Quite a number of us have heard about genes; from biology classes to the doctor’s, to our everyday conversations, genes are a component of our body that we are quite familiar with. But then, however familiar we are with the word ‘genes’, there`s still a whole lot more to learn. Get ready!
So what Are Genes?
Our genes are located on the chromosome in the nucleus of a cell, and they are simply the functional units of heredity. Genes are essentially made up of the DNA and they function as instructions to help us make proteins. Genes may be of various sizes, ranging from a few hundred DNA bases to about 2 million bases and normally, we have about 20 to 25,000 genes.
The word ‘gene’ was first introduced by the Danish botanist and geneticist, Wilhelm Johannsen in 1905. It has its root in the Greek word: ‘gonos’, meaning ‘offspring and procreation’. Therefore, genes are primarily concerned with how we make our offspring and the traits they bear. Each of us has two copies of every gene, each inherited from one parent. While sexual organisms inherit genes from their parents, asexual organisms simply replicate a complete copy of their parents` genes to form theirs, which is why such an offspring is an identical copy of the parent.
Let’s Talk about Alleles
Quite simply, an allele is a variant form of a gene. The words ‘allele’ and ‘gene’ are frequently mistaken by a whole lot of people and we’ve got to demystify the concept of alleles. Let`s go!
Let us take X, a 6.4-inch-tall person, for instance. If X has got the genotype ‘Tt’, then ‘T’ is the allele for tallness while ‘t’ represents the allele for shortness, provided the allele for tallness is dominant over the one for shortness. Now, X’s ‘T’ and ‘t’ alleles must have come from their parents’ genes—either the ‘T’ from the father and the ‘t’ from the mother or vice versa. These alleles then interact during mating.
Dominant and Recessive Genes
A dominant gene is that which expresses itself phenotypically (physically), whereas recessive genes are those genes that cannot be expressed in the presence of the dominant ones—that is, a recessive gene can only be expressed when both alleles that make up the gene are recessive. For example, if the allele for tallness (T) is dominant over the one of shortness (t), then one can only be short when both genotypes are ‘tt’, otherwise one is tall.
Also, dominant genes are the superior genes—they express themselves even in the presence of recessive genes. For example, if an individual has the genotype ‘Tt’, such an individual is likely to be tall—because the allele for shortness is recessive in this case, so it cannot be expressed, except they have a poor nutritional health or their environment is not favorable for growth. If another person has the genotype ‘TT’, they are also likely to be tall. There is no general yardstick with which we can determine the dominance or otherwise of a gene except by an individual’s physical appearance or through medical examination.
What about Genotypes?
Genotypes are the expression of genes which determines the functioning of our bodies, our physical appearance and a whole lot more. Genotype covers a whole lot, from sickle-cell to blood group, and other traits.
Talking about the blood group genotype, three alleles control the determination of our blood groups: the A, B and O alleles. O is the recessive allele while A and B are dominant. Therefore, if one has the AO genotype, they would exhibit the characteristics of one who has the A blood group. In like manner, if an individual has the BO blood group genotype, such individual will exhibit the traits associated with the B allele. However, if there’s a situation where one has the genotype AB, then there is the concept of incomplete dominance.
Incomplete dominance occurs when we have two dominant alleles in our genotype. In such a case, neither of both alleles expresses itself completely.
The sickle cell genotype is also characterized by two alleles: the A and the S alleles. In stark contrast to the blood group genotype, there is no recessive allele here. The genotype ‘AA’ represents normal hemoglobin, the genotype ‘SS’ represents sickle-cell hemoglobin, while incomplete dominance occurs with the ‘AS’ genotype.
Based on this explanation, we see that a person’s physical appearance is primarily the sum total of the person’s genotypes, except for environmental influence.
Ever Heard about DNA and RNA Genes?
DNA genes are present in the chromosomes of most living creatures, except a few viruses: the RNA viruses and retroviruses. Organisms which have the DNA genes copy their coding sequence to form RNA, which are then translated to proteins which eventually lead to what we call genotypes.
However, a few viruses (such as the Herpes virus) do not need to follow these steps to get their proteins synthesized. Since there are no DNAs in their genes, the process of transcription—which is the conversion of DNAs to RNAs—is bypassed. Hence, their hosts help them synthesize their protein without the delay in carrying out transcription.
There is another class of organisms: the RNA retroviruses (such as the HIV), which require a reverse transcription of their genes from the RNA form into the DNA form before their proteins can be synthesized. These are usually the most difficult viruses to get rid of.
This is another term that many of us are quite familiar with. Mutation is simply an alteration in the DNA sequence in our genes. Mutation can also mean an error that occurs during DNA replication. It is estimated that in each generation, our genes are likely to experience 1-2 mutations. Mutations, especially the larger ones, usually occur during recombination of the DNA pair to cause abnormalities in the chromosome, leading to deletion, inversion, translation or duplication of a greater portion of the chromosome. These mutations are caused by chemicals that cause cross-linkage of DNA strands, and excessive exposure to radiation.
Genetic mutation leads to a number of disorders, such as cystic fibrosis, sickle-cell anemia, and color blindness. These diseases are caused by the mutation of a single gene, in which case it is the mutation of the hemoglobin gene for sickle-cell anemia, the CFTR gene for cystic fibrosis and the red and green cones for color blindness.
A few mutations that we humans are currently undergoing are quite beneficial to us. Examples of these mutations include the Apo-AI Milano gene which reduces the risk of heart diseases and the LRP-5 which reduces the risk of getting osteoporosis.
How Does Our Environment Influence the Expression of Our Genes?
Our environment influences a whole lot in our lives, one of which is the exhibition of genotypic traits. Our genes are greatly affected by our internal environmental factors such as hormones, metabolism and our gender. Gender-influenced genes are known as sex-linked or sex-influenced genes. An example of a sex-linked gene is the gene for baldness, which is influenced by the actions of testosterone and dihydrotestosterone. Since these two hormones are present in higher quantity in males than in females, males tend to be bald, while it is rare to find the condition among females.
The external environment, on the other hand, is also a deciding factor in the expression of genes. For example, melanin is a pigment responsible for the color of the skin and an individual who lives in the tropical region is likely to have higher melanin levels than another fellow who lives in the temperate regions and as such, is more likely to have a darker skin color, irrespective of the genetic make-up of such individual.
Our genes are what make us. The kind of food we eat and our environment matter a lot in the expression of our genes. Now that you know more about genes, eat good food, stay away from radioactive substances, live under favorable conditions, and stay healthy!