Genes are the basic units of heredity within which the characters or the traits pass on from one generation to another. Genes are the DNA sequences that are stored on chromosomes, and each gene codes for a specific feature like Eye color or diabetes or any other trait. Alleles are alternate forms of a gene and are the variations of a gene. Also, it is the basic key to all the variation that we see in humans and all other organisms. These are the alleles themselves that determine whether you have one of many possible eye colors in humans or no eye color at all. In simple terms, it differentiates between individuals’ visible traits that can sometimes be non-visible as well.
In General, when we think of the nature of the alleles, we usually think of them being a Dominant one or Recessive one. But the working of an allele is not that simple. The alleles for a gene can also have an additive effect as well and not only this but the phenotype of an individual from the action of the additive alleles mainly depends on the number of alleles that are present at a time. A good example that could be taken here is that the continuous variation that we see in the skin color is also determined by the additive alleles. The more alleles we have for high melanin, the darker will be our skin color; and lesser the alleles for high melanin contained within, the lighter will be the skin color in appearance. Additive alleles also may differ from the dominant alleles in the fact that it only takes one of the dominant alleles to inherit the phenotype in an organism. So, in this sense we can say that if the skin color was determined by a dominant allele then it will not matter how many alleles for the dark skin color you may have. If you have only one dominant allele of a darker skin color then your skin color will be dark irrespective of how many alleles you have for the low melanin content. Additive alleles show levels of intensities to be shown and not only show the presence or absence of any phenotype.
Additive genetic effects are said to take place when two or more genes gives a single contribution to the final phenotype in an individual or when alleles of a single gene, in case of hetero-zygotes, combine in such a way that their combined effects will be equal the sum of their individual effects. In a broad sense, many phenotypes or many of the observable traits, are not primarily determined by a simple dominant or a recessive allele. People show a huge variation in many things which can even be in small aspects such as skin tone, hair color, height and weight.
EFFECT ON SKIN
If we talk about the additive effect in the skin, then there are nearly three genes that determine how much melanin is present in the skin, so in this case each person will inherit six alleles. This leads to a wide variety of skin tones in all individuals. As we have mentioned earlier the more allele you inherit for high melanin content, the darker the skin color will be and the more allele we inherit for a low melanin content, the skin color will be lighter. A case could also be seen where a parent who has all three alleles constituting high melanin and other three alleles constituting low melanin, such parent could have their children with skin tones which could range from very fair to very dark color. This is one of the reasons why the twins sometimes are born with an entirely different skin tone.
When we think about the genetics of a character, then we usually think about the relationships between the alleles. The relationship between two different genes is also an important factor in determining the physical features of an individual. Diversity arises because multiple genes interact to determine these kinds of traits. When more than one gene is determining a trait or character, the outcome is said to be determined by the interactions of the genes altogether and this is called an as polygenic trait. When one gene determines may character then that is known as the Oligogenic trait which is just reverse of the polygenic trait.
As we know that the genes can have an additive effect as well so when we have a gene that simply codes for the same trait and has the same outcome or a similar result is obtained on the phenotype then it is called the additive gene effect. The genes that determine how much melanin content a person has in their skin is a good example of the additive gene effect.
Among a set of the population there are uncountable potential sources that give rise to phenotype variations. Each of these sources shows a separate underlying origin. These sources of the phenotype variations decide that whether the character can bring a change and holds an ability to respond to a natural and artificial selection or whether it can respond to the surrounding environmental variations. A large number of investigators around the world are aiming to determine the significance of both the genetic and environmental factors and the role they play in regulating the phenotype expression of a particular trait. An additive genetic variance occurs mainly due to the genes which tend to show an additive effect on the quantitative trait. It measures the degree to which the phenotype differences of an individual can be prophesied due to the additive effects of allelic changes.