked genes. If two genes are very close on the same chromosome, they’ll be inherited as a unit, so the association in these cases isn’t independent.

Natural selection and genetic drift

All these definitions and terms are interesting, but how do they apply to living things? The answer is complex, but we’ll try to explain it in these last lines.

Natural selection is an evolutionary phenomenon that refers to the differential reproduction of the genotypes of a biological population. Using the same example as before, we could speculate that the yellow color of the peas highlights the seeds in the environment, so they’d be more likely to be preyed upon by herbivores.

Therefore, the yellow seeds (AA) or (Aa) would hardly leave offspring. They’d disappear before they could become plants. Green seeds, on the other hand, would transmit their genotype (aa) throughout the generations, because even if some were preyed on, the rest would go unnoticed long enough to become plants.

This is natural selection. Mutations arise spontaneously in living beings and some are deleterious, others are mute, and others are beneficial. Those positive adaptive traits end up being fixed in populations, as the individuals that present them reproduce more.

Another very different concept is that of genetic drift, as we’re dealing with a random process by which living beings lose and gain alleles based on stochastic sampling. For example, if out of 10 peas, only one is green (aa) and by chance, a bird eats it, if the rest are homozygous yellow (AA), then the green character would disappear forever.

Genetics and the key to life

We hope that the world of genetics has become clear by means of the principles established above. In any case, it should be noted that genetics is, of course, much more complex, as in many cases, genes code for more than one character or don’t follow Mendelian patterns of inheritance.

These examples are useful to know the terminological bases of genetics, but by no means can they be applied in all cases. Of course, heredity and its mechanisms present more mechanisms to unravel, although one thing is clear to us: The genome is the key to life.